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01-10-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 101
Post ID: 9374
Reply to: 9321
Some talks about the new production tubes
fiogf49gjkf0d

It is 300B but I do not think that there is any difference.

http://www.enjoythemusic.com/magazine/equipment/1201/300b/

http://www.enjoythemusic.com/channel/ch300b/

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-10-2009 Post does not mapped to Knowledge Tree
drdna
San Francisco, California
Posts 505
Joined on 10-29-2005

Post #: 102
Post ID: 9375
Reply to: 9374
New 2A3 tubes
fiogf49gjkf0d
You will be happy to hear I have got the new 2A3 tubes. I have spent the evening listening to them all. Some very interesting results! I will be posting soon...
Adrian
01-10-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 103
Post ID: 9380
Reply to: 8932
Some thoughts about one-IHT vs. multiple-DHT
fiogf49gjkf0d

 nl wrote:
I have used Russian high-gm tubes like 6C45 in amps before, including single-stage designs very much like Romy's single-stage Milq. These have many attractions. However, in my experience it just didn't have the tone. Jim de Kort of VT52.com came to the same conclusion, regarding his WE437 spud using amorphous output transformers, on his 107db horn speakers. He went back to his 26-10-45 combo, three stages instead of one.

  Jim de Kort's wrote:
Even a single tube performing all the tasks can't come close to my three tubes per channel DHT amp system. There is no getting around the immense difference in sound. Yes, the 437A and 6S45P sound incredible and perform a unique task, but compared to a DHT it really can't hold water. I was doubtful about the comparisson before, but now I am fully convinced. Even three stages of (good) DHT's can't be beaten by a circuit using only one IHT.

 Romy the Cat wrote:
Meanwhile the upper range of PP2000 is back completely and oh boy – I do not like my MF channel now. I clearly would like it to be loaded to 2-3 time higher impedance; I mean much higher, the way how I use to have it 2 years back. If you remember my progress with S2 driver then you will see how I fought with it “liveliness” by loading the stage that drives S2 harder and harder. The first Melquiades was driving S2 with 1700R that is VERY high impedance for 6C33C.  The best load I go at the time was at 1350R, it was very high as well. Then I started to cure (as I presume now) the electricity problems with damping of my MF driver. I was driving with 1000R, 650R and in the end with 550R. When I circulated my MF transformer for single-stage 6E6P I made it 15:1 or relatively heavy loaded. Now I can go for much-much higher loading – the electricity from PP2000 shell be able to care it.

In a light of my recent electricity-inspired dissatisfaction with my MF driver I begin to think about return to the two stages Milq and as it might be understood from this thread I am contemplating to use DHT in the Milq’s output stage. Still, the single-tube quandary is extremely lucrative. Jim de Kort (and I do not know who he is) said that a single tube is performing all the tasks but something he did not like in it. However, we under impression that a single stage high gain IDT and DHT shell be identically loaded in respect to own plate impedance.

Let me to prose another alternative. How about if we load the high-gain single-stage IDT against very high impedance, much higher than a common wisdom would suggest? Sure it will eat a lot of power and the small IDT do not have a lot of it but I drive only HF and near 110dB sensitivity then ever 0.5W would be enough and I might do away with it. The 5E5P load to 30/40:1 transformer might be a ridiculously lucid way to experiment.

Sure, the DHT shell be tried and it highly highly-possible that just two-stage DHT will do just fine. Still, the single stage IDT with completely idle anode might have own undiscovered kink that shell be also considered.

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-11-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 104
Post ID: 9385
Reply to: 9293
The Sun Audio with 6E5P
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 Romy the Cat wrote:

BateryBais.jpg

Funny it took 2.5 hours to build the San Audio Kit for 6E5P. The 6E5P is at 150V (a bit low for it but it is OK), loaded with 15K and coupled with 2A3 via a cap. The 6E5P is biased with one 3.6V lithium battery sitting in series with grid. I drooped the 2A3’s bias resistor to 150K and the rest it the basic San Audio Kit.

I was not wild with original San Audio Kit sound, I did not like when I redid is with better parts and I frankly not a hugely enthusiastic about the sounds of San Audio Kit with 6E5P driver.  I can’t say anything certainly as I have no full-range speaker. Unit I have the MiniMe I decided to dump the 6E5Ped San Audio Kit on somebody else’s laps.  I have a local guy with San Audio amp built by original circuit with two 6SN7 stages and I will give to him my amp to play. It might be very interning to hear how in this configuration the buttery-biased 6E5P will punch up against two 6SN7 stages. The two 6SN7 stages have 40 times gain, the single 6E5P has 30 times gain - really close and it might be a very interesting rivalry.

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-12-2009 Post does not mapped to Knowledge Tree
drdna
San Francisco, California
Posts 505
Joined on 10-29-2005

Post #: 105
Post ID: 9392
Reply to: 9385
Multistage DHT circuits
fiogf49gjkf0d
 Romy the Cat wrote:
I was not wild with original San Audio Kit sound, I did not like when I redid is with better parts and I frankly not a hugely enthusiastic about the sounds of San Audio Kit with 6E5P driver.
After thinking about this for a while, it occurred to me that each driver stage imparts its own character to the sound. It may be that the 6E5P is not a good driver to complement the 2A3 or that the topology needs some modification to improve the sound.

Adrian

01-13-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 106
Post ID: 9394
Reply to: 9392
Wrong tube or wrong topology – here is the question.
fiogf49gjkf0d

Possible, I never was a big fun of 6E5P without Milq type of bias. I remember what when I did the simple fixed bias when a few year back I was trying to make Milq to sound “contestable” with ML2 it was racy and jumpy sound but nothing special. I feel that with buttery the 6E5P sound too flat and too blended. I can’t be certain as I have no full-range sparkers to get objective picture. I however an not planning to change anything in my version of Sun Audio until I have the MiniMe done.  That is what I will give the amp to a guy who has the same Sun Audio amp with 6SN7 and let him to recognize the difference.

BTW, I run today the Chinese 2A3 and they were  much better then Sovtek tube, much-much better, in fact I was very surprise how bad the Sovtek were.

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-13-2009 Post mapped to one branch of Knowledge Tree
drdna
San Francisco, California
Posts 505
Joined on 10-29-2005

Post #: 107
Post ID: 9395
Reply to: 9394
Emission Labs 2A3
fiogf49gjkf0d
 Romy the Cat wrote:
BTW, I run today the Chinese 2A3 and they were  much better then Sovtek tube, much-much better, in fact I was very surprise how bad the Sovtek were.
Having received a bunch of tubes, I have spent some time listening. Tubes have gone up in price a lot in the last ten years! I will preface this by saying listening was done with FM, vinyl, and CD's. I listened, and I also had my girlfriend listen. This is important because she is a professional musician, and we listened to some of her CD's. I know what she sounds like, I was there at the recording sessions, and I spent many hours listening to the tracks over and over during mixing and mastering. This gives me an advantage, so that I can truly see what changes connect me to the original musical composition and performance on many levels.

So this is what I had on hand:

The Sovtek 2A3 was probably the worst. The sound was congested with a spatial constriction and a dull and sluggish presentation. The sounds were very uninvolving and overall I wanted to turn off the music.

The Chinese 2A3 is what I had originally in the amplifier. Brighter, but also more grating, more fatiguing. The sound is quite dry, and there are issues with microphonics. Nice enough but unrevealing. Lacks detail and has an unnatural feel. Nice stereo sound, but not the Sound.

The Emission Labs mesh plate 2A3 is in a separate universe than the other tubes. There is no comparison. Much much better! The music comes alive and is liquid and involving. There is a dramatic increase in inner detail and a reduction in noise. The tubes are built like a tank by the way. Excellent timbre with much better highs and lows. Much improved frequency extension. Really wonderful. However, there is something not quite right. It sounds better than I remember the recording session. It is like coming home to find a cake you baked, and you are sure you did not put frosting on it, but now it has frosting. My girlfriend confirmed this (and this is all double blind FYI). She said it sounded like a more perfect polished recording of the sessions, not like the actual sessions. There is an added Koetsu-like glaze over the music. Everything is sweet and wonderful, candy-like, concupiscent. But it is not really a true connection to the composition. It is like a siren song. I have no doubt a lot of people will go ga-ga over this tube, the same that love the Koetsu cartridges. Wonderful but not neutral and not true to the Sound.

On an even higher level are the KR 2A3 and the Sophia Mesh tube. These are the pinnacle for 2A3's, but I have not yet finished my listening, so this will be in the next post.

Adrian
01-13-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 108
Post ID: 9398
Reply to: 9395
Some comment and questions about then 2A3 talks.
fiogf49gjkf0d

 drdna wrote:

Having received a bunch of tubes, I have spent some time listening….

Yes, it looks like the Sovtek 2A3 are remarkably bad, even I with my stinky Tannoy was able to get it.  Regarding the rest of your comments I have a few questions to ask.

1)      The frosting that you detected on Emission Labs mesh plate 2A3 – is it the same frosting on the all type of cakes of the ingredients of the frosting falcate with respect to the tease of cake?

2) When you use your 2A3 then what it driver? I mean what channel/channels it drives, what the impedance and how they crossed?

3) Do you know what impedance your 2A3 sees; or what transformer ratio it has and what taps it uses if it has taps?

4) If you use your 2A3 with two channels then how do you fell your comments would be altered if you observe the 2A3 sound in context of just HF channels with no LF support?

5) You told that you went after PP2000 regenerators, have you were able to employ it yet during your observation of the 2A3 sound?

6) I am a bit confused with the whole saga of single plate and dual anodes on 2A3. Do the single plate and double plate run at the same current? It shell not be this way. Do you change currant what you go from single plate to double plate?

7) Do you have a couple of common double-plate old RCA, Sylvania, Raytheon etc tubes from 30-40s? They are not too expensive, about $100 per tube but it would be interesting to throw them into the play.

The caT


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-13-2009 Post does not mapped to Knowledge Tree
JJ Triode
Posts 76
Joined on 09-12-2007

Post #: 109
Post ID: 9400
Reply to: 9398
Which Chinese 2A3?
fiogf49gjkf0d
Romy and drnda,
When you speak of the "Chinese 2A3" which one do you mean? Romy, if you have the Shuguang 2A3C then I understand why you find it better than the Sovtek, no argument there! drdna, the Shuguang is a little more extended on top than the Sovtek but it is not harsh, so I wonder, is your "Chinese 2A3" the cheap Valve Art biplate? Those have a reputation as you describe. Incidentally the Sophia meshies are also Chinese (Tianjin I think.)

Romy, as far as I know the single and double-plate tubes are designed for the same total plate current and other specs, though the new-production single plates mostly have higher max dissipation ratings. That is, the individual plates in the biplates must have twice the plate resistance of the plate in the singles, then are connected internally in parallel to give the same external plate resistance. I have not used any old stock 2A3s yet, myself.
01-13-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 110
Post ID: 9405
Reply to: 9394
OK, for a first time I like the San Audio Kit.
fiogf49gjkf0d

If I say that it was horrible with San Audio 2A3 driven with buttery biased 6E5P then I really meant it.  I got home today tune it on and it was so bad that decided that it shell not use it as is. Even though it an experimental amps but still it beyond any acceptance. I run Tannoys from full-range Milq. With all imperfection of Tannoy RD it was Sound instead of the crap that 2A3 outputted. Could the 6C33C be so much superior to 2A3? I do not think so. The sound from 2A3 was truly broken – it was flat, blended and extremely boring.  The 2A3/45 that I heard out there did not sound like this. It was obviously a bug somewhere in it.

Then I begin to rollback in memory what I got this sound. Evan the original San Audio 2A3 with 6SN7 was more exiting in my view. I concluded that I got that sound when I introduced my 6E5P to San Audio 2A3. Does it mean that a pair of 6SN7 stage is better driver for this amp? Hm, I would argue with this assumption.

When I put the 6E5P in the game I used battery bias. I used very briefly a fixed bias with original San Audio 470R resistor and the garbage bypass caps that they supplied the kit. It was not good and then I move for Able Lithium 3.6V battery sitting on the grid. I did not go got a full scale Milq bias in San Audio as I did not want hassles with negative supplies, gas tube, dividers  etc.... Still, right along with introduction of 6E5P the indifferent sound came along…

I was thinking about this: how come that driving 6C33C (in fact driving anything one can imagine) the 6E5P was very fine but driving 2A3 it was dull like hell. I opened the amp and tested all – everything was perfect but there was no sound. I desired that before I put in there the full scale Milq bias I need to try one more time a cathode bias only with good parts. I kicked off the Lithium battery, leaving only 100R resistor as grid stopper (the 6E5P is very fast tube). In cathode of 6E5P I put 562R very good resistor and bypassed it with unavoidable 25V/1000uF Nichicon Muse KZ. The whole procedure took 15 minutes. I measured what I got. It was 4.1V on 6E5P’s cathode and 170V on plate – perfect. I closed the amp and plugged it in.

Abracadabra!

Well, the very first note I hear I immediately knew that the sound went back. I do not want to describe what got better – it was just very different sound – better or worse but it was already the Sound that might be worked with.  So, it was the battery bias that screwed me up. In the past what I true it did not like the result but it was not even close in misery to what I go now. Perhaps it was the brand of the battery or it might be that all battery bias work this way. I do not know that answer but I do not care now and I do not use battery bias anymore…

I still do not know if I like the San Audio amp but now it does not have the revolting sound. Back to business to make the amp to sound “interesting”…

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-13-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 111
Post ID: 9407
Reply to: 9359
The YO186 Strip Show
fiogf49gjkf0d

 Romy the Cat wrote:
http://www.goodsoundclub.com/pdf/YO-186.pdf
Here is the lit sucker that I told you before. It is Russian impersonisation of Telefunken AD1. It was made from 1932 to 1963. It is smaller than 2A3, the size of the larger version of AD1, has slightly less gain them AD1 and slightly more play impedance. Reportedly it has “mystical sound”. It is “mystical” because one of 4 possibilities:

1)      Whoever heard it already dead

2)      Whoever heard it have no idea what they are taking about

3)      Damn Russkies did not save money at that time, did in fact good tubes and YO186, along with GM57 are super kinky pinnacle of Soviet tube design (and they did have phenomenal capacity to do it)

4)      It has good sound but not match to best western alternatives

We will see…

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-14-2009 Post does not mapped to Knowledge Tree
drdna
San Francisco, California
Posts 505
Joined on 10-29-2005

Post #: 112
Post ID: 9410
Reply to: 9398
More on the 2A3 comparisons
fiogf49gjkf0d
 Romy the Cat wrote:
I was thinking about this: how come that driving 6C33C (in fact driving anything one can imagine) the 6E5P was very fine but driving 2A3 it was dull like hell. I opened the amp and tested all – everything was perfect but there was no sound. I desired that before I put in there the full scale Milq bias I need to try one more time a cathode bias only with good parts. I kicked off the Lithium battery, leaving only 100R resistor as grid stopper (the 6E5P is very fast tube). In cathode of 6E5P I put 562R very good resistor and bypassed it with unavoidable 25V/1000uF Nichicon Muse KZ. The whole procedure took 15 minutes. I measured what I got. It was 4.1V on 6E5P’s cathode and 170V on plate – perfect. I closed the amp and plugged it in.
I knew it must be the topology but I still say the tube must have a lot to do with the character of the sound. The circuit may still need refinement to optimize the operation between these two tubes.

 JJ Triode wrote:

When you speak of the "Chinese 2A3" which one do you mean? Romy, if you have the Shuguang 2A3C then I understand why you find it better than the Sovtek, no argument there! The Shuguang is a little more extended on top than the Sovtek but it is not harsh, so I wonder, is your "Chinese 2A3" the cheap Valve Art biplate? Those have a reputation as you describe.
Yes, it is the Shuguang tube. I describe it as harsh, but it is a matter of opinion. Certainly it is not so abrasive I wanted to smash it like a Krell-Martin Logan installation or something. It is a matter of being artificially bright.

 JJ Triode wrote:
Incidentally the Sophia meshies are also Chinese (Tianjin I think.)
Yes, I know, and so it is a big surprise that they beat the Slovak tubes, at least to me. More about the other tubes to come soon.

 Romy the Cat wrote:
The frosting that you detected on Emission Labs mesh plate 2A3 – is it the same frosting on the all type of cakes of the ingredients of the frosting falcate with respect to the tease of cake?
No, in this respect it is not exactly like the Koestu, which made everything romantic. Instead the sounds come out perfectly -- too perfectly. The soundstage is obvious and huge. The frequency response is even and the mid-bass is very strong. Delicate recordings are perfectly delicate. Dynamic recordings are super dynamic. Hard rock albums rock hard. But it is wrong. Not every note on every recording is perfect; all recordings have flaws. When I listen to my girlfriend's recordings, it does not sound as I remember the sessions; instead it is a perfect ideal version. Like a Technicolor movie. Hard to explain, and honestly a small difference between the three top tubes, but still easy to consistently pick out which tube is which in double blind tests.

 Romy the Cat wrote:
I mean what channel/channels it drives, what the impedance and how they crossed?
I have listened on the Edgarhorn Titans, driving both the total system (except subwoofer) and also only to the midrange driver alone. It is slightly less than 8 ohm impedence.

 Romy the Cat wrote:
Do you know what transformer ratio it has and what taps it uses if it has taps?
It has the Magnequest DS-025 OPT, connected to the 16 ohm tap.

 Romy the Cat wrote:
If you use your 2A3 with two channels then how do you fell your comments would be altered if you observe the 2A3 sound in context of just HF channels with no LF support?
I have done this. My comments are based on this also.

 Romy the Cat wrote:
You told that you went after PP2000 regenerators, have you were able to employ it yet during your observation of the 2A3 sound?
It arrived today. I will set it up this weekend, but I am still not done listening to the other tubes yet, and I will wait until that is over to avoid confusion. I will go back a re-listen after adding the PP2000.

 Romy the Cat wrote:
Do you have a couple of common double-plate old RCA, Sylvania, Raytheon etc tubes from 30-40s? They are not too expensive, about $100 per tube but it would be interesting to throw them into the play.
I do not have them any more. I had listened to them in the past. My recollection is of burnished brass. Warm, brilliant, rounded. Excellent sound in their own right, and again it is splitting hairs between these and the other top grade modern tubes, but that is what I am doing. It is like comparing the chess games of Alekhine or Tal to that of Kramnik or Kasparov. All are geniuses, but the differences between the modern and hypermodern styles are obvious.

Adrian

Adrian
01-14-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 113
Post ID: 9411
Reply to: 9410
It will be interesting….
fiogf49gjkf0d
 drdna wrote:
It has the Magnequest DS-025 OPT, connected to the 16 ohm tap.

So, it is 2.5K primary and you drive 8R speaker with 16R tap. That makes the 2A3 loaded in your case with 1750R. I think it is necessary to know in order to put the things in perspective. In my case I think I will (at least try) to drive 2A3 with muck higher impedance, as high as S2 “break up” would allow me to.

http://www.magnequest.com/025.htm

 drdna wrote:
It arrived today. I will set it up this weekend, but I am still not done listening to the other tubes yet, and I will wait until that is over to avoid confusion. I will go back a re-listen after adding the PP2000.

Hm, I think it might be very interesting ride and I think you might as the result to go much bolder with loading, it you have power to drive the Titan upperbass horn.

 drdna wrote:
I do not have them any more. I had listened to them in the past. My recollection is of burnished brass. Warm, brilliant, rounded. Excellent sound in their own right, and again it is splitting hairs between these and the other top grade modern tubes, but that is what I am doing. It is like comparing the chess games of Alekhine or Tal to that of Kramnik or Kasparov. All are geniuses, but the differences between the modern and hypermodern styles are obvious.

Very cool association. Thanks, Adrian.

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-14-2009 Post does not mapped to Knowledge Tree
drdna
San Francisco, California
Posts 505
Joined on 10-29-2005

Post #: 114
Post ID: 9413
Reply to: 9411
OPT impedence
fiogf49gjkf0d
 Romy the Cat wrote:
 drdna wrote:
It has the Magnequest DS-025 OPT, connected to the 16 ohm tap.

So, it is 2.5K primary and you drive 8R speaker with 16R tap. That makes the 2A3 loaded in your case with 1750R. I think it is necessary to know in order to put the things in perspective. In my case I think I will (at least try) to drive 2A3 with muck higher impedance, as high as S2 “break up” would allow me to.The Cat
I chose this for a sound most free from "graininess" which may change with PP2000. However, what is the rationale to change the impedence loading on the tube, from your perspective? As you know the Magnequest OPT has taps for 4,9, and 16 ohms. Your suggestions for an experiment after PP2000 is installed?

Adrian
01-14-2009 Post mapped to one branch of Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 115
Post ID: 9414
Reply to: 9413
More about the Plate Loading
fiogf49gjkf0d

 drdna wrote:

I chose this for a sound most free from "graininess" which may change with PP2000. However, what is the rationale to change the impedence loading on the tube, from your perspective? As you know the Magnequest OPT has taps for 4,9, and 16 ohms. Your suggestions for an experiment after PP2000 is installed?

Yes, I do feel that with PP2000 the negative influence of “graininess" that you describe might be changed. It is not truly "graininess" but some kind of “diaphragms noise” that is most like the signs of bad electricity. With good electricity the "graininess" goes go up with increase of impedance but it in my view allow to increase of impedance higher with less negative feeling about the "graininess".

Generally with increase of impedance (more idle tube) you have less distortion, faster “sound”, l less harmonics, less power, and higher perception of transients among many other things. The lower impedance or the heavier load on the plate work in opposite way – slower transients, more power, more damping of diaphragm, less imitate sound, more masking effect over the driver imperfections. The right balance in my view is the loading state where the max transients are available but where the harmonics of the tone still maintain its acoustic signature. There is not rules in there and there is no way to measure it – it have to be done by ears as there are so many ingredients evolved into it, to name a few: type of tube, plate currant, type of the OPT, type of the transformer core, presence  and type of feedbacks, characteristics of the speakers drivers, type of the drivers, drivers integration methodology, listing distance, type of the speakers and so on and so on… As you underrated that rules of the game what you use multi-amping are very different as you have a chance to surgically load each narrow-bandwidth channel and shape the harmonics not only by loading but also buy the channel integration and creative crossovering.

So, what you begin to use PP2000 you might try to jump to lower tap of your Magnequest OPT. You will lose 3dB with each tap and will have “faster” sound. It is highly possible that you will not have the "graininess". What I think you would need to look is the state where the MF begin to be breaking apart and harmonic tail will not be any longer fill the gaps between  he transients. This parabola, with which tone roles to its pitch shell be a hill not a valley (unless you listen rock, pop, jazz crap). if you can grasp it right the way then let you girlfriend to hear the “flatness” of the tone and she will get it right the way. It might take for a while to properly load MF driver, I uselessly suggest going for faster sound and slowly step back, instated of going for heavier load and moving up in impedance.

BTW, the PP2000 does absolutely amassing thing – it laniaries harmonics across bandwidth, so you might find that it is fun to play with it.  

The Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-19-2009 Post mapped to one branch of Knowledge Tree
floobydust


currently roaming the US
Posts 62
Joined on 01-19-2009

Post #: 116
Post ID: 9464
Reply to: 9414
Some feedback on designing with the 45 and 2A3 DHTs... might prove interesting.
fiogf49gjkf0d
 This thread has been pretty interesting overall. I've read most of it and would at least like to provide some feedback on the technical side of designing with the 45 and 2A3 DHTs... at least from my personal experience. A few years back I decided to do a clean-sheet design for a DHT amplifier after building a Shishido 2A3 schematic using a 45 instead (and proper circuit changes). I wasn't overly impressed with it despite using very good quality parts and OPTs, hence the clean sheet design.

 I'm old school... so I started with tube specs and working from old published plate curves. After much searching I eventually found Hashimoto transformers and purchased a set for testing... (great choice I've never regretted). I built a breadboard for and open build and easy change and started with the power supply and the output section. I'm lukcy to have a large stash of NOS 45 and 2A3 tubes along with some 6B4G... so I started with the 6B4G. After much testing and measuring (again, just the output stage), several things became clear:

1- AC filaments on the 6B4G are a disaster for output hum. The reason became clear... the 6B4G is essentially a dual-section 2A3 where the filament sections are wired in series and the grids plates are paralleled. This makes it impossible to balance the hum out with an AC filament. A simple full-wave DC supply was added and things quieted down so I could resume design work.

2- Recommended plate load impedances vary quite a bit with operating points. Finding an optimum load and staying within recommended ratings was desirable, so a fair amount of time was spent optimizing the output stage. When I say optimize, it is to ensure the best possible output power with the minimum amount of distortion, symmetrical clipping (i.e., operating point balanced against the transformer's magnetic saturation and field collapse) and be assured that the tube will have adequate life in actual operating conditions.

3- Input capacitance has a much greater affect on the driver. As is common with triodes, the input capacitance is greater as is the required input voltage swing compared to most beam-power tubes (for similar output power) this places a much greater demand on the driver circuit to ensure proper frequency bandwidth and slew rate for a crisp and clean rise/fall time.

4- Fixed versus cathode biasing. More of a personal choice here. In general I prefer cathode biasing for single-ended designs and fixed bias for push-pull designs. For this design I chose cathode bias. More on this later.

 For the 6B4G (and 2A3) I found that optimal loading impedance was 3500 ohms. Note: the same testing on the 45 resulted in 5000 ohms. This yields the best overall performance and lowest distortion by a measurable amount. Yes... I use some simple and proven techniques to measure with. A clean sine/square wave generator, dual-trace scope and accurate AC RMS meters and distortion analyzer (all HP and Tek gear). Whlie this is not same as listening, I still believe that each stage of a good design should be able to pass some simple measurements and get them right, otherwise you wind up with a cascading of stages which each adds it's own coloration (i.e., distortion) and it's own short-comings in being capable of driving the next stage.

 As the Hashimoto SE OPTs invert polarity I eventually opted for a 3-stage design as I wanted to maintain the phase and AC grounds on the transformer. I spent considerable time on designing the input/driver, again using some previous experience of various dual-triodes I've used over the decades. Initially I started with a 6FQ7 which is (essentially) a 6SN7 in a 9-pin miniature size (many vintage tube manuals refer to the 6SN7 plate curves). While this did work quite well, I wasn't completely happy with the performance and noise levels despite having a large stock of NOS tubes to pick from... I even picked up some additional ones without any significant change.

 I later swapped to a 5814A (rugged version of the 12AU7) and again started with the plate curves and re-biased the same topology. Success! Output noise was lower, gain was about the same (140-150 volts/volt) and measured distortion (at the 6B4G grid) was also lower and at the same or lower drive current. Further testing and some slight value changes resulted in very consistent performance with a wide range of 5814A, 12AU7, ECC82 and 5963 tubes of different brands. This is always a design goal to have a circuit which will result in expected performance with unit-to-unit consistency as parts tolerances can skew things out the window. I also swapped many tubes thru the output stage with repeated measurements to prove the same.

 After much effort, the design firmed up and I eventually built final versions using the 45 and 2A3. I ditched the 6B4G with it's DC supply and went on to develop a better balancing arrangement to ensure the output stage is quiet with AC filaments. By definition, the 45 can be the quietest as it is always a single triode section with a simple "M" filament design. The 2A3 can be almost as quiet, but ONLY in the dual-section paralleled types. The original single-plate triode has some advantages but also has disadvantages which makes an AC filament impossible. After a while, I devised a simple technique which I've never seen on any design before (and I've seen hundreds or more over the past 40 years). Someone mentioned he had seen it, ws nothing new and he also used it... but he could never point me to anything to confirm... so who knows... in any case, I adopted a split balance technique. It's ability to acheive better hum balance is unquestionabe... it works with all good quality 45 triodes and dual-section 2A3 triodes.

 The technique is simple and I would recommend anyone with a 45 or 2A3 AC-operated filament SE amplifier to try it. It only works with cathode bias due to the topology but it is effective. You also need a good quality filament transformer with an accurate center-tap. As usual, the filament transformer drives the filament of the 45/2A3. The center-tap is used for the biasing resistor (ONLY) to ground. This becomes a "fixed DC balance" for the directly-heated cathode. As the gauge of the filament is accurately maintained during manufacture, it is reasonable to believe that maintaining a centered DC balance provides an even DC current thru the filamentary cathode.

 The next part is the AC balance which is adjustable. As the filament wire itself is coated with rare earth elements and other compounds to increase it's ability to become an effective cathode for electron emission, the coating is not 100% even. This results in the filament AC voltage being imposed in the electron flow. I used a 100-ohm wirewound potentiometer with the ends in parallel with the filament. The wiper of the balance potentiometer goes to ground thru the cathode bypass capacitor. As this circuit can not pass any DC current, it becomes effective only for AC balancing. I also pad the potentiometer with a pair of 12-ohm resistors, one from each end to the wiper. This reduces the effective resistance of the 100-ohm pot to less than 4 ohms (from the "cathode" to the bypass cap) and increases the wiper range so you don't have a "knife-edge" adjustment on the balance pot.

 So, how effective is it?? With good quality 45 triodes, I've had the output noise (at the speaker output) better than 85dB referenced to 1-watt output. For an 8-ohm load, measured output voltage is less than 125 microvolts. As a result, I would not bother to use a DC filament supply on a good quality 45 or dual-section 2A3 tube. I think the AC filament on these tubes is better technically as you come closer to acheiving a similar operation to a unipotential cathode.

 The final measured specifications for the 45 amp were:

Rated output: 2 watts RMS clipping at 2.25 watts RMS
Frequency response at 1-watt: 25Hz to 45KHz within 1dB
Power Bandwidth at 2-watts: 50Hz to 45KHz within 1dB
Distortion (THD): 0.35% at 1-watt, 0.75% at 2-watts (slightly higher below 100Hz at higher power, less at higher frequencies)
Signal-to-Noise: 85dB with good tubes... some samples are better 90dB below referenced to 1-watt output

 The design uses all AC filaments, no regulators anywhere and zero feedback. So it is possible to have a wide-bandwidth, low distortion and quiet SET without any fancy circuits for filaments and/or plate supplies.

 Some information regarding some other DHTs I've used... namely the single-plate 2A3 (original RCA/Cunningham) and the 300B (Western Electric version). Both of these tubes are NOT candidates for AC operated filaments. Both of these tube designs use what is known as a "center-tapped" filament. Yes, there are only two pins for the filament, but electrically there are two halfs to the filament arrangement and they are strapped in parallel. Western Electric will confirm their 300B (old and new manufacture... as it's the same design) uses a center-tapped filament arrangement. If you look closely inside the WE300B, you'll note the filament takes the form of a double "M" arrangement. The bottom center where the two "M"s meet is the center-tap and goes to one of the filament pins. The two far ends of the filament are tied together and goes to the other filament pin. Because you effectively have two filaments operated out of phase within a single grid and plate structure, it's impossible to balance the hum out.

 The single plate 2A3 is similar... I've actually seen some where half the filament was gone where the other half (side) lights up. The arrangement is somewhat different as there are 8 runs of the filament wire and a single bar tensioned by two coil springs up top to maintain wire tension. It suffers from the same basic problem as the WE300B tube, but it's actually worse (and easily demonstrated). While the WE300B has a double-"M" filament, each loop is tensioned by a separate "fishing rod" tension wire. The single-plate 2A3 has 8 loops all on a common tension bar. As the filament heats up and each run conducts current, minor variations cause unequal expansion of the wire loops. This results in some wire loops being loose enough to to exited by small mechanical vibrations. With a scope on the amplifier output, a handclap within a few feet can set them off. They will also resonate at certain frequencies as well. I played a couple scales on a trumpet within 6 feet and certain notes drove them to extreme levels of exitation. This could, perhaps, be one reason that many people claim the single-plate version to have a warmer sound or whatever terms you'd like to apply... bottom line, you have mechanically induced distortion to the output stage. And these versions go for major dollars too!

 I've not had much experience with the new range of tubes being manufactured by EML, KR, the Chinese or the Russians, etc. but I did buy a new pair of EML-45 solid-plate tubes a couple years ago. They are well made, huge (by contrast to the original 45) and butt-ugly (personal view). Two things that I don't like about them. First, they exhibit the same mechanical vibration as the single-plate 2A3 noted above. Second, they sound dull and lifeless when plugged into the same output circuit as any of my good NOS 45s. A conversation with Roger Modjeski resulted in me increasing the idle current closer to 40ma (from around 34ma). This made a huge improvement sonically but alas they are still easily exited mechanically.

 So, sorry for such a long post from someone who's new at Romy's place.... but hopefully some of you find the above useful to some extent. I also have much feedback regarding 45 and 2A3 brands and internal mechanical structures but will save that for another post. Needless to say, I've narrowed my acceptable choices on which 45 and 2A3 tubes I keep and which ones I don't like. If anyone has interest, I'm happy to post those findings as well.

 Regards, KM




... just because you're paranoid, it doesn't mean they're not after you ...
01-19-2009 Post does not mapped to Knowledge Tree
drdna
San Francisco, California
Posts 505
Joined on 10-29-2005

Post #: 117
Post ID: 9465
Reply to: 9464
2A3 data
fiogf49gjkf0d
 floobydust wrote:
The 2A3 can be almost as quiet, but ONLY in the dual-section paralleled types. I adopted a split balance technique. It only works with cathode bias due to the topology but it is effective. You also need a good quality filament transformer with an accurate center-tap. As usual, the filament transformer drives the filament of the 2A3. The center-tap is used for the biasing resistor (ONLY) to ground. This becomes a "fixed DC balance" for the directly-heated cathode. The next part is the AC balance which is adjustable. I used a 100-ohm wirewound potentiometer with the ends in parallel with the filament. The wiper of the balance potentiometer goes to ground thru the cathode bypass capacitor. I also pad the potentiometer with a pair of 12-ohm resistors, one from each end to the wiper. This reduces the effective resistance of the 100-ohm pot to less than 4 ohms (from the "cathode" to the bypass cap) and increases the wiper range.

Some information regarding some other DHTs I've used... namely the single-plate RCA 2A3 and the WE 300B. Both of these tube designs use what is known as a "center-tapped" filament. Yes, there are only two pins for the filament, but electrically there are two halves to the filament arrangement and they are strapped in parallel. If you look closely inside the WE300B, you'll note the filament takes the form of a double "M" arrangement. The bottom center where the two "M"s meet is the center-tap and goes to one of the filament pins. The two far ends of the filament are tied together and goes to the other filament pin. Because you effectively have two filaments operated out of phase within a single grid and plate structure, it's impossible to balance the hum out. In the 2A3, the arrangement is somewhat different as there are 8 runs of the filament wire and a single bar tensioned by two coil springs up top to maintain wire tension. It suffers from the same basic problem as the WE300B tube, but it's actually worse (and easily demonstrated). While the WE300B has a double-"M" filament, each loop is tensioned by a separate "fishing rod" tension wire. The single-plate 2A3 has 8 loops all on a common tension bar. As the filament heats up and each run conducts current, minor variations cause unequal expansion of the wire loops. This results in some wire loops being loose enough to to exited by small mechanical vibrations.

I also have much feedback regarding 45 and 2A3 brands and internal mechanical structures but will save that for another post.
Very informative, I'll stay tuned.

Adrian
01-19-2009 Post mapped to one branch of Knowledge Tree
drdna
San Francisco, California
Posts 505
Joined on 10-29-2005

Post #: 118
Post ID: 9466
Reply to: 9410
The New 2A3 tubes
fiogf49gjkf0d
Having listened for a while with the tubes, I have come to some conclusions. The three tubes I have seriously listened to are: Kron, Sophia, and Emission Labs. All the tubes are much better than any other modern 2A3 tubes I have tried. I felt my initial post was a bit premature as the tubes needed also to break in. Keep in mind all three tubes are excellent and the differences I am talking about are very tiny. It is easy to tell the tubes apart when you listen to them, but it is hard to easily point to a best tube.

Sophia Mesh Plate 2A3. This tube has a very large degree of mechanical resonance, even with tube dampers, which creates distortion in the music. The frequency range is somewhat limited, with the highs and lows being obviously lower in amplitude. Thus the midrange is emphasized. This in combination with the mechanical resonance and harmonic distortion gives a warm character. The tube however reproduces the signal extremely accurately. Restricted to the midrange, for simple pieces like guitar and voice, this tube is clearly superior to all others I have tried, reproducing this part of the frequency range with clarity, definition, and a profoundly low noise floor that leads to a delicate lifelike sound like no other. However, it falls apart on more complex orchestral pieces, the lack of highs and lows gives a recessed, rounded feel to the music. The HF reproduction, given the mechanical and harmonic distortion is rather intolerable. The midrange is wonderful; everything else just sounds horribly "wrong."

KR Enterprises 2A3. Minimal mechanical resonance. Solidly built. Very good, even reproduction of music across the frequency range. There is minimal harmonic distortion. The HF, midrange, and LF are correct. The reproduction of the waveform is not as accurate, with some slight sluggishness in the attack and decay. This leads to a less delicate sound that can be "steely." This tube is engineered to compensate for the problems with the recording process and the artificial nature of stereo. This compensation leads to the audio sounding most like the original musical event in the recording studio. The sound in the mixing sessions is the same sound here now. Despite its defects, more than any tube, it opens the window to seeing into the musician's soul.

Emission Labs Mesh Plate 2A3. This tube is gigantic and extremely well made. There is minimal resonance; tapping the tube yields a dull thud that dies down quickly. Equal to the KR tube in this respect. There is minimal harmonic distortion and music is reproduced extremely evenly across the frequency spectrum, even more than the KR tube. The waveform is reproduced well, although not quite as well as the Sophia tube (however the fact that it does so over a much wider frequency range makes up for it.) In the first week, there was a sense of "cake frosting" on the music, which disappeared over time. The presentation of the music is very different from all other tubes. The soundstage is much larger, deeper, taller, and more accurate; yet it is not an artificial soundstage -- instead it is very natural. The presentation is very different. At first everything sounded "too perfect" with this. I now think I am hearing the recording process more accurately with this tube; it is transparent. Every mixing editing decision comes through clearly and obviously. Bad recordings are intolerable. Good live recordings are transcendent. All small things in recordings, the equipment, the electricity: all are revealed on an accurate scale. A musician shifting in his seat is heard, but quietly, distantly and exactly, just as in life. The EL tube is a neutral device, I think, revealing exactly what it is given, but making no accommodation for any weaknesses. Is that waht we want? Truly, la belle dame sans merci.

Adrian

01-19-2009 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,690
Joined on 05-27-2004

Post #: 119
Post ID: 9467
Reply to: 9464
How many stages I need for 110dB sensitivity, small room and above 3200kH?
fiogf49gjkf0d

Thanks, floobydust.

There are a lot of interesting points in you post to consider. What speaker you drive with your 2A3? If they are relatively high sensitively (like compression drivers) then did you consider going with a single stage? I think I have no power for my very high desirable load  in the tube that I use as my single driver  but  am still is debating of a more powerful IDHT tube in single stage application would be for me a better way to go then DHT two stages. (I am taking juts about driving a MF channel from 3.2kHz and up)  I think I would need 30W plate dissipation with 1K of plate impedance, or higher impedance and higher power and I will be able to employ such a tube.

I am not against DHT solution but it has a lot of intellectual ballast with it. Most stages, more cathode caps etc, etc. Then the coupling. The DC coupling is kind of freaky. Besides the subjects of stability it has strange impact to the lower region of sound. It is like you are thirsty and are drinking water but you have a large hole in the belly and the water run away from you do not letting you to be pleased with the satisfaction of the thirsty. The capacitive coupling has own problems. Leaving aside the fact the caps all have sound do not forget that DHT are low biased tube if they jump into A2 then the cap begin to recharge. The transformer-coupling is also problematic. The inductance acts as a choke and screw up the current transients. I mean this is all just taking but a single stage DHT is immune from all of those problems...

I would agree that some DHT has own signature that is very nice to capitalize upon, and it is possible that I would go there with my MF channel amp, or probingly return to the two stages with 6C33C. I do not know yet. Your 45 amps look interesting; I do appreciate that fact that you contrary to the most of even very experienced and knowledgeable fool present the frequency response of your amps at full power. The power at 1W shell not even be mentioned – I wish people understand it. I hate to see the Morons claim that their 20W SET run from 2Hz to 250kHz at 3W. What the holly foolishness! They need to mention that at 0.000034W their SET might run a video signal….

Anyhow, the heaters comforting with AC - is looks like you do what everyone else does. Di you try to use UHF power supply for filaments yet? Anyhow, my most interest is to get you take about one good DHT vs. the multistage DHT.  The guys in the mid of the thread did expressed their views. Still, take a the best very original WE, Klangfilm or RCA power amp (Joe Roberts, do not read further – you will have a hard attack :-) and put it next to Lamm ML2.0. The damn Lamm (with all it “horrible IDHT”) will chew and spit all those “good vintage amps”.  My post is not the tribute to 6C33C (there are tone of very bad amp with this tube) but I still am not convinced that multistage is a direction for me to go if I need to drive 110dB sensitivity in a small room and above 3200kH.

Rgs, Romy the Cat


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-20-2009 Post does not mapped to Knowledge Tree
floobydust


currently roaming the US
Posts 62
Joined on 01-19-2009

Post #: 120
Post ID: 9469
Reply to: 9467
How many stages and why... and who makes that 45 or 2A3
fiogf49gjkf0d
 First, Adrian,

 Thanks for your feedback on the newer designed 2A3 tubes... very interesting. I've looked at all 3 of these brands before but have not as of yet purchased any (I have a large collection of 45 and 2A3 of various NOS US brands). As much as I wanted to like the EML solid-plate 45 it fell short of my expectations. The inherent mechanical resonance issues and the fact that I need to alter some circuitry to use them. Even then, the sound was not as clear as my favorite NOS samples, possibly due to the mechanical resonance issues and the coloration (i.e., distortion) it adds. I've also concluded that you can't damp the glass envelope in cases such as this, the problem is the filament wires resonating and damping the glass won't do squat in this case. Still, you've given me cause to re-investigate the KR and EML 2A3, so thanks for that.

 Second, Romy,

 Thanks for your response also. Referring to DHT filaments... I feel that the simplicity of a basic AC heater is best provided you can balance the residual hum to inaudible levels. I don't do the same as all others however... the following are differences:

1- A very accurate filament drive using two accurate windings to form the two 1.25 volt halfs (Hashimoto power transformer)
2- A very high-quality, accurate and inherently noiseless cathode resistor (Caddock MP820)
3- A high-quality film capacitor for bypass where the value is calculated to tailored low-frequency response
4- The split balance technique described above which does result in improved hum balance with good tubes.

 These all add up to a very quiet amplifier as noted in the specifications. The added complexity of a DC filament supply or a high-frequency supply will probably not make any measurable difference in signal-to-noise and thus should not have any positive effect sonically. Having a high-frequency oscillator in the middle of the amp also requires additional design choices to contain it and prevent it from being imposed on the rest of the circuit. Whenever possible, I opt for simplicity provided it can meet the design criteria.

 The main component that limits low-frequency response in any SET is the OPT. For these initial designs, I used the smallest Hashimoto SE OPTs... their H-203S for the 2A3 and their H-507S for the 45 which are rated at 7-watts at 50Hz. Despite their small size they are excellent in quality, both mechanical construction, shielding and sound quality. Tailoring the low-frequency response for any amplifier should be a design factor so you don't over-run the OPT. Such is the case in my design for these. I purposely wanted to roll them off on the low end so you don't have issues with core saturation and/or field collapse within reasonable listening levels.

 Some thoughts on direct-coupling.... for all of the pluses people rave about regarding sonics and such and the removal of the alleged "evil" signal coupling capacitor there are also some negative points which you have hinted at. First, DC coupling does present some additional challenges in the design and maintaining proper DC biasing of the cascaded stages. In many cases, additional circuitry and components are used to compensate for this, many with adjustable resistances to lock it into spec. So that raises one initial question... does the addition of several resistance components where they are of different construction (wire-wound pots, cermet-film pots, wirewound resistors, metal-film, etc.) and/or regulator circuits to maintain DC biasing compensate for the removal of one single coupling capacitor? Which is worse, the cause or the cure? Second, as you noted there is something different in the sound at times. One possible cause is the fact that ANY DC offset and/or low-frequency underlying signal (like record warp) can push the circuit out of it's linear operating region causing high distortions at these extremes and also push the OPT into core saturation and field collapse. This is yet another reason to tailor the low-frequency response of low output amplifiers so they are not pushed out of their linear operating region from signal anomalies.

 Multiple stages versus one. Well, again, a choice that has to be made somwhere in the signal chain from start to finish. Let's start at the output stage... using my little design above, and the 45 triode. In the operating point I use, grid bias is approximately -60volts. Simple math shows that you need to be able to drive the grid at +/- 60 volts or 120 volts peak-to-peak for maximum power without driving the tube into A2 on positive swings or cutoff on negative swings. Due to the multiplying effect of input capacitance, you also need a suitable driver for the output stage. If you go thru the required math and use 20KHz and 60 volts swing you can calculate the required slew rate and minimum driver current to ensure you drive the grid properly... there's no free lunch here. The signal level and drive current must be met or you won't have a clean output stage. In a single stage design, you are completely reliant on the driving source. How many preamps can provide 120 volts peak-to-peak under these conditions? In such a design, the overall sound quality now becomes completely reliant to the what's driving it. Granted, if you design the entire system and take all of these points into consideration for design and implementation than you can manage it. If you plan on using other components... you've lost control.

 As for the input/driver stage I eventually settled on, it has some excellent operating attributes. First, the driving stage (one triode of the 5814A) has ~2.8ma of current and can easily drive 150 volts peak-to-peak cleanly into the grid, meaning it can drive the 45 or 2A3 fully and at wide bandwidth as the response is flat past 40KHz. Yes, you use more drive currrent, but it is somewhat akin to killing a fly... you can use a fly-swatter or a bazooka... end result is still the same... dead fly. You can also look at it's square-wave response and note that it's crisp and clean which supports my calculations on drive current versus required slew rate and voltage swing. The input stage is the other half of the 5814A. The output of this stage is DC coupled to the driver... but, the operating points of each stage are selected to ensure it does not push the driver out of it's linear region. The input stage is biased at ~1.2ma of current and can handle a input voltage of 8 volts peak-to-peak without being driven into A2 or cutoff. Of course, this would result in pushing the driver stage well beyond it's maximum output level, but the design goal is to ensure the input stage has no possibility of being driven out of it's linear region or placing an odd load on whatever drives it, as class A2 would result is increasing current demands from the input. Another part of the design is the input bypass capacitor being chosen for a specific roll-off point, again to help reduce large low-frequency input offsets like record warp or rumble. The driver stage bypass capacitor is actually calculated for 1Hz or less as I don't want it adding anything like phase shift but simply following it's input signal. And lastly, the coupling capacitor to the output stage (it's the only one in the design) is also chosen for a specific roll-off point. In all, there are 3 time constants and they are not stacked but each is calculated for a predictable response on low-frequency. The end result was as required, 1dB down at 25Hz, which is 1-octave below the spec'd power level of the OPT.

 There is also another point to consider here. As the deisgn has no feedback, having two cascaded stages which essentially match each, albeit out of phase, some of the inherent non-linear characteristics will be cancelled out. The proof is in the pudding as they say.... the specs of the amplifier validate the design choices and operating points chosen... or, you can't argue with success. I would have to admit that the only downside is the simple fact that the circuit has a fair amount of gain and can easily be driven beyond full power by any line-level source. Of course, this is something that needs to be factored into the overall system design. If this is done properly, the concept of 1, 2 or 3 stages becomes a moot point. The gain has to come from somewhere. One concept was to use low-mu tubes in each stage. This provides lower gain and lower noise in each stage and when measured results in lower distortion from input to output thru the 3 stages.

 So, what am I driving with these little guys... it varies. I do have quite a few speakers at home. I have Quad ESL-63 US Monitors ,but they are far too low on sensitivty for any appreciable SPLs.... but they are gems in their own right with enough power. For single drivers I have a pair of Fostex F120A in 10-liter enclosures I built which are also on the low side, but a few dB more sensitive than the Quads. However, the Feastrex D5nf setup in solid red oak BVR enclosures does have sensitiviy around 95dB. The amplifiers are okay provided you don't push too loud. Granted, having 100dB+ speakers would be a huge plus, but I can monitor output levels with a Mytek DDD-603 and ensure the amps can never be driven into clipping (with digitial sources only). They get loud enough for some very critical listening but of course not loud enough for a symphonic orchestra level. I'm not entirely done on speaker choices yet, but have additional higher powered SETs on the design board. Overall I would certainly recommend the smaller 2-watt 45 amp for driving a high-sensitivity tweeter in a multi-amp system. It's overall linearity is excellent.

 Finally, any design has to be engineered so it can be assembled and this creates yet another challenge... the physical layout of the chassis and components and the selection of each component and preparation of each. For this design, I used Caddock TF020 resistors for all circuitry on the input/driver. Caddock MK132 resistors were used for padding the hum balance pot and a Caddock MP820 for the cathode bias resistor. In the power supply Mills resistors were used, the MRA5 where non-critical (non-signal path) sections were needed and a MR-200 foil resistor for voltage drop and supply decoupling for the input stage. I opted for all Axon capacitors.... their 630V High-Volt films in the power supply, their 250V True-Cap for cathode bypasses and their 630V tin-foil and film for the single coupling cap. All signal path wire was pure stranded silver in teflon and a single isloated ground in 12ga pure solid silver. For part preparation, I actually take each component and polish the leads to a smooth shine and then "spin" a skin-tight teflon sheath on each one. This does two things: first, it remove the oxidation, crud and contamination from each component lead and two, protects it so it won't oxidize over time. This helps to provide consistent performance over time... talking years or decades, not hours or days.

 Regards, KM




... just because you're paranoid, it doesn't mean they're not after you ...
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