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01-02-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,723
Joined on 05-27-2004

Post #: 21
Post ID: 6257
Reply to: 6250
Many fast tubes and the Lynn Olson Syndrome

 hagtech wrote:
Yes, it is an E55L.  I wonder how stable it is.  Interesting to me how many of these "uber-tubes" are designed for RF operation.  Should make a wonderful oscillator. 

Go figure not stable are they. All those MHz tubes are very fast and defiantly need a good shock-observant mounting, rid stopper and elevation of anode over screen to minimize possible oscillation. How stable are they after that? I think it is a model by model, brand by brand and in many instances a tube by tube. The 7788 and particularly made by a good brand is very good from this perspective and from my view it has the best transconductance vs. noise ratio.

 hagtech wrote:
The other find I came across was a pair of 6CL6.  Makes me want to design all kinds of new amplifiers.  The video amp schematic in the 6CL6 data sheet shows the exact series-shunt inductive peaking I used many years ago, but with transistors

Well, here were we are differents – I never have any desire to design/build “all kinds of new amplifiers”. I know many people do, it is what I call the Lynn Olson Syndrome – I do not suffer from it. I need juts one amps and one speaker and I do not found a pleasure to intermingle again and again and again with the amps and speaker. For the Olson-type of people the ceremony of intelligent BS on the subject is meaningful and self important. I am in contrary found that result is something then more motivates me. So for me that fact that there is another good tube out there or another good driver out there is not the solicitable factor. To me more important the specific problems that I recognize currently with sound and the specific direction where I would like to lead my Sound. To massage everything that is massageable, as Lynn Olson and many others do at his DIYaudio site is too tedious and ignite no interest to me. I am certainly not against the 6CL6 but it would be more useful to me if you say what is wrong with your sound NOW and how you feel the 6CL6 should change it. Well, you use 7788 with 1V of bias at line level input – I know what and why it should be wrong with your Sound now… :-)

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
01-02-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006

Post #: 22
Post ID: 6259
Reply to: 6257
L-O-S
I need juts one amps and one speaker


I don't design amplifiers for myself.  I design them for others.  And I am always on the lookout for something new or different or any possible improvement.  Not all customers want for the same thing.  That is where I am coming from, the viewpoint of a manufacturer.  And so coming across a tube like the 6CL6 brings new challenges, even if merely theoretical.  I wonder how I might use it.  What would it be well suited for?  Is it for driving, gain, buffer?  Is there something in the datasheet that gives me an idea for something else? 

Meanwhile, I am not experiencing any oscillation troubles with either the 7788 or the 6E5P.  Although I am always on the lookout for it.  One thing I did change, was that for triode mode, I took pin 8 on 7788 to anode, not ground. 

jh
01-02-2008 Post mapped to one branch of Knowledge Tree
Lucian
Posts 3
Joined on 01-02-2008

Post #: 23
Post ID: 6264
Reply to: 6250
E55L
Hello first, as I am new on this site...

I have tried a number of high Gm pentode/triode tubes, including E55L and, of course, 7788. All are very linear when used as triodes, but do not bet on very low noise, if that is what you would be after. These are very high bandwidth tubes, so be on the watch for RF behavior (see grid stoppers, decoupling, heater RF filtering, etc).
Great linearity for E55L/8233, not necessarily at very high anode currents (20-30mA usually are enough if a bit of conductance loss is tolerable, but maximum linearity is reached at about 50-60mA and 125-130V on the anode, whether triode or pentode connected). Great line driver and, why not, can be used for spud SEs (E55L can be run at 200V maximum). Also great perveance, E55L will swing between 25V and 200V at 50mA, if pentode (was used by Tektronix as a deflection amplifier up to 30MHz and more). Good current source too, but relatively noisy.
The only drawback: they are VERY expensive and fabricated no more. For fans of high Gm tubes, a few recommendations:
1. 6J51P (EF184) - Russia
2. 6J9P (E180F) - Russia
3. 6J52P - Russia
4. 6J53P same
This list is only a very small one out of the many tubes like this out there.

While there is no known correlation between Gm and sound quality, the increased conductance/perveance guarantees driving difficult (read reactive/complex) loads and also increases the efficiency of the smallest amount of current feedback, if used to increase the linearity. As a matter of my own personal taste, I do not use tubes that have less than 10mA/V and plate resistances larger than 10K, so I am basically confining myself to triode connected pentodes and high Gm triodes.

As a last comment, many reputable designs have used pentodes as drivers, but there are design rules to that, if the very high output impedance of a pentode is not to become a problem. See Citation II, Radford STA100, etc. The fine point is to use pendodes into a "zero impedance" NFB summing point: thus a pentode is used in the ideal way - a constant current source into an integrator input guarantees maximum bandwidth, lowest possible distortion, but noise can become a problem because it is presented also as a noise current source to the said summing point... but I should keep that rant for another occasion :-)...

My two cents.
Lucian
01-02-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,723
Joined on 05-27-2004

Post #: 24
Post ID: 6266
Reply to: 6264
A correlation between Gm and sound quality?

Lucian,

here is where the beauty of 7788 comes - max Gm with absolutely lowest noise ( the area where Russian tubes reportedly can hardly compete). What is particularly cool about 7788 that that are dead quiet one after another. I have near ten different 7788 brands all of them are dead quiet.

What you said is very interesting: “… there is no known correlation between Gm and sound quality…” This is a whole big subject. With transconductance of 55.000 in one stage and with very low signal from MC cartridge or a microphone where would be a virtue of “sound quality”? Will two gain stages of half transconductance be inherently inferior juts because it will have one more active stage? It is hard to say.

I have Dima’s designed 7788-7721 phonostage with 65dB gain in juts two stages. It is very good; would it be the same if it were 3 stages? I doubt. BTW, the 7788 is very sensitive to the “quality” and type of screen voltage, sometimes I will play with this subject more.

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
01-03-2008 Post mapped to one branch of Knowledge Tree
Lucian
Posts 3
Joined on 01-02-2008

Post #: 25
Post ID: 6269
Reply to: 6266
High Gm Tubes and "Sound Quality"
Hello Romy,

Quite clearly, a high Gm device will have a higher voltage gain than a lower Gm (for pentodes, gain is Gm*Rload if single ended) and this is something we are after in a MC stage. Noise performance is crucial and below I am analyzing a bit of that...

Like any other amplifier, a tube has an input noise voltage (mostly defined by an RMS sum of the thermally generated noise, mechanically generated noise and internal noise voltages like partition noise for pentodes plus other sources of noise - 1/F noise, cathode interface noise, etc -) and a (relatively low) input current noise, whose origin is in the thermally and electrically generated leakages in the grid(s)... Let us just see whether the high Gm is warranting good noise performance...

As defined by the datasheet, E810F has a conductance of 42-58mA/V, an Ri=42K if pentode and a gain µ1µ2=57. Excellent choice for a high gain stage, it seems, specially if we do not ignore the equivalent noise resistange of 110 ohms at 45MHz... Does all this promise good noise performance in a WB application? Clearly, but only when used at maximum gain (i.e. as a pentode or cascoded with another one like it). You obviously can see that this performance is relative (to use an Atkinson's favorite, "good for a tube amplifier" :-)...

Let's make some measurements, then: an E810F measured for noise shows about 50-100nV/sqrtHz@20Hz, 10-20nV/sqrtHz@1KHz and about 10nV/sqrtHz for the rest of the higher frequencies noise voltage densities as measured with my HP3585 spectrum analyzer. These values are practically the same for a very high quality ECC83 (TFK used as a comparison), but generally 10-30dB worse for the garden variety ECC83/12AX7 bunch.

The ECC83 has a 1.6mA/V Gm vs. 220K load, while the Gm of E810F is 50mA/V vs 20K load... these values will convert to a (calculated) input noise voltage of 1.1µV for ECC83 vs. 1.4µV for E810F. When taking gain into account, the output noises are 55µV for ECC83 and 38µV for E810F, not much of a difference, but clearly better for E810F.

What my is point, considering the measurements above? The Gm of a tube is not so clearly illustrating the noise performance of a tube by default, but only in a direct comparison (1.6mA/V versus 50mA/V only yields a noise improvement of 2.5dB, while the direct Gm increase is 29dB).
Not to mention that the noise densities (measured) of a 2SK170BL JFET are 10nV/sqrtHz@20Hz, 0.4nV/sqrtHz@1KHz and the rest, yielding a WB input noise of 0.33µV with a Gm of 80mA/V :-)... Is the JFET better? Noise performance-wize, vastly, at only 2.2µV output noise (+24dB SN improvement as compared to E810F or about +21dB including the -non-trivial- noise current of the JFET).
Is the JFET better? Noise-wize, far better if the JFET is used with very low source impedances such as MC.

Does it also mean that the JFET will "sound" better? Now clearly this is a whole totally different sort of discussion and, as you have said, we must tackle this (very interesting) subject separatedly :-)...

Best regards,
Lucian
01-13-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006

Post #: 26
Post ID: 6341
Reply to: 6269
7788 ringing
I am having a LOT of microphonic trouble with the 7788s in a high gain stage.  I run a 40mV input signal into a 7788 triode at 15mA CCS.  Plenty of gain, nicel output.  But dang, if I bump the tube or board or chassis, the grids inside ring like a bell.  More of a shimmer.  I can measure one having a very noticeable 5kHz tone.  It's a shimmer with a strong 5k resonance, like a tuning fork.  This is not a ping that damps out quick.  It lasts for 10 seconds or more.  One sample is worse than the other.  On a scope it looks like I have a 5kHz carrier tone riding on top of the signal.  I checked the usual suspects (fluorescent lighting, etc.), but it came down to the tube itself.  No RF oscillations.

I'm not sure I can use this tube in such an application (like a phonostage).  Romy, how do you keep yours quiet?

jh
01-13-2008 Post does not mapped to Knowledge Tree
Lucian
Posts 3
Joined on 01-02-2008

Post #: 27
Post ID: 6343
Reply to: 6341
7788 Ringing
Hello,

While the question was addressed to Romy, may I be allowed to barge in :-)?...

A question before anything: do you feed the 7788 DC or AC on the filament?

Many tubes are perfectly able to oscillate mechanically on 1KHz--20KHz range, specially when they are very high gain (see your CCS load...). This self- or microphony- induced oscillation seems to be excited further by the 60Hz heater power (electromotor induction into the cathode). DC on the filament tends to minimize/eliminated this electromechanically induced oscillation.

Obviously, against microphony there is the normal array of measures that one can try: soft "lantern" shock absorbers on a separate tube plate and mesh shielding, but all this can only do so much... Microphony seems to be one unavoidable evil with tubes...

Lucian
01-13-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,723
Joined on 05-27-2004

Post #: 28
Post ID: 6344
Reply to: 6341
My experience with 7788 is very different.
Jim, I do not know what to say: I never experienced any nose problem with it. A few years ago I built 2 mono phonocorrectors with 7788 at input as a pentode both of them were just nice and quiet and did not give me any problems in noise department. The phonocorrectors are still in service. Over the year I was trying a few different 7788 brands: Philips, Raytheon, Telefunken, Siemens, few otheres and none of them were noisy. I drive the 7788 with DC of course, typical for me SS rectifier with LCRC. The last cap is 80.000uF. The tube has 150V on plate and slightly less on screen, both drivers from the same 400V power supply, SS bridge – LCRC with an enormous last cap. The tube has one ferrite bid on grid, 50R grid stopper and uses Pearl’ shock-mount tube socket. I use as grounded tube cooler around the tube. Pretty much it is all that I did. I have no microphonic at all and noise at 150MHz (~200MHz in realty) scope looks very noise and random (I have no faster scope). I do pick microphonics if I hit the tube but it immediately dies. I never ever seen my 7788 get aroused itself in the way how 417A or 435A did. What is also amassing that all my 7788 were practically identical in gain, plate impedance and etc… Sorry, I am not useful in it as I never develop any experience to fight with 7788 as I never had any problems 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
01-13-2008 Post does not mapped to Knowledge Tree
Paul S
San Diego, California, USA
Posts 2,238
Joined on 10-12-2006

Post #: 29
Post ID: 6345
Reply to: 6341
The trouble with tubes...
Why do we even bother with tubes for low levels?  Of tubes generally, I have used 6SN7s, 2A3s and EL34s with no known-to-me problems, but lots of various noise trouble with certain tubes, including 6DJ8 family, KT88s, and on and on, especially annoying in cases where I like other things about the tubes.  I have found quiet 6DJ8 tubes, but I have to go way out of my way for them.

One thing I have noticed, though, is that some problem tubes quiet down when operating points are lowered, where this is an option.  Of course, not all tubes are mechanically quiet, and most were not designed to be, either.  I use the old dark plastic (PETA, or something like that) Herbies dampers on tubes that are SUPPOSED TO BE quiet.

Jim, I know it's a stupid question, but how many 7788s have you tried, and from how many sources?  Haven't we all gotten/tried several bad tubes in a row at some point?

I don't know the 7788, but I am generally suspicious of too much gain from too low level, which is one reason why I prefer to jump start with a transformer.

I know you will find a solution!

Best regards,
Paul S
01-13-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006

Post #: 30
Post ID: 6346
Reply to: 6345
More Whine
I guess I need to try out some more samples.  Only have a few here.  And only one has a long time constant 5kHz ring.  Heaters are 6.2Vdc with 20mVrms sinewave ripple at 120Hz.  Fed from schottky - L - C.  Other side of heater is grounded.  Operating points are low, with only 80V on plate, at maybe 13mA.  Sort of in a bind here if the 7788 doesn't work. I'm re-using a low voltage B+ supply (only +105V) from another project.  With a choke on plate or CCS (line level output) I still get enough headroom.

Maybe I could use some step-up tranny.  Although they would be dangerously near the power supply chokes.  It's a small box.

jh
01-13-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,723
Joined on 05-27-2004

Post #: 31
Post ID: 6347
Reply to: 6346
High transconductance and the Hawaiian archipelago.

Jim,

in my corrector (Dima’s design with R2=50R) I made many experiments adjusting the VR1. Dima proposed 150V on 7788 plate purely arbitrary, what he did it he never ever seen 7788 and it was just a concept RIAA for him. After it was built and I played with it turned out that 150V was a dead right voltage and I do swear that I was able to head what the voltage change to 160V and to 140V. So, you idea to “re-use a low voltage B+ supply”  might not be so economical after all…. BTW, I know that you are creasy person who like to put low bias tube at high-signal input… therefore I  hope you you’re your 7788 with it’s sub 1V of bias input at phono/mic preamp input not at line-level. At line-live that tube shell melt… It should not see more than couple hundreds mV AC on it’s grid…

Jim, I am very confident that you do something very wrong. I have practically no skills in the field of electronics DIY and the damn 7788 gives absolutely no problems to me – that makes this tube absolutely fool-proof even I was able to cook a phonocorrector with it. I mean you with you expertise shell be able to use the 7788 with no aggravations of any kind… Something you do is just wrong or you just have a bad party of the tubes.

BTW, I can testify that I did experience bad 7788. 5-6 year ego I bought on eBay a party of 150 tubes, they were all 7788 Amperex, all pulls and the price “Buy Now” was ridicules – like $40 or something like that. When I got the tubes a half of them were almost back – they were VERY severely burned out. I tested a few dozens of them that had clean glass and the almost all of them has no transconductance.  Among all party I pick 4 of 5 tubes that were more or less useable.  The rest I put back on eBay as pulls and went to somebody in Europe for $350. I do not know, I teach people for year never buy anything from me as whatever I sell is crap that I hate – people just do not listen. Anyhow, this good Amperex that I still have also had no noise problems. So, it is something on your side… You live on a volcanic island, perhaps the screen of the 7788 is too sensitive to those dally harmonic tremors…

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
01-14-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006

Post #: 32
Post ID: 6353
Reply to: 6347
Buy It Now!
I'll buy some more 7788 and give them a try.  I'd hate to change tubes at this point.  My triode connection has pin 8 (g3) to anode, not ground).  I wonder if that makes a difference.  Also, the power supply chokes are nearby, perhaps they are putting a 60Hz or 120Hz motor vibration into the chassis, which stimulates the microphonics.  Well, off to eBay.

jh
01-17-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006

Post #: 33
Post ID: 6368
Reply to: 6353
Too much gain
Maybe I was a little hard on these tubes.  I have this habit of listening to noise floor at extra high gains.  Maybe 20dB above loud.  It is part of the design process for me.  I like to hear all of the hiss, whine, buzz, hum, shimmer, and ping of everything going on.  I tap on all parts of the chassis and tubes.  This is much more effective than running measurements.

Anyway, I finally had the chance to listen to the tube.  The hum was not so noticable at normal playback levels.  And the tube microphonics were also much less than I had eluded to.  I could still hear the 5kHz tuning fork between songs.  However, then one channel went very bad.  The sound was like a hair stuck under a stylus while playing LP.  Huge distortions, most notably on the louder passages.  Funny, it took me awhile to cath on, as the other channel was crystal clear, and the brain was trying to sort things out and run real time corrections.  A quick swap of tubes from left to right and the problem went with it.  So I have a very baaad tube.  Sadly, the one with the 5kHz resonance is the good sounding one.

jh
01-19-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006

Post #: 34
Post ID: 6390
Reply to: 6368
Good News, Everyone
Romy, you were right.  I just got in another pair of 7788 from eBay.  This time, one tube has a little bit of microphony, not bad, really.  The other was dead quiet!  No ping at all.  Very impressive, that sample.  So now I guess I have to buy batches and select...

jh
01-19-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,723
Joined on 05-27-2004

Post #: 35
Post ID: 6392
Reply to: 6390
The "art" of buying high transconductance tubes.

 hagtech wrote:
Romy, you were right.  I just got in another pair of 7788 from eBay.  This time, one tube has a little bit of microphony, not bad, really.  The other was dead quiet!  No ping at all.  Very impressive, that sample.  So now I guess I have to buy batches and select...

Well, it how it should be. The 7788, according to people who knows, at its max gain has the lowest noise among any other reasonable tubes. I have to admit that with exception the large party of the pulled Amperex tubes that I bought in the past (that were juts all severely burned out), and among all others 7788 that I have (perhaps 40-50 tubes of the few brands) I never come across any 7788 that was     microphonic. I did not plug all of my tube in my phonocorrector but I did plug one or two of each brand (perhaps a dozen brands and vintages).  Perhaps it was just luck or perhaps it was just the way how I buy them.

Talking about 7788…. I do not buy used tubes; I do not buy tubes that were measured; I do not buy tubes that have no original boxes. I do not buy single tube or a pair of tubes. I tend to buy the 7788 from a seller who has a party of NEW 10s or 100s of them. All of them should be sealed and then I buy my few tubes from that party. I tend do not buy the high transconductance tubes if a seller has a part of I would say 20 tubes and 2-3 of them without original box. To me it is bad sign saying that somebody who owned the tubes cared about them. Well, perhaps I am wrong, but not doing audio for living I know that if I sell anything then it is something that I really hate or do not need. Perhaps I am wrong… but I never seen the bad sounding or noise 7788. My experience with WE 417A/437A was very much different and at that times (~2001) I was buying variety of those tubes that did not comply with my “rules” above  and as a result was horrible – 80% of my WE tubes were horrible. I am not saying the exceptions are not possible but… still I would be very anal if I was in market for high transconductance tubes, particularly since the price for 7788 went up last years and a dozen of a good 7788 might cost over couple hundred dollars.

Rgs, 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
01-19-2008 Post does not mapped to Knowledge Tree
N-set
Gdansk, Poland
Posts 473
Joined on 01-07-2006

Post #: 36
Post ID: 6393
Reply to: 6392
Other high-gm choices?
Guys, appologies for drifting a bit off topic (Romy fell free to remove the post).
Are there any other interesting high gm tubes?
Mu of 30-50 and Rk around 1.5k
is what I'm looking for.
Any experience with
the standard 6C45P, EC8010, etc?

Thanks,
jk


Cheers,
Jarek
STACORE
01-19-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006

Post #: 37
Post ID: 6394
Reply to: 6393
Other Tubes
That 8233 might do it.  Also noticed the 6CL6 in triode mode will get you almost there, but only a mu of 20.  The 6688 could be a substitute.

jh
01-19-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,723
Joined on 05-27-2004

Post #: 38
Post ID: 6395
Reply to: 6393
High gain tubes...
http://www.pmillett.com/pentodes.htm

http://www.pmillett.com/images/pentodes.PDF


"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
02-17-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 9,723
Joined on 05-27-2004

Post #: 39
Post ID: 6684
Reply to: 6368
Read my leaps: Absorb-GEL

Jim,

I do not know if you still fighting with 7788 noise but let me tell you something: those Allnic shock-absorbing tube socket adapters are insultingly affective. I act lay made some measurements and I was VERY surprised how much the Absorb-GEL eats vibration, I think it should a must for phonostage/mic first tube. In fact the effectiveness of the Absorb-GEL change the ways how the chassis design might be viewed…

http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=3333#3333


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
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  »  New  Where are our good phonostages?..  Omnigon Tubes...  Analog Playback Forum     61  511999  05-31-2004
  »  New  Shock-absorbing tube sockets...  Skinny pins and cheap-o sockets...  Audio Discussions  Forum     11  64600  12-21-2006
  »  New  The 6E5P tube data...  Bartola Valves: 6e5p beam tetrode SPICE model...  Melquiades Amplifier  Forum     44  381178  07-23-2007
  »  New  The Shielding Condoms on those tubes......  The Shielding Condoms on those tubes.......  Melquiades Amplifier  Forum     0  19987  09-21-2009
  »  New  An interesting Russian pnonocorrector...  Uber-tweeky phono topologies deconstructed...  Analog Playback Forum     9  67885  03-01-2010
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