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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: AaarrggghhhhhPosted by anthony on: 1/4/2019
 anthony wrote:
I woke up this morning to have the brightest thought that my DHT issues are probably caused by how I have wired up the filament transformers. 

After rewiring the filament transformers this morning, I have spent all day trying to figure out why there is next to no current flowing across the cathode resistor of the DHT tube.  Wiring was checked every which way, measurements were taken, exasperation mounted, everything was ruled out, I even rewired half of the circuit just in case I had a broken wire somewhere...India is thrashing us in the cricket...nothing was going to plan.  I was miserable.

Then I swapped the used YO186 I purchased just for this testing stage and put in a NOS 1939 vintage tube and the circuit worked straight away with all voltage and current targets just perfect.  Put in a 45 and it was just as perfect.

Good to have it working though...just wish it had been 8 hours earlier!  So all 6 channel of the first DSET are now operating.

The DC Offset would not trim to zero and 0.1mV was as low as it would go before running out of trimpot.  The pot from one end to the other would only adjust the DC Offset by 0.7mV, which is much less than I thought it would.  My 0A2 are quite closely matched for voltage and are within 0.3V of each other.  Swapping the tubes had no effect on the DC Offset so I might have to play with another tube to cancel that offset to zero.

With the input shorted I made some quick noise measurements at the output of each channel:
A - 0.2mVAC
B - 0.2mVAC
C - 0.3mVAC
D - 0.25mVAC
E - 6.7mVAC - I assume this is hum and it was a little dangerous to reach around to the hum pot to try to eliminate it so I did not - should be able to do better than this
F - Held the Lazy Ribbon to my ear and could hear not a thing.

The noise numbers are quite good so I don't think there will be any major wiring changes required for the second DSET, although I will reposition the amplifier so that I am able to try out the hum pot for the DHT Channel.  Next up I will make it safe and get a mate to help me lift it upstairs and into position.  Once there I will see if I can figure out the audio analyser and make some more checks and measurements.  I really want to make a detailed input impedance graph and I have the software here to do it...I think you would be interested in that plot Romy.


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