70's Twin Reverb loud circuit disturbance during warmup

[jvm]

I have a 70's master volume Twin Reverb that works great other than a very loud hiss/crackle during warmup. I have isolated the noise to one side of the output section. I have swapped tubes from one side to the other (in pairs, new tubes) with no affect. Bias current on both tubes bounces from approximately 40mA to 70-75mA per tube while this is happening then stabilizes at 40mA per tube. The bias VOLTAGE never fluctuates. I have replaced both coupling caps feeding the output stage with no change. This only occurs when first turning on the amp and for only about 60-90 seconds but is VERY loud and VERY annoying. Anyone seen this particular problem before?

[Enzo]

A noisy tube or something? Sure we have all seen it.


I get if you pull the PI tube it won;t happen. My first thought when I read your title was: noisy phase inverter tube. Pull the reverb tube and stick it in place of the PI tube. ANy difference?

[jvm]

I have pulled the PI and isolated the noise to the output section. I can watch my bias probe and see the current draw bounce around only on the left pair of 6L6's. The other pair are stable while this happens. New output tubes but the amp was doing this with the old tubes also. I've re-soldered everything on both sockets and re-tensioned all the tube sockets also.

[Enzo]

OK then. ANy noise when amplified will also show as a current spike through the tube. Tube current is how it amplifies anything.

You have four power tubes, and you have isolated the problem to them by pulling the PI and the noise remains. You have further limited it to two tubes on one side. So pull one of those at a time to see if both tubes are involved or if really only one of the bad pair is noisy.

You have eliminated the coupling caps? Then the next suspects are resistors. If both tubes can individually still make the noise, I'd look at the plate resistor for the PI. If only one tube does it and its friend is innocent, then you have the grid stopper resistor and the 470 ohm screen resistors. How is the resistance of those screen resistors? Carbon comp? Noisy resistors are a common enough repair.

In the preamp, we can often ground a grid with a clip lead to see if noise is coming through that tube. We can't ground off the grid of your 6L6 because it need the -50v there to keep it running right. But we can still ground off the noise. If we add a series cap - I use 0.047uf because I have a bunch - to a clip to ground, we can then ground a 6L6 grid with it. The cap will ground off any noise signal, but won't affect the DC bias. Using such a tool will help isolate where the noise signal is travelling.

[jvm]

I replaced PI plate resistors, screen resistors, and grid stoppers. No change. I also grounded the grid as Enzo suggested. No affect. I finally decided to change the tube socket and the noise is gone. I see no signs of arcing or damage to the socket but it apparently was the issue. Thanks for your advice Enzo!

[Randall]

In the preamp, we can often ground a grid with a clip lead to see if noise is coming through that tube. We can't ground off the grid of your 6L6 because it need the -50v there to keep it running right. But we can still ground off the noise. If we add a series cap - I use 0.047uf because I have a bunch - to a clip to ground, we can then ground a 6L6 grid with it. The cap will ground off any noise signal, but won't affect the DC bias. Using such a tool will help isolate where the noise signal is travelling.

I Love It! Buy that man a beer!

[jvm]

Thanks, Enzo, for your willingness to share these invaluable tips to help us all better understand how to troubleshoot these amps. I learn something from every one of your posts. You are the man!!!