PP 6b4g bad / distorted channel diagnostics

2channels · 25th September 2011, 03:40 PM

Trying to diagnose one of the channels on a push-pull 6b4g amp, the amp uses 12sl7 and 12sn7 for the input and driver stages.

The symptoms are that the bad channel sounds "dirty" / distorted and is weaker than the other channel. The symptoms become more pronounced when the amp has warmed up for a few minutes or I turn up the gain control on the amp.

I'm far from an expert on tubes but I tried doing the following: swapped tubes, cleaned all tube sockets and pins with deoxit, cleaned the volume pot, checked resistors, looked for bad solder joints... all to no avail...

The amp isn't very old so I don't suspect that the caps would be leaking or be bad. I don't see any obvious leaks, bulging, etc. Unfortunately I don't have the equipment to properly test the caps.

Any suggestions as to what / where to look for the source of the distortion?

MrCurwen · 25th September 2011, 03:59 PM

A simple multimeter is enough to diagnose most problems. Simply measure voltages at various points, for example a leaky cap would result in a non zero voltage at the following grid (assuming cathode bias). Cathode, grid & plate voltages will go a long way to seeing if something is wrong.

25th September 2011, 03:40 PM	#1
2channels diyAudio Member Join Date: Aug 2010	PP 6b4g bad / distorted channel diagnostics Trying to diagnose one of the channels on a push-pull 6b4g amp, the amp uses 12sl7 and 12sn7 for the input and driver stages. The symptoms are that the bad channel sounds "dirty" / distorted and is weaker than the other channel. The symptoms become more pronounced when the amp has warmed up for a few minutes or I turn up the gain control on the amp. I'm far from an expert on tubes but I tried doing the following: swapped tubes, cleaned all tube sockets and pins with deoxit, cleaned the volume pot, checked resistors, looked for bad solder joints... all to no avail... The amp isn't very old so I don't suspect that the caps would be leaking or be bad. I don't see any obvious leaks, bulging, etc. Unfortunately I don't have the equipment to properly test the caps. Any suggestions as to what / where to look for the source of the distortion?

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25th September 2011, 03:59 PM	#2
MrCurwen diyAudio Member Join Date: Feb 2010	A simple multimeter is enough to diagnose most problems. Simply measure voltages at various points, for example a leaky cap would result in a non zero voltage at the following grid (assuming cathode bias). Cathode, grid & plate voltages will go a long way to seeing if something is wrong.

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