HI!. After working for a long time in this amp, and searching for a hum issue, i realised that i have a small voltage at the grids of tube 1. What can be the cause of this? How can i trace it?. Voltage at input jack is zero, but at grid 1 is 7mv. And the real problem is at grid 2. i have 60mv, and if i disconnect the whole eq and leave just the volume, with gain at zero i have no voltage, with gain maxed, i have 160mv. I also took out the neg feedback. there's only the 220k and bypass cap at that stage's input, and of course the gain pot. I've changed tubes, and the problem persists. Any clues?.
Also...when i touch the chassis anywhere the hum seems to be the same, but if i touch it near the first preamp tube, it disappears...
All preamp tubes use dc filament supply, reference to ground as in tut books.
Thanks a lot for your help and consideration...
small voltage at grids 1 and 2...help!
Moderators: pompeiisneaks, Colossal
Re: small voltage at grids 1 and 2...help!
swt, It 's possible you have leaky grid caps. disconnect the grid side of the cap and measure DC voltage from the loose cap lead to ground. Except for the initial kick, if you read any at all the caps are leaky. hth
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Darkbluemurder
- Posts: 584
- Joined: Tue Jan 03, 2006 7:28 pm
Re: small voltage at grids 1 and 2...help!
swt,
are the tone controls or the input trimmer scratchy? This would also be an indication of leaky coupling caps. Please note that the tone caps do also work as coupling caps in the circuit.
Good luck,
DBM
are the tone controls or the input trimmer scratchy? This would also be an indication of leaky coupling caps. Please note that the tone caps do also work as coupling caps in the circuit.
Good luck,
DBM
Re: small voltage at grids 1 and 2...help!
thank you guys for the replies. It was a whole batch of EH tubes. Put a sovtek in there, and the problem was solved. But the hum/hiss issue is there anyways. SO...i'll keep the search on, because this is the best amp i've ever played, and i wanted sounding ok at the least. thanks again.
Re: small voltage at grids 1 and 2...help!
I didn't jump in on this one because I typically see a few mv on the grids. I think it's grid leakage and is acceptable.swt wrote:thank you guys for the replies. It was a whole batch of EH tubes. Put a sovtek in there, and the problem was solved. But the hum/hiss issue is there anyways. SO...i'll keep the search on, because this is the best amp i've ever played, and i wanted sounding ok at the least. thanks again.