New Amp, DC on grid, microphonic plate, cathode, grid wire/coupling cap.

[cboysen]

Hi, I have a problem with my newly built amp. Schematic and picture attached.

At full volume, I have about -30mV of DC grid voltage on V2A, feeding the cathode follower, making my volume pot scratchy as it turns. I have ruled out leaky coupling cap C2 from V1, since the voltage persists when lifting the cap from the pot. The open end of the cap then displays variations of +-15mV. I can get to a scope next month to clarify.

Furthermore, thumps/microphonics can be heard when tapping around V2, especially plate resistor R7, and wires for pin 1, 2, 3 and 8. Shielded grid wire from vol pot, does not thump. Coupling C2 thumps loudly too with 130V on one side, and is prone to noise when a hand is close by. The Cathode Resistor for V2A, R7 has 0.735V on one side, and is placed right next to C2, but does not thump. R8, the cathode resistor of V2B also has 130V, and I fear capactive coupling between C2, R8 and R7, or one of the parallel lines to V2. I highly doubt a cold solder joint in the new built, but it is possible.

Supply line E to V1 and V2 runs all the way across the board, and could be a problem, when V1 and V2 is filtered with 10k/10uf. Pulling V1, from the amp, has the odd effect of motorboating the amp at full dial. I believe the issue is unrelated, and is solved when inserting V1 or increasing the filtering of node E. Pulling V3 and V4 (reverb circuit) does nothing. Swapping out different tubes for V2 has no effect either.

Here are the DC voltages for V2,
1 134.7V
2 -0.030V @ Full Vol
3, 0.735V
4, 5, 9 - 50V (floated heater)
6, 230V
7, 134.7V
8, 134V

Any suggestions?

Thanks for the help!
Regards, Christian.

[SILVERGUN]

Hi, I have a problem with my newly built amp. Schematic and picture attached.

At full volume, I have about -30mV of DC grid voltage on V2A, feeding the cathode follower, making my volume pot scratchy as it turns. I have ruled out leaky coupling cap C2 from V1, since the voltage persists when lifting the cap from the pot. The open end of the cap then displays variations of +-15mV. I can get to a scope next month to clarify.

How about C5?


I just noticed you have 3- C5s...I'm talking about the .047 in the loop

[cboysen]

I have edited the schematic a bit, and forgot to relable some of the capacitors. In fact, the C5 capacitor in the loop, is not connected. It is switched out via a Dpdt switch, that changes between no NFB, 10Meg and 20Meg NFB resistor. When engaging the loop, the motorboating stops, even with V1 out, but it does nothing to the microphonics or grid voltage.

kind regards
Christian