5F1 Build Thumping Problem?

[oogabooga23]

Hello everyone! I just finished my 5F1 build, and it works and all, but I don't seem to be getting different volumes with different inputs. That's a lesser problem, however, as the main problem is whenever I turn up my volume to about 10-12, there is this loud, consistent thumping sound. It mainly starts whenever I hit a bass note and if I keep hitting it, the thumping stops eventually just to return. Does anyone what this is and how to possibly fix it?

[oogabooga23]

So right after I just posted this, I fiddled around in the amp and found that whenever I hit the V1A Cathode and Cathode Bypass, it fixed the solution. The only problem now is the same volume in both channels and that I realized that the lamp flickers every now and then.

[oogabooga23]

Disregard what I said as the thumping did not get fixed. I am assuming it is the cathode resistor or cathode capacitor that is making this noise. Can I remove it altogether?

[sluckey]

A 5F1 has no cathode cap on V1A.

[oogabooga23]

The mojotone wiring diagram has one though, unless I am thinking of something else

[sluckey]

I don't seem to be getting different volumes with different inputs.

That usually happens if you forget the jumper between the input jacks (see attachment).

loud, consistent thumping sound

Disconnect the NFB wire between the speaker jack and the 22K on the board (see attachment). Any better?

[oogabooga23]

The jumper wire is hooked up, and I should of mentioned before that I don’t have a NFB resistor in series with the 1.5K going to the speaker jack. Maybe installing it would help? Does it also make a difference that I used an input jack as a speaker jack?

[PRR]

> I don’t have a NFB resistor in series with the 1.5K going to the speaker jack

The mojotone wiring diagram does not have it that way (I hope!). Leaving out the 27k(?) NFB resistor will kill gain in the final (enough to motoboat) *and* short-out the bias on the triode (overload easy).

[oogabooga23]

Hooking the NFB resistor didn't help at all, but it did help it conjunction with me clipping a .1 uF capacitor going to pin 1. Is there a reason? The motorboating seems to only happen whenever I play a note now and almost full volume.

[oogabooga23]

Alright so I think I got it down to the two 0.022 uF capacitors. I clipped some orange drops parallel to each capacitor and the thumping stopped! The only problem I have now is that I can't seem to get different volumes in the two inputs.

[sluckey]

The only problem I have now is that I can't seem to get different volumes in the two inputs.

Two jacks, three resistors, and a bit of wire. Should be easy to fix.

[oogabooga23]

Yep, I managed to fix it that problem as well. One of the grid resistors had a broken end so I replaced that, and I also removed the bypass cap. Thanks everyone!