The Amp Garage

Reversing the cap is worth a try. Is the radio station noise going up while the hum goes down when you ground to the bus? It's odd that you could kill the hum by grounding the Gain 2 pot... that cap was still there with a high resistance from the grid of V2A to ground. This whole thing is weird lol...

Have you reversed the cap orientation to make sure the outer foil is on the ground side? Just a thought. Nope. I'll try that. Someone else also suggested I split the 150K grid stopper up into two 68K resistors and place the cap at their junction. So a grid stopper is blocking any additional noise f...

Now that I've identified the problem, I need to figure out how to solve it haha. That cap should be in the circuit. It's part of the tone of the amp. But why is it adding noise in this build but not my others? I tried grounding it at the bus bar on the turret board instead of locally and it reduced ...

Ok, I figured out the source of the problem. But I don't know how to fix it just yet. It's the cap to ground on V1b. The 0.001uF cap from pin 7 to ground. Not sure why it's causing hum. But removing it totally eliminates the noise. That cap is integral to the circuit. It's in my other builds and it ...

Tried reversing the phase on the heaters. No change.
Tried a snubbing cap across V1a plate-to-grid. No change.

Sorry, on the back panel pot I meant "up" on the schematic, grounding the grid. The behavior is as expected there. Since the hum is stopped by grounding the input to Gain 2 that pot is okay, and that pinpoints the source as V1B. That leaves the tube, socket (poor contact perhaps), and the filament ...

Some folks on another forum have me on a wild goose chase that I'm not really going to do lol. They think it's heater hum. But I'm running DC on V1 and V2. And my other builds don't have hum. They want me to try reversing the phase on the filaments. Not sure why. That would require ripping out all t...

Ugh... I've been over every single connection in the amp. Everything appears correct. No idea what's causing this noise haha. Maybe a bad pot or switch? Would a short between one of the DC heater wires and ground and/or a short between DC filament on pin 9 and pin 4/5 of V1 or V2 cause noise? Or wou...

Does the hum go away if you turn the pot on the back panel all the way up? All the way up? As in maxed out? No, the hum doesn't go away. But it's not like that pos is causing it. With that pot all the off, there is no hum. But that makes sense since there would be no signal with it off. Same thing ...

Hmm. So where are you grounding for these tests? And is that the same place that the input or wiper of gain 2 was grounded when the hum stopped? Doesn't matter where I ground. The result is the same. I have a test probe with an alligator clip on the other end that I can clip to the chassis anywhere...

What if you ground the terminal strip lug where the 150k grid resistor on V1B is connected? Does that make it silent? If so then that would say it's coming in through the coax. That terminal strip is a rogue ground point too, but if your other amps are quiet it must be okay. Nope. That amplifies th...

martin manning wrote: ↑Sun Aug 20, 2017 10:49 pm The hum you are chasing is 60 Hz, correct?

No idea what its frequency is; don't have an o-scope.

Ok, work your way upstream by grounding the wiper of Gain 1. If the hum is still there then the source is V1B or perhaps a filament lead too close to its grid lead. Grounding the wiper or input of Gain 1 doesn't reduce the hum. In fact, it actually creates a slightly different hum frequency on top ...

Ok yeah, grounding the wiper of Gain 2 stops the hum (same goes for grounding the input lug).

Ok, thanks. So, if you ground the top or wiper of the Gain 2 pot, does the hum go away? The h-k insulation in the CF triode (V2B) might be under a lot of stress (don't know the voltage there), but if you can kill the hum by grounding upstream of that it's ok, at least for now. You said you swapped ...