1972 Twin Reverb Hum

[manolo]

listen to reverb pot up and down..

[Chuck H]

I have no audio right now.

We've eliminated ground loops, the pan and the recovery circuit completely. Now I'm especially perplexed by the 400Hz!?! Nothing in the amp itself can do that. 400Hz would sound about like an "A" note played on the guitar (a little lower). Hardly a hum.

Plug the driver tube back in. If you have a 400V+ electrolytic cap tack solder it from the reverb transformer HV node to ground. What happens?

If you don't have the cap just skip that and try lifting one end of the coupling cap to the reverb driver grid. What happens?

[manolo]

try lifting one end of the coupling cap to the reverb driver grid. What happens?

v3 in and lifting the 500pF = same hum!

[Chuck H]

What is the resistance from the driver grid pin to ground?

[manolo]

its 1M

[Chuck H]

That's odd. It's not often that a resistor measures it's exact spec

I was requesting that you take an actual measurement regardless of how it's wired or what the schematic says.

[manolo]

with the 500pF is 1.098M
without the 500pF is 1.12M

[Enzo]

And with that cap lifted it still makes the noise?

I mean there are only so many ways an amp can make noise. Scope the B+ for this stage. I mean the B+ supply node, not the tube plate. Is this noise signal riding the B+? Like the filter cap is not working?


I can't see why it would matter, but is the secondary side of the drive transformer grounded?

[manolo]

And with that cap lifted it still makes the noise?

yes

is the secondary side of the drive transformer grounded?

yes

Scope the B+ for this stage. I mean the B+ supply node Is this noise signal riding the B+? Like the filter cap is not working?
how do i do that?

[manolo]

between groung and B+ measures 440V DC and 3.3V AC!

[Chuck H]

Somehow half of your filament winding is on that supply node as well as that node not decoupling well. This would be from a mistake in the wiring or a bad ground involving both the filament winding and that HV node. How that ends up as 400Hz I don't know. It could be a sum/difference thing but my brain can't figure out just how.

EDIT: Check to see if you have the same 3.3VAC on any other nodes. You probably don't because the filters should decouple it, dangerously I would think. Check the 100R resistors for the false CT on the filament supply. Make sure they are grounded. Check the decoupling cap for the screen/trem/reverb HV supply. Make sure it's grounded. Look for mistakes along the way. If you didn't replace all the tubes, including the power tubes, there may be a bad tube leaking AC onto the heater circuit? Basically, you've narrowed down what to look for and found that there is AC where it shouldn't be. Find out why and how to fix it and that'll probably fix this problem and perhaps others you didn't even know you had.

[manolo]

3.3Vac 100hz have to A+ and B+ . A few millivolts on C+ and D+
Filaments have 56 Ohm to ground each (2x100Ohm) one is 3.4Vac and the other is 2.96Vac both 50hz
The hum is there with new and old power tubes

[Chuck H]

If the filter caps aren't wired wrong they are bad. You said you put in new electrolytic caps. How old were the new caps? If they're NOS or have been sitting around for years they may be bad/new. More likely there is an error in the wiring. Did you check polarity on the filter caps? Did you measure from the B cap "-" terminal to chassis?

[g1]

Not sure why your heater supply is unbalanced (3V and 3.4V), lift one end of each of the 100 ohm and check them, one of them may have drifted.

[Chuck H]

Agree. Needs to be checked out. But that shouldn't cause the issue at hand. 3.3VAC on a filter node HAS TO BE the root of he problem. Something is wired hinky. I'd bet on it.