Friedman BE-100 hum problem

[sorin65]

The amp got a loud 50 Hz hum noise. The noise is not affected by volume controls. also, when I insert the guitar in FX Return, still there. even without any tubes in preamp. disapear only when I remove the PI tube.
I changed the power EL34 tubes with a new set - same problem.
The amp, upon serial number, made in October, 2015: preamp tubes heaters are in DC 12V. power tubes and PI, 6 V AC, with mid tap to ground.
I have not any more clues to resolve the hum presence - heaters circuit seems ok, tubes are ok either.
Should I check anything else?
Thanks in advance!

[nickb]

Have you tried a different tube for the PI?

Not sure if yours is the same the schematic below. Try tacking another 100uF in parallel with C30 in the power supply to the PI (although that would be 100Hz hum).

[sorin65]

oh, yes, I tried new sets of tubes. very trusted tubes.
just right now I discovered 181mV Ac over the 37 V DC on the gate of powetubes. I put just for test a 50 micro el cap to ground, and AC goes down to 80mV AC and hum goes lighter.
I ll replace both el caps for the bias circuit... but will have a lot to work because the amp is not a "real" point to point.

[catalin gramada]

Man...Usually the bias circuit is a half wave rectification it is as normal to have some ac ripple there.For the sake of tone some valves like a little bit ac into the grid. Change it with exactly you found if want to preserve the tone. Decouple with a big value and will change it functionally speaking. From my opinion loud 50cps hum can be a ground reference missing somewhere. This I.ll check in first instance. Perform heaters voltage checkin in respect with chassis first...Maybe is a ground reference bolted in a point which becomes loose.

[sorin65]

I put the 50 cap only for test. I know that larger values ruins the bass response and the feel of the amp. true also for big caps on ht rails, in modern era.
the heaters circuit is very simple, with big gauge wires direct from PT.
anyway, changing elcaps in bias circuit didnt help, 50 cycle hum still there.

[catalin gramada]

Hey, nickb pointed an important observation: are you sure is 50 and not 100cps hum? 50cycles hum should be easy to track. Now seen from nickb posted schematic the heaters are elevated by a divider from screens node point.Check the circuit and it's decoupling caps

[sorin65]

yes Cātālin, is 50Hz. very clone G1 note pitch. . I verified on the guitar tuner. when I find this problem I assumed that is an 10 minutes repair. usually 50/60 cycle consist only in the heaters circuit... but I verified all , anyway is easy to track - large gauge wires direct to heaters and tap 1/2 to ground. ok, the guys from Friedman didnt torsion each other the heater wires, but the amp was heavy used around two years on stage without problem.
to much hype in BE-100

[catalin gramada]

You said if take out the PI tube it stop humming. Did you tried to shunt to the ground PI input to see if is stop humming ? Just shunt to ground the PI input before those 22nf cap, not from tube socket side. Check first if you don.t have any dc there (should not be)

[xtian]

Man...Usually the bias circuit is a half wave rectification it is as normal to have some ac ripple there.For the sake of tone some valves like a little bit ac into the grid.

Wait, what? That’s a new one on me. Please explain.

[sorin65]

yes. anyway, with the Return Level pot, when is to max low, the 22 nF PI cap goes to ground..
I think are some shorted coils in the PT, on the 2x3VAC secondary. I isolate the center tap, trying to high lift the tap, or i put 100 ohm trimer between heaters to ground. worse!

[catalin gramada]

In all guitar amps I found bias circuit have to be as dynamic as it was designed for cause power supply act in a dynamic way.The half wave rectification is the traditional way to ensure negative voltage to power grids. Usually the caps was chosen to decouple enough the circuit and not necessary to ensure ripple free dc voltage as signal noise ratio is not a concern for application. Make it stiff or full rectified and will change the original response.In a good or bad way, that is debatable, but will be changed for sure. Sorry for offtopic

[catalin gramada]

I have a suspicion.Can you try to lift ,just for checkin, the earth wire or to use a power cord without earthing ?

[catalin gramada]

Man, I have no other ideas, time to get a scope Btw a neon bulb can generate hum into signal path if is to close to sensitive traces, but think is not the case...

[nickb]

yes. anyway, with the Return Level pot, when is to max low, the 22 nF PI cap goes to ground..
I think are some shorted coils in the PT, on the 2x3VAC secondary. I isolate the center tap, trying to high lift the tap, or i put 100 ohm trimer between heaters to ground. worse!

If there were shorted turns on the heater winding the transformer would be running very very hot. I seriously doubt that is the issue. I'm not convinced by the 50Hz test using the tuner either. I think it might simply showing you the lowest frequency and not the loudest.

Bias circuits feed both grids equally and so any ripple tends to be cancelled in the output transformer. The same is true to some extent of the HT supply to the pi but you haven't done anything to test for that yet (see post #2) so give it a try.

If that didn't help, then with both clean and drive channel volumes set to zero, what effect does the channel switching have?

If the same then lift one end of C31 to see what the effect is.

Another possibility for hum is instability in the power amp, probably ultrasonic. Best to check for that with a scope but it might go away if one pair or the other of the EL34's are removed. Did this start after a repair, perhaps the EL34's were changed?

Also, how much hum are we talking about here? Measure the ACV across the speaker.