1972 Twin Reverb Hum

[manolo]

I replace filaments resisrors with 2x120ohms and the voltage went to 6.14Vac both (balanced) but nothing to do with hum...
Then i take some measurments from my good old super reverb.
A+ is 510Vdc with 5.3Vac! (problematic twin has 450Vdc with 3.3Vac)
B+ is 508Vdc with a few milivolts of Vac!(twin has 450Vdc with 3.3Vac)
The super reverb's CHOKE is 105Ohm but the twin's CHOKE is 12ohm!
im gonna try a choke from another twin and tell you the results...

[Chuck H]

12 ohms is definitely too low. The choke is the same part# for both amps BTW. I might expect a little AC at the first filter node because there's just so much ripple there. By the time you get to the second node it should be squelched under a volt though. So this is probably the problem. Lack of isolation so the filters can do their job independently. I have to give you kudos for perseverance. And my apologies for assuming a wiring error, but that wasn't assumed to be your fault considering the condition you received the amp in. I hope the substitute choke fixes it. If you don't want to bastardize another amp for this a 10W 1k resistor should work too. Though you'll definitely want to put a proper choke in it eventually.

[manolo]

Unbelievable!
The f$%^ CHOKE!
Swap the choke from my SR and it sound beutiful...no hum at all ... dead silence
Now i measure at A+ 450Vdc with 7.03Vac and B+ 447Vdc with a few millivolts of ac!
125C1A

[manolo]

Thank you guys!Great forum!

[Tom Phillips]

...Swap the choke from my SR and it sound beutiful...no hum at all...

Glad you found the problem. I think the report of "400 Hz 'Hum' " threw people of the track. It would have been excess 120 Hz at the power supply node. Why your Fluke gave you a 400Hz reading at the speaker terminals remains unknown. Perhaps it just got confused trying to measure the frequency of the resulting waveform after the 120 Hz passed through the reverb springs. At least the problem is fixed which was the main goal.
Cheers,
Tom

[manolo]

here it is .
see the burnt area on the chassis amp

[52 Bill]

Glad you found the problem. I think the report of "400 Hz 'Hum' " threw people of the track. It would have been excess 120 Hz at the power supply node. Why your Fluke gave you a 400Hz reading at the speaker terminals remains unknown. Perhaps it just got confused trying to measure the frequency of the resulting waveform after the 120 Hz passed through the reverb springs. At least the problem is fixed which was the main goal.
Cheers,
Tom

Tom, I think that he said that he is in a location where the ac is at 50Hz, so it is a multiple of that.

[Tom Phillips]

Tom, I think that he said that he is in a location where the ac is at 50Hz, so it is a multiple of that.

OK. That makes more sense then. For a while I considered that the frequency counter was locking on to the 2nd harmonic of 100Hz but I still think it was just confused by the waveform created by the reverb turned up high. It may have locked on to 100 Hz if he put the Fluke freq meter on the drive side of the reverb circuit.

[Chuck H]

It may have locked on to 100 Hz if he put the Fluke freq meter on the drive side of the reverb circuit.

I think he did. That's why I mentioned sum/difference possibilities. I stopped believing it was actually 400Hz once the pan was ruled out.