135 watt Twin Reverb loud hum

[lo-watt]

SO,,i had a late 70's Twin Reverb come in with a loud hum.

It had been taken to another shop and had the following replaced: all filter caps,all bias caps and resistors,the cathode bypass caps,power tube screen and grid resistors(470 and 1.5k),the big 10 watt resistors in the power supply,bridge rectifier diodes,hum balance pot tested and working, tube matching pot tested and working,bias in acceptable range. New matched quartet power tubes,all preamps pulled and hum still exists. Speakers test well(JBL orange frame) Plate load resistors test well

At this point im thinking an issue with a transformer. Not much else left?

[nosaj]

SO,,i had a late 70's Twin Reverb come in with a loud hum.

It had been taken to another shop and had the following replaced: all filter caps,all bias caps and resistors,the cathode bypass caps,power tube screen and grid resistors(470 and 1.5k),the big 10 watt resistors in the power supply,bridge rectifier diodes,hum balance pot tested and working, tube matching pot tested and working,bias in acceptable range. New matched quartet power tubes,all preamps pulled and hum still exists. Speakers test well(JBL orange frame) Plate load resistors test well

At this point im thinking an issue with a transformer. Not much else left?

Depends on the hum, a carelessly soldered connection will make all the hum in the world. Divide an conquer, is it in the preamp or the power amp?

nosaj

[lo-watt]

Since the hum exists with all the preamp tubes out,id say power supply section.

[Enzo]

Absent preamp tubes - and I assume you include the phase inverter in that - that leaves the power amp.

I don't know how it would be a transformer.

Does it hum with power tubes removed?


I don't care what they replaced, don't assume anything is OK. Is there ripple on the bias supply? Scope it.

Are any of your power tube screen resistors open?

[lo-watt]

Yes, I pulled all six preamp tubes. It does not hum without the power tubes. I alligator clipped in new F and T caps under the dog house one by one,to make sure it was not a bad filter or bias cap. No change there. I verified all the screen and grid resistors were within spec and voltages on the power tubes were within spec.

[lo-watt]

l also swapped the new power tubes with another known good quad with no change in hum. Thepower cord was also changed and the outlet and ground switch bypassed and new power and standby switches installed as well.

[The Dude]

100 ohm resistors for the filament artificial center tap good?

[Enzo]

Bias caps not in the dog house.

But that is why we scope the supplies. Ripple is ripple, whether it makes sense to be there or not.

Is it 60Hz or 120Hz? Be sure.

[lo-watt]

100 ohm resistors for the filament artificial center tap good?

This amp has the hum balance pot tied into the heaters at the V1. No resistors

[The Dude]

Ok. So does the pot test good?

[nosaj]

Ok. So does the pot test good?

At this point the scope will be your best friend. Once you find the answer you'll wonder how you looked over it so much. If you have had the amp for a while put it away go watch a game quit thinking about it then go back fresh eyes.

nosaj

[lo-watt]

Bias caps not in the dog house.

But that is why we scope the supplies. Ripple is ripple, whether it makes sense to be there or not.

Is it 60Hz or 120Hz? Be sure.

On this one they are in the doghouse. This is the one with the rectifier diodes to the right of the powertransformer

[66 Kicks]

Bias caps not in the dog house.

Wow.

[nickb]

How loud a hum are we talking about? With all the preamp tubes still out report the ACV across the speaker. Check the frequency while you are at it.

[HaroldBrooks]

This amp has the hum balance pot tied into the heaters at the V1. No resistors

I am a novice, but is the hum balance pot tied to an elevated ground ? If that connection is bad, you won't get the same degree of cancellation.

But take what I said with a grain of salt if it was never tied to ground in the first place. Unlike others here, I am still taking shots in the dark...