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FDP Forum / Fender Amps: Vintage (before 1985) / Twin Reverb suddenly humming



Apr 27th, 2019 05:45 AM   Edit   Profile  

My Twin Reverb Silver Plate (how do I identify it so that I find the right circuit diagram spot on?) suddenly had a silent but audible humming.

Can anyone help o identify? There is a T-4 stamped on the chassis rear.
together with another stamp: 0822
Serial is F066xxx

I took out the amp chassis and put it on my workshop table, connected a speaker and the hum is loud enough to be considered as disturbing.

On the other hand, it is playing loud as normal.

I also found a white wire going from the "Bright" switch on the left channel to somewhere - assume middle of pot beside it - and is hangin in the air. Most probably broken off a cold solder junction an the pot beside it.

First off I would appreciate to find the matching circuit diagram in the net.

Help welcome and thanks in advance,




Apr 27th, 2019 07:13 AM   Edit   Profile  

Was able to find the culprit: Output tube balance pot needed some correction.

In this vein I found that the 100Hz (50Hz country) sawtooth ripple on the grid bias (70µF) capacitor 62V before the 2.7K/10W resistor was around 300mVpp at a total height of -60V DC. Is that ripple tolerable? After the 2.7K resistor there was practically no measurable ripple when looking at the scope at 20mV/div.
At most it could have been 2mV.


Contributing Member

American Patriot

About as ordinary as you can get.
Apr 27th, 2019 09:33 AM   Edit   Profile  

Glad you found it but I was thinking electrolytics at first.

Contributing Member

Planet Peegoo

Rhythm & Lewd Guitarist
Apr 27th, 2019 02:31 PM   Edit   Profile  

Outstanding Christoph!

Sounds like you know how to use a scope too. Huge plus when troubleshooting signal issues in an amp.



Apr 30th, 2019 01:25 PM   Edit   Profile  

The story isn't finished yet. Today I went back to my rehearsal room with my arm getting longer and longer while carrying the amp chassis with one hand. The twin reverb with the two E120 is a monster. The amp chassis alone is half one.

Ok, I built it in. Fired up the amp with the Rhodes connected. But a slight hum was audible again.

How could? I had adjusted it to silence at home.
Now it just took a screw driver to make it silent again. But after some minutes the him was there again.

Could zero it out again. But what if this continues to happen all the time?

Should I change the 6L6 tubes? Reseat them in their sockets?

Or might it be that (strange) pot with the stationary extra tap? Could it be deteriorated?



May 2nd, 2019 07:36 AM   Edit   Profile  

It could be the power tubes. Are they old, worn and in need of replacement? Are they a matched quad?

Contributing Member

Whitehorse Canada

I don't get out much
May 3rd, 2019 01:13 AM   Edit   Profile  

Don't condemn your power tubes.
Take the 100 ohm "hum balance" pot out of the circuit (you may have to add some heater wire to the heater circuit) and replace it with two 100 ohm resistors.
Connect one resistor to each of the tabs on the pilot light assembly, twist them together, and solder them to the case or solder them to a lug and connect the lug to the nearest power transformer bolt.
The resistors should be perfectly matched....a little above or below 100 ohms is not as important as having matched resistors.

FDP Forum / Fender Amps: Vintage (before 1985) / Twin Reverb suddenly humming

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