Gibson GA-20 Hum problem

[Sonny ReVerb]

I tried moving the filement ct to ground yesterday as I was suprized that they would have it on the cathode, but it didn't seem to help any.

Referencing the filaments to a positive DC source, instead of ground, is a common way of reducing hum. In a cathode biased amp, the point between the cathode and the cathode resistor provides a good source of DC for this purpose.

http://www.freewebs.com/valvewizard/heater.html

[stompjunkie]

Sorry for being away for so long. Work got in the way of amp repair....

Phil_s ...The hum is 120 Hz. How might that narrow things down?

I have tried three rectifiers, with no luck....The ground is not a floating one (I second the yuck) so that isn't a worry. I also tried battery powering the filaments with no luck.

My camera isn't currently working. I'll see if I can borrow one so you can all have a look. I'll also try to get some voltage readings asap.

Thanks again for the help and for putting up with my gone for a week responses....

[stompjunkie]

Ok some quick voltage readings...

6v6 no 1.

pin 3 plate 321.3v
pin 4 grid no2. 249.1v
pin 5 grid no1. .009v


6v6 no2.

pin 3 plate 326v
pin 4 grid no2. 248.97v
pin 5 grid no1. .4v


Why would pin 5 give a positive reading? Could that be the clue to the big hum issue?

[jjman]

Looks like the “normal” model references the heaters to the cathode of the output tubes. The Tremolo model has no reference for the heaters (floats.) The Reverb model uses 2 100ohm resistors (artificial CT.)

http://www.schematicheaven.com/gibsonamps/ga20.pdf
http://www.schematicheaven.com/gibsonamps/ga20t.pdf
http://www.schematicheaven.com/gibsonamps/ga20rvt.pdf

120hz hum means it's related to the power supply and not the heaters. Filters would be a common target as well as grounding as already mentioned. If one of the OP tubes wasn't running, for whatever reason, you would hear 120hz PS hum that would normally cancel in the OP tranny.

Negative side of the speaker output is properly grounded? I've read that can sometimes be an issue.

[Phil_S]

Ok some quick voltage readings...
<snip readings for brevity>
Why would pin 5 give a positive reading? Could that be the clue to the big hum issue?

Could be your meter. Anyway, this is close enough to zero that I wouldn't focus on it. This amp is cathode biased. What is the cathode reading?

How about preamp tube readings?

If it's 120Hz, then it is not your heaters, it is something about the B+ rail or filtering of the DC on the B+ rail. Try a chopstick or other non conductive (not a pencil) and use it to probe and move signal and/or B+ wires on an open live chassis. You might solve this with a bit of lead dressing/relocation.

[stompjunkie]

I finally figured it out.... nearly drove me mad.....

The new CE filter caps that I used were measuring out fine, and I quadruple checked them too, but one had a short in the lead right by the base of the cap. Apparently it was in the right position to short when it was mounted in the amp .....

A quick swap with another cap and I was back in business....

Thank you all again for all of the help! I wish I could have gotten back sooner, but work has been keeping me really busy.

I guess this goes to show that sometimes strange things happen even when you check and check and check....

[LOUDthud]

Don't know how I missed that you found the problem.

[Phil_S]

Yup, 120Hz = filter cap problem. Good job fixing it!