Ampeg VT-22 on the bench for repairs...hum

[TubeGeek]

Hey guys,  I have an ampeg vt-22 on the bench for repair.  I have seen this amp a few times over the past 3 years or so.  This time it's in for excessive humming. I made a short video clip of what I am dealing with.

https://youtu.be/BpbGiAaAzlg

I am going to trace out and study the power supply section first as I think the hum is coming from there.


I may need some help with this beast.


http://el34world.com/charts/Schematics/files/ampeg/AmpegVT22_V4_RevA.pdf

[jjasilli]

My V4 has developed a similar hum.  An input jack is loose which may pose aground in issue.  Otherwise I suspect filter caps which are original from the early Seventies.  (That said, always suspect tubes first!) Even though you replaced caps in 2014 there could be a bad cap or ground in the PS.  Anyway I agree with your planned approach.

[TubeGeek]

Well the problem wasn't much of a big deal.  v1 - 12ax7 had the hum.  With a new tube installed and the hum balance pot adjusted, the hum is acceptable now.

[PRR]

> hum is acceptable

These amps will never be "no hum" amps because the heater lines run on the PCB.

It was amazing how the "big hum" heard when you put your ear in "my" VT-40 really vanished a few feet away from the open-back cabinet. Don't think I'd want to hear it jacked into a large closed-back cabinet.

[jjasilli]

Well the problem wasn't much of a big deal.  v1 - 12ax7 had the hum.  With a new tube installed and the hum balance pot adjusted, the hum is acceptable now.