Mesa 20/20 Loud Hum Poss. Grounding Issue

[Thomas Sandstrom]

Hope to get some input from other 20/20 owners. I had purchased the 20/20 power amp to go with my formula pre that I have been using for recording. As soon as I hooked it up with the Formula Pre there was significant noise coming through the speaker and it sounded like a grounding issue - tested it on other outlets and even tested the outlets with tester to confirm the outlet being used was grounded correct. Also engaged ground lift on the Formula Pre and no luck. The formula has never been noisy and I used to run it with a fifty-fifty - also with no noise issues. I then decided to remove the preamp and listen to the 20/20 power amp with only a speaker plugged in and no input. Still the same noise. The Seller claims that the noise is normal and should go away when a preamp is plugged into it - I call BS - I have never heard that - I have several power amps that do not need a preamp plugged in to solve the noise issue. I was hoping if any other 20/20 users have had such an experience with noise on a 20/20 or can confirm a preamp does not eliminate hum in a power amp. (NOTE - IT IS NOT THE FAN - I HEAR THE FAN - YES IT IS KIND OF A NOISY FAN IN THE 20/20 - BUT THIS ISSUE I AM DEALING WITH IS NOT THE FAN)

 

[Nandrewjackson]

If that's an early one, it's pushing 30 years. Electrolytic cap failure?

 

[gtrnstuff]

There's also a ground switch on the 20.20. Try that, too. On mine it was a mini toggle in the tube cavity, IIRC.
If it was lifted, plugging in a grounded preamp might have quieted the hum, as your seller says.
As posted above, I wouldn't rule out electrolytic caps, if they are the originals.

 

[austin sonar guru]

If that's an early one, it's pushing 30 years. Electrolytic cap failure?

This. Original owner of 20/20 and Blue Angel. Sprauge filter caps from that period are some of the best made - however 20 plus years is too long.

You might pass the noise through a spectrum analyzer to see if you have a peak at 120 Hz. You should also visually inspect them by taking the top cover off (care with high voltage stored in those capacitors required). Many web resources on discharging capacitors in guitar amps and how to inspect for leaking caps safely

In any event you should have them all replaced - if they aren’t already leaking - the are bound to leak in the near future. Don’t want your pcb’s covered with failed capacitor gunk.

I’m replacing my Blue Angels now.