Twin Reverb 65 RI - Reverb Hum

[pickles_man]

I've searched this forum for an answer, but couldn't find one. Here's the deal:

When I turn the reverb up, I get 120 Hz (first harmonic of 60 Hz) hum. It is independent of the main volume, ie it always stays at the same level.

The reverb otherwise sounds great. I've checked that both RCA cables are fine, with a multimeter. When I physically remove the tank but keep it connected, I can get the hum to change and get slightly better depending on the orientation of the tank. This leads me to believe that it's acting as an antenna. Are there any options for better shielding of the tank? (also I noticed that only the output jack's ground connects to the actual tank chassis).

To complicate (or maybe clarify ) things, I'm pretty sure the grounding in my appartment is bad to non-existant. My roommate has a power-bar with a ground tester, and it does not light up when plugged into my outlet. Also when I used this very same amp at a friend's place, there was no hum in the reverb. This could be due to the fact that it's a less electrically noisy environment, or due to the fact that the chassis is actually grounded.

So in summary, do I have any options here? can I manually ground the chassis by running a wire to a water pipe? Should it still be hum-free even without grounding? I've never used an old amp without the ground on the plug, but did/do they hum all the time?

could it be a filter cap or other component not doing its job? basically I'm looking for some approaches to take to try and reduce the problem. Thanks!

[Enzo]

If it were a filter cap, it wouldn't care where you took the amp.

If pulling the pan out of the cab reduces this hum, they try turning the pan 180 degrees. You want the end marked OUTPUT to be as far from the power transformer as possible.

Since it matters where you are, then the environment around it is involved. Certainly non-grounded mains power makes a diffference, but so do things like flourescent lighting in hte room.

[pickles_man]

thanks Enzo. Yeah re: filter cap, that makes sense. So my problem is definitely grounding/sheilding related. I didn't mention it before, but my pilot light also fades sometimes, and flickers on and off every now and then. I also get hissy crackling, which I suspect is due to some of the preamp tubes needing a change. Other than that, the thing sounds great

[Tom Phillips]

Originally Posted by pickles_man

…also I noticed that only the output jack's ground connects to the actual tank chassis.

That’s normal by design.


Does the reverb circuit hum reduce if you unplug the foot switch assembly completely from the amp? If so, you could be a problem with the shielding in the reverb lead of the footswitch and the footswitch wire is acting as an antenna.

Regards,
Tom

[pickles_man]

Originally Posted by Tom Phillips

That’s normal by design.


Does the reverb circuit hum reduce if you unplug the foot switch assembly completely from the amp? If so, you could be a problem with the shielding in the reverb lead of the footswitch and the footswitch wire is acting as an antenna.

Regards,
Tom

Yeah i figured it was normal.

As for the footswitch, I leave it unplugged, as I'm not currently using the tremolo.

I'm not sure what to do at this point. For any recording I do, I'll just keep the direct signal, and then re-amp it at an electrically quieter location. I may look more into the grounding situation in my appartment as well.