Spring Reverb Tank noise in a Head

[stephenl]

I've been trying to come up with a way to reduce radiated EM coupling to a reverb tank in a head that I built a while back. The source of the noise is definitely the coupling to the transformer. If I move the tank about 2 - 3 feet away it completely kills the buzz.

I added a steel cover over the amp's toroid PT, which made the biggest difference but didn't attenuate it to my satisfaction. BTW, toroids really aren't as low in the radiated noise dept. as I believed. I've tried adding a steel cover to the open side of the tank and I added copper foil tape to no avail. Mu-metal is expensive and I'm not sure it's really a cure.

I love the sound of the reverb but the buzz is annoying enough that I think I may pull it out of the amp all together and make it a separate unit. I've played around with my "Hall of Fame" pedal followed by a Boss DD-6 in the effects loop...and it sounds pretty close to as good. There's just something about a really good spring reverb, maybe...

Has anyone here found a creative solution for a buzz kill?

We really do spend a lot of time chasing noise.

[xtian]

Rotate the tank on every axis. You may be able to find a position that kills the hum. Don’t fret about the spec’d orientation of the tank.

[Stevem]

The output side of the tank needs to be as far away from 50 to 60hz sources as you can make it!.
In 1968 when Fender stuffed reverb into the bandmaster and Duel showman heads they had to increase the how tall the heads where by over 3 inches just to get back to the noise floor these amps had before reverb!

Try out rapping the whole tank in several layers of Aluminum foil and then use a clip lead to ground it out.

[Orson Maxwell]

If the buzz is 50/60Hz and not 100/120Hz, the aluminium foil won't work because it would more likely be magnetic interference.

[TUBEDUDE]

Are you sure all the noise is being coupled into the coil, and not a combination of ground noise and hum from the singleended drive of the tank in combination? I've found I can shed noise by driving the tank differentially with a push pull driver instead of the common paralleled gain stages. Sometimes changing the grounding of the drive or recovery side of the tank can eliminate a loop and make it quieter, making the coupled noise less intrusive.

[stephenl]

Thanks all for the suggestions.

• The tank driver circuit is push pull, and I used a matched, low noise 12AT7. The power supply is well filtered w/ a cap multiplier.

• I've tried changing the orientation of the tank in free air next to the head, which makes some difference, but in the head shell there are limited options.

• When I move the tank a few feet away from the amp, it's almost dead quiet with everything turned up

[TUBEDUDE]

Yep, induced for sure. Mu metal may be the last option.

[stephenl]

Is lead sheet an effective shield?

[Orson Maxwell]

No, you need highly permeable magnetic material such as transformer steel.