hum in replacement reverb tank ...

[Boogs]

hi folks.

i bought a three-spring Accutronics reverb tank to replace the stock two-spring one on my Classic 30. i think it sounds quite a bit better, but there is a persistent hum, not present in the original tank, which increases as i turn the reverb up. at first i wondered whether the tank could benefit from shielding (try not to laugh), and tried several layers of aluminum foil (didn't want to waste copper foil tape on a shot in the dark), which caused no change or improvement.

any idea what could be causing the hum? again, it's present only on the replacement tank, and the tank's wiring seems to be fine....


thanks,
David

 

[John Phillips]

Two possibilities:

Either you have a ground loop because both jacks on the new tank are connected to the casing, and only one on the old tank was - easy to check, with a continuity meter. If it's this, you need to remove the ground from one connection (usually the send is better). Many amps do have both ends of the tank grounded with no hum though.

Or the impedance, placement or orientation of the new return transducer is different, and it's picking up hum from the power transformer. No amount of shielding will help, it's magnetic hum. If the impedance is the same, you may need to try moving the tank somewhere else - tricky, with a full-size one; that's one reason a lot of amp companies use the small tanks. Turning it round so each transducer is at the opposite end might help though.

 

[Boogs]

Mr. Phillips to the rescue once more!

thanks, i'll investigate those possibilities and post back - thanks very much.

David

 

[Boogs]

well, i tried re-orienting the tank in the case, and could only make it worse.

i took a look at the wiring on the tank, and the ground wasn't connected to the casing that i could see. i tried desoldering the ground on the Send cable, which completely switched off the reverb, then resoldered it. should i have removed the ground wire on the inside of the casing, or have i just completely mis-interpreted what you've said? i don't want to remove wires internally unless i'm sure i've understood you.

sorry, i'm a real babe-in-the-woods when it comes to this stuff.

 

[John Phillips]

The ground is connected via the RCA jacks being riveted to the casing, but on some tanks one end has an insulating spacer. On others, one cable is not grounded at both ends.

If disconnecting the ground on the send cable stopped the reverb working altogether, you definitely haven't got a ground loop. You don't need to take any wires off inside.

It must be just the orientation of the tank or the precise characteristics of the transducer. They do vary - ones designed for solid-state reverb usually have higher send impedance and lower return impedance; the C30 has solid-state reverb even though it's a tube amp. Using a tank with too-high return impedance could possibly cause excess hum.

 

[Boogs]

thanks very much for your response. i can at least know the underlying causes at work now, and possibly try another model Accutronics.