Home Brew SVT Hum Issues - Suggestions

[jakejoseph]

I have made a homebrew SVT clone. It's a 100% clone, exact to the Ampeg schematics for a 6550 amp. Like the original, the pre amp is in a different chassis than the power amp.

The power amp section is dead quiet with nothing plugged into the input - I used a shorting jack. When I plug in the pre-amp section, it hums. The pre-amp section works great. Even if I remove the final 6C4 or short the output, I get hum. I can get rid of the hum by shorting the input jack to the power amp, but not by shorting the output of the 6C4 to ground. TO me, this means a ground loop somewhere - a differential in voltage between the pre-amp ground and the power amp ground.

I star grounded both amps, and they are connected through one ground, a 12 gauge wire that comes straight from the negative side of the power supply cap in the pre-amp and goes straight to the negative side of the power supply caps for the power amp. If I disconnect this ground wire, the hum drops to acceptable levels, but obviously(?) since this is the only ground connection, the ground on the pre amp is floating. I did not try to pass a signal with this wire disconnected.

Any tricks for reducing this hum with the two chassis?



Disconnecting the chassis ground on the pre-amp makes no difference.

Any suggestions? Should I relocate the ground wi

[Steve Conner]

If I disconnect this ground wire, the hum drops to acceptable levels, but obviously(?) since this is the only ground connection, the ground on the pre amp is floating.

Well, didn't you use a screened cable and jack to connect the preamp to the power amp? The screen of that cable is the second ground, and there's your ground loop. You never said, but I think it's likely. If the wire really were the only ground, disconnecting it would make the hum much worse, and other bad things too.

Assuming you do have that ground, it is OK to disconnect the ground wire for test purposes, but it poses a safety issue in the long term, because unplugging the jack will make the preamp chassis and your bass float up to B+. You should look for an alternative place to connect the safety ground wire that doesn't cause the ground loop.

Hints: The wiring around the power supply cap negative terminals can carry heavy ripple currents, and if the ground loop passes around the PT, a lot of hum voltage will be induced in it.

For me, the no-brainer point to relocate the ground wire would be the ring terminal of the power amp input jack. And tie it to the screened cable between preamp and power amp, to minimize the loop area. (But it may not be a good idea to tie the heater wiring into the same bundle.)

[jakejoseph]

Well, didn't you use a screened cable and jack to connect the preamp to the power amp?

Yep - but only one end of that cable is grounded. Thought about that possible loop when I designed it. The end in the power amp section is grounded. The jack going in to the pre section is a Cliff, and the ground tab isn't connected to anything.

If the wire really were the only ground, disconnecting it would make the hum much worse, and other bad things too.

I though so too. I must have another ground SOMEWHERE, but couldn't figure out how. I have 2 heater wires, 1 B+ wire, 1 ground wire and 1 signal wire connected to the power amp section, which houses the power supply. No other wires are running through it. The pre amp section is physically about 1 foot away from the power amp section. no contact between the amp sections.

Assuming you do have that ground, it is OK to disconnect the ground wire for test purposes, but it poses a safety issue in the long term, because unplugging the jack will make the preamp chassis and your bass float up to B+. You should look for an alternative place to connect the safety ground wire that doesn't cause the ground loop.

Hints: The wiring around the power supply cap negative terminals can carry heavy ripple currents, and if the ground loop passes around the PT, a lot of hum voltage will be induced in it.

OK, so move the ground point to get away from that ripple current. Gotcha.

For me, the no-brainer point to relocate the ground wire would be the ring terminal of the power amp input jack. And tie it to the screened cable between preamp and power amp, to minimize the loop area. (But it may not be a good idea to tie the heater wiring into the same bundle.)

I'll give it a shot. Thanks so much for your help.

[jakejoseph]

OK, so move the ground point to get away from that ripple current. Gotcha.

Aaaaand... It's fixed! Awesome! amp works 100% now.

Gonna assemble it and run it through its paces!

J

[Steve Conner]

Glad you got it working!

I've always wanted to build a big tube bass amp. I managed to get most of the parts including a 15lb output transformer, but I never got round to it.