Ampeg SVT

[soundguruman]

I have to revive this thread because this amp has a horrible hum after I put in new tubes. I think the old ones were so weak the hum wasn't bad but with new winged c sextet it's unbearable. It sounds like heater hum. LOUD heater hum. I've got 4.5 v ac at node A ?! I've got .2vac at node D but 1.8vac on pin 7 of the 12bh7.

Help!

Forgot to mention. The amp sounds very good now after all the work I did to it. Except for the hum of course.

You have to set all the balance and bias controls, properly. Otherwise it will hum.

[leydenjar]

Thanks SG.

I received this amp without a hum balance pot. In it's place the amp just has 2 100 ohm resistors going to ground. Do you think I need to install a hum balance pot where the old one might have been to eliminate the hum?

I have no idea how many people have been in this amp doinking about.

I have a 100 ohm pot on hand but I 'm not sure of exactly how to install it. Is it as clear as it looks with the wiper terminal going to ground and the two end terminals to one side each of the heater winding?

[leydenjar]

Thanks SG.

I received this amp without a hum balance pot. In it's place the amp just has 2 100 ohm resistors going to ground. Do you think I need to install a hum balance pot where the old one might have been to eliminate the hum?

I have no idea how many people have been in this amp doinking about.

I have a 100 ohm pot on hand but I 'm not sure of exactly how to install it. Is it as clear as it looks with the wiper terminal going to ground and the two end terminals to one side each of the heater winding?

[soundguruman]

Thanks SG.

I received this amp without a hum balance pot. In it's place the amp just has 2 100 ohm resistors going to ground. Do you think I need to install a hum balance pot where the old one might have been to eliminate the hum?

I have no idea how many people have been in this amp doinking about.

I have a 100 ohm pot on hand but I 'm not sure of exactly how to install it. Is it as clear as it looks with the wiper terminal going to ground and the two end terminals to one side each of the heater winding?

Yes it's a pot with the wiper to ground. Adjusting it will help "some."
In that amp, it may be helpful. Or, one of your 100 ohm may be opened up.
You have to verify that all the parts and resistors in the circuit are within tolerance, and the filter caps are good.
Just changing the tubes may not be enough.
However the SVT does hum, especially if the covers are removed. Especially if the volume controls are turned up, with no input connected.
The balance controls have to be calibrated correctly. everything must be just right,
The ground from the power receptacle has to be correct,
but still there will be some hum, even when everything IS correct.
The SVT is kind of squirrely, the AC heater tracks are run side by side with the audio. So, there is hum, even when it's correct.
The trick is tweaking it to minimize the hum, cause it will never be eliminated completely.

If the amp is all dinked around with, start at the output and verify everything is in tolerance.
Then work your way back...
The power supply filters are very old, and probably need replacing...there is no way you can expect it to work right unless all the parts are functional. It's a big amp, with a lot of parts, and it takes a lot of patience and time and effort AND money.
But, it's worth it. They don't make em' that good anymore.
But NO, it's not "just" the tubes. It's a lot of things...

[leydenjar]

I've rebuilt the power supply this month. No problem there. The amp sounds seriously good now. It's just the hum.

[soundguruman]

Start at the input, make sure the input normal jack is grounding when the guitar is unplugged.
besides a bunk 12DW7, besides a bunk preamp tube, the next source of hum that comes to mind is the 6C4 driver tube.
It's really hard to find a good one. They are extremely microphonic, and prone to noise.
What happens when you use the external amp inputs J1 J2, and take out V5 6C4? Do you still hear the hum?
unplug V1, V3, V4, now try the channel 2 input with just V2, V5.
Now try the channel one input with V1, V3, V4 and V5 tube installed, and V2 removed.
Do you have test equipment?

[Jazz P Bass]

A 2 w pot should do the job.

[leydenjar]

How many watts is a standard amp pot? Will this one work?

[Jazz P Bass]

I think the AB type J pots are rated at 2 watts.
Research the part # to be sure.

[Alex R]

if it hums with all the controls on zero the chances are there's an imbalance in the power amp. If an SVT isn't perfectly balances bias-wise it will hum, simply because there's so much current to balance. Sometimes you have to swap output tubes around to get it within range, then level it out with the bias pots.