How much hum is normal in SVT?

[Randall]

I just recapped, and did a full re-tube on a 1987 SVT-HD limited Edition. Customer bought vintage NOS for the preamp and new Tung-Sol 6550 matched set for the PA. It biases up nice and balances to 0v. When I fired it up it had a monster hum, but the hum balance was almost all the way to one side, and it quieted down a lot when I adjusted it, but not all. It's really not that bad, but it's there.

So, I don't have much experience with SVT's and I'm wondering, how much hum is 'normal' for these? I have read about the preamps being hummy, but this sounds like PA or PS to me.

[Leo_Gnardo]

I'm wondering, how much hum is 'normal' for these? I have read about the preamps being hummy, but this sounds like PA or PS to me.

The inductor in the midrange circuit loves to pick up hum, whether you cut or boost. Sometimes you can get a little relief by rotating the inductor on its mounting bolt. In some extreme cases, I've added extra length to the inductor leads and moved it to somewhere else in the preamp chassis where hum pickup is minimized. The hassle is, you have to mock up the pre chassis propped atop the power amp chassis, with a couple sticks or cardboard bits holding it in the position they will be relative to each other when mounted in their box.

As mentioned in another recent thread, there's a pair of pre out jacks on the back of the power amp. Some SVT's have a pre out and power amp in jack. These should be mounted on insulating washers. Output jacks may also need insulating washers.

Finally, it's a 300ish watt amp - noise relative to such a huge amp is bound to be more than your typical 50 & 100W tube heads. So there's a point of diminishing returns, chasing the hum down to - almost - nothing.

[The Dude]

I'll add: Keeping bias as low as reasonably possible will negate a slight amount of hum also.

[Randall]

I biased per the directions on the back panel and schematic. 72mA per side, at 24mA per tube, that seems pretty low enough. I think I'll go with, that's not a lot of hum for a beast amp like this. I already have sooo much time into this one, I really don't want to re-engineer it if it came like this from the factory.

[The Dude]

That's plenty low enough. There are some that insist on the 70% rule (or higher), which will increase hum slightly.

[Leo_Gnardo]

Concur with The Dude, close enough for rock 'n roll.

Owning an SVT is like having a locomotive. Expensive to maintain but awesome when it's working. That reminds me, I gotta fix mine up & sell the dam' thing... let it be someone else's headache. oops I mean prize.

[xtian]

I've worked on three. In each case, there was some hum, but the fan was louder.

[Randall]

Yes, the fan is louder in this case.