Hey everyone, I'v'e got a problem (Warning! Hi fi Content)

[FE911]

So, I've got a pretty high end version of the dynaco st-70. A lot of hiss/buzz coming from it. Normally, I tear things apart and have at it. This time I think I'll ask for a little help. Too nice to screw it up! It's a sonic frontiers sfs-40. It uses 4- 6dj8 tubes with a quad of el34's. I have no way of getting a schematic, and so I'm just hoping some one might have a good game plan for me.
First thing I noticed was the dc supply to the filaments was a little off. I'm getting 2.1vdc, and 4.3vdc. Usually I get roughly 3.1 on each. Never had that before. The electrolytics are huge 1700uf/300v x 2 in series. When I turn all the bias pots off, there is no hum. When I increase any and all, the buzz cranks up evenly.

[Structo]

Those amps are very sweet!

Not sure if you have the manual.

http://archive.anthemav.com/OldSitev1/pdf/sfs40.pdf

Have you tried new power tubes.

I have had an amp hum pretty bad when the power tubes are out of balance or one is bad.

Check all the tubes and inspect the screen resistors.

How old is that amp?

[dynaman]

Dang, I've owned over a dozen various ST70s and that's a far cry from a Dynaco! Looks nice...and expensive! Have you tried posting at Audio Asylum? Pretty hard to stump them fellers....

[Bob-I]

Have you tried a new quad of EL-34's? I once chased a hum for hours on an amp I just finished only to find it was a bad power tube. In your case it could also be one of the PI tubes since when you shut off the power tubes with the bias pot the hum is gone.

I'd also look over that DC filament supply. If the voltages are off there could be more problems with the supply causing some noise. Track it down and see if you can locate the regulators and filters and you can check for the source of the problem.

[FE911]

Thanks for the help. First thing I did when I got home was try a cheater plug that disconnected the ground pin. No hum whatsoever. Sounded really good compared to my st-70 that it is replacing. Just doesn't have the gz34 rectifier, so it was a lot tighter in the low end. Wonder if I should use the cheater plug, or try and track down the ground loop issue. I've been told to leave it the way it is.
I tried a few sets of el34's, and they all worked the same The bias wanders a little bit on the two front tube, not more than 4 or 5 millivolts. It's nice to have the front meter mounted to it, might try it on my next build. As far as the PI tubes, they are new russians, and I don't have any spares to try them against. Might have to pick up some more, but NOS telefunkens are getting mighty pricey!

[Structo]

Hmmm, I wonder why it would develop a ground loop?
Did you build this amp?

I assume that this amp worked fine for quite a while before this?

[dynaman]

Well, it's common practice in a hifi setup to have only one ground in a setup. I never used three prong cords in my hifi amps, only for my preamp. Everything else was two prong. My system was absolutely dead quiet. I only gut hum when I tried using overpriced fancy three prong connectors on multiple units.