Ampeg SVT Bias...


Hi y'all:

Gotta quick Ampeg Bias problem. The amp is within spec with no load.
.072 v on the power tubes and balances fine. I changed the bias sense resistors to 10 ohms.

So it is all happy until you run your test signal through it and it the bias goes to hell with 1/2 the output tubes glowing read. I"ve tried fooling with the bias resistor values but it seemed to make it worse.

I run out of room on the Balance pot (VR3) under load, to keep 1/2 the tubes from red plating. No melt down, but it wouldn't be be longer.

Yes, I have Tube Amp Talk etc, and Groove Tube, Tube Amp book and read Ken Fischers notes. It doesn't seem to help. Swapping tubes from
side to side doesn't help either.

I've tried putting a 1000 ohm resistor in VR3 also but it didn't help either.

The rest of the amp is in spec until this last step.

Ideas anyone?


A couple of things:

Never run a tube amp with no load.

Having it come up to .072v on the tubes doesn't mean much with no load hooked up. These amps are a PITA because everything will seam fine, but it just won't bias correctly.

Changing to different values for the bias usually doesn't work. These amps are set to run very specifically for the bias, and changing things around usually just makes things worse. Get everything within spec for the bias (and the rest of the power supply, and power amp section. Check EACH part). Also, make sure you are running a good matched sextet of tubes. If one is off it screws everything up.

Jerry Glass

When you say "no load", I assume you mean with the speaker attached properly but with no signal applied; this would be your quiescent current and .072 volts is the correct setting.

I am also further assuming that when you apply a signal to this same condition, you lose control over one side of the amp.

Also, make sure you are running a good matched sextet of tubes. If one is off it screws everything up.
This is what you need to look at first; if one of the output tubes has an excessive grid current issue, it will drag down the bias voltage on the other two tubes on that side of the power amp.



Bias fine with no signal; my bad it has a load.

Also the amp is in spec during idle conditions. Then when a signal applied,
the one side will not balance up. All the componant parts test and
measure out fine. I also replaced the plate load resistors with new
600 ohm instead of 22 ohm. New diodes 1n4007s were installed along with them. Everything reads to spec.

Then with load, and signal, trying to balance the two sides
to minimize distortion, it will not balance. One side of the amp
won't bias up and starts to red plate.

That side is with the driver tube V2, the cap side of the amp.

The tubes test out fine, and when swapped out from side to side
the V2 side is the side that redplates. It consistently starts with
the middle power tube, of that triad. then the tube to either side starts reddening.

As you can imagine I don't have a lot of time when that starts happening
to shut the amp down.

Drivers check out fine too. I did install a larger resistor (1000 ohm) to the other side of the VR3 and didnt seem to help.

I'm just thinking about it and open to ideas.


Go back to the stock parts. Fix amps by replacing the broke and out of spec stuff, not modding the parts that aren't broke.

Running it without the power tubes in place, see that the bias supply remains stable on both sides when running signal and that the problem side driver is not passing HF oscillations on to the power tubes. IOW measure DC volts on the grids and scope for oscillations under operating conditions.

Those two are pretty much the sources for that sort of problem. We'll leave out the possibility of a problem with the OPT, fairly rare, difficult to accurately diagnose and bloody expensive, so we go there last.


I would also suggest putting the amp back to stock and get it working before doing any mods. Not sure what version of the SVT you have but you might want to check all the 5 ohm resistors on the power board. If these open up it can cause redplating of the tubes when they are pushed . They are a PITA to get to but they do fail a lot.

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