'72 Ampeg SVT problem child

[rf7]

I've got a vintage SVT on the bench that goes into an oscillation whenever I hook up my scope ground to the chassis when it's fully powered up. I do get a spark when I touch the scope ground to the powered on amp chassis. I measure an AC voltage offset of .024V between the scope ground and the amp chassis. My scope is ground lifted. I have installed a grounded AC cord. Any ideas?

The issue I was chasing is that the amp only puts out 225 watts. It's not the tubes, I've swapped them all out with known good ones and it made no difference. The amp has all new power tube plate resistors and screen grid resistors. I changed the screen grids to 1.5K ohm. That shouldn't cause the low power issue, right?

[Enzo]

A compromised output stage could cause low output power, but really, anything limiting the drive to those tubes can as well. Are your drive tubes OK and in the right sockets? In modern SVT-CL amps I have to replace more 12AU7s that output tubes.

[loudthud]

Did the amp originally have 22 Ohm screen resistors? I think 1.5K may be too high.

[rf7]

I've got a 69 in here that works fine and I swapped the tubes between the amps and I still have low output with the 72. So I think the tubes are not the issue. Yeah I'll go back to the 22 ohm screen grid. Most of them were pretty bad and had to be replaced. Thanks guys!

I was trying to scope inside the power amp stage, but with the scope ground oscillation issue, I couldn't really do that. I may just pull the power tubes to see how the drive tubes are working, if changing the screen grids doesn't help.

[MarkusBass]

You haven't specified which version of the amp it is (I know of at least two different versions). Do you have diodes parallel to the screen resistors? The resistors were initially 22 Ohms but later Ampeg suggested to replace them with 220 Ohms (and remove the diodes).
Regarding the ground issue - as far as I remember the power amp is grounded through mounting posts to the chassis and if there is no good contact, you may get various problems similar to the ones you have.

[Enzo]

He said it was a 1972, which predates the SLM versions, and I think the 22 versus 220 and diodes are issue from the much later SLM versions.

I suggest you cure the oscillation thing first, it may be at the heart of your low power issue, whatever causes the grounding to trigger oscillation. Have you checked for oscillation at RF?

[TimmyP1955]

I'd float the ground of the amp to see if there's any voltage(s) on the chassis. Hammond, on the old organs, tests with a 2W 10K ohm resistor in parallel with a 0.01uF capacitor, connected between the (floated) chassis and earth ground. If there is more than 4 VAC across the resistor, they consider there to be a safety hazard.

[rf7]

I put in 22 ohm screen grid resistors, and am now getting full power. So the 1.5K was definitely too high.

With the chassis floated, I measure 24Vac between the chassis and earth ground. That is not so good, I guess. But with the 69 SVT, I measure 42Vac between the floated chassis and earth ground. And the 69 does not oscillate. I'm confused.

The oscillation looks to be a 60Hz square wave.

[soundmasterg]

Does the 69 use 6146 tubes or 6550's? I've found that the 6146 SVT's make more power and have more of a mids focused tone.

Greg

[g1]

Have a look at both amps with regard to insulation/grounding of jacks to chassis.

[rf7]

Both amps have the speaker jacks floated, and the inputs along with the loop jacks are grounded to the chassis.

[g1]

The grounding to the preamp is all good? I've seen issues with that umbilical as far as the connection between the 2 chassis.

[rf7]

The 1.5K screen grids were causing the low power issue. I went back to 22 ohm screens and got it back up to where it belongs.

[g1]

What about the oscillation and sparks?