Marshall 1959rr no play with Fryette power station

[tdlunsfo]

This is the 4th time this amps been on my bench. Never could get it to fail before, but it keeps coming back with blown fuses. Finally we put a new quad in the amp and it immediately blew a tube (tube cracked and went to air). Customer stated he runs it through a Fryette PS2 power station so I had him bring it over. Today I am doing some testing with my dummy load and noticed that this power station issentially "absorbs" your amps output then takes a line level and reamplifies it so you can control the output. So, I have the Marshall about halfway up and all of sudden 2 of screens resistors open. Odd. I replaced them and checked the screen voltage drops to be 78v while on the power station and 36v with it bypassed. Obviously they opened because they were pulling 7.5W... but why? Anyone ever deal with this? I guess I'm not smart enough to wrap my head around this one. I'm reaching out to Fryette for a response, but in the meantime I thought I'd check here to see if I'm missing something obvious.

[Helmholtz]

Not sure what a 1959rr is.
As the Fryette PS acts as a reactive dummy load, there's a possibility of HF oscillation.
Do you have a scope?

[tdlunsfo]

It's a 100w EL34 Marshall with no master volume. I do have a scope. In fact, I had it up while on the dummy load but I wasnt looking at it when the resistors failed.

[The Dude]

I'd pull the output tubes and measure bias, let it get good and warm and measure again. Possibly, you are losing your bias voltage causing output tube over-conduction.

[Helmholtz]

Any signs of HF oscillation?

[tdlunsfo]

While playing I did get some slight noise on the waveform but it never went into full blown oscillation. I was actually measuring bias current/voltage, B+, screen while testing on the dummy load. Amp was at 65W with a 500hz sine wave, here are the numbers:

B+: 398
Screen; 311
Bias Current: 4mA
Bias Voltage: -45

Same amp settings but Power Station bypassed:

B+: 356
Screen; 326
Bias Current: 92mA
Bias Voltage: -42

4mA is not a typo above. I checked muliple tubes and same. I chalked it up to something the power station was doing.

[tdlunsfo]

I remember looking at bias voltage when the screen resistors opened and recall a -36v reading. I didn't write it down, so I may be off a little.

[Helmholtz]

DC voltage and current measurements should always be done without signal.

Best way to test for instability is a square wave signal.

What do you mean with "Power Station bypassed"?

[tdlunsfo]

I'm aware on the bias. I just biased this amp a month prior with new tubes and it was the same as I left it minus the bad tube. My test setup above was to catch what was going on with the amp under load and on the power station as that is the only time it fails. The power station has a bypass switch that removes it from circuit. I don't have a schematic for the power station, but I assume it's a true bypass around the circuitry.

[The Dude]

For clarity: Are you talking about the Power Station, which is an amp, or a Power Load, which is a load box? At least, that's what I'm seeing on their site.

Edit: Oh never mind. I see the amp also acts as a load box.

[tdlunsfo]

It's the Power Station PS-2.

[Helmholtz]

The PS-2 connects to the speaker output of the amp. It consists of a reactive dummy load and a power amp which amplifies a sample of the attenuated amp's output signal.
This way the input impedance stays constant independent of the output level.

[pdf64]

So what load does the amp get when the Power Station is switched to bypass?

[pdf64]

While playing I did get some slight noise on the waveform but it never went into full blown oscillation. I was actually measuring bias current/ voltage, B+, screen while testing on the dummy load, here are the numbers:

B+: 398
Screen; 311
Bias Current: 4mA
Bias Voltage: -45

Same amp settings but Power Station bypassed:

B+: 356
Screen; 326
Bias Current: 92mA
Bias Voltage: -42

4mA is not a typo above. I checked muliple tubes and same. I chalked it up to something the power station was doing.

Does 'bias current' mean anode or cathode current, at idle or with audio signal?
Are the number per valve or for more than one valve?

[tdlunsfo]

So what load does the amp get when the Power Station is switched to bypass?

This would be a 16ohm resistor. Bias Current is one of those inline socket adapters that puts your ammeter between the plate and B+. So I guess "plate current" instead of "bias".