1976 Marshall JMP 2203 - High B+ - too high?

[Greg_L]

Hello I hope this forum works again.

I recently acquired a 76 JMP 2203 for a steal. It was made for the Canadian market but apparently before they added all those fuses and omitted the 16 ohm tap, so it's like a "normal" Marshall JMP 2203.

Anyway, it had been sitting forever and it's pretty dusty and crusty. I recapped the entire thing, also replaced the screen resistors and dropping resistors. Repaired some hack wiring mods. I slowly powered it up through a variac and I'm getting what seems to be very high B+ at full wall voltage. I'm seeing 530 VDC at the output tube plates and screens. I know some vintage Plexis had over 500 volts on the plates. I'm not aware of mid-70s master volume units having that much plate voltage though. I'm wondering if maybe this amp is one of those oddities made with leftover parts? It does have a laydown power transformer which is sort of odd for the time period of this amp. And it appears to be the original transformer. It still has the dye on the solder joints etc. Heater voltages are pretty normal though - 6.6 VAC. I'm wondering if this is just a crazy hot power transformer on the secondaries? The PT does have a very faint buzz to it at full wall voltage.

Anyway...I'm running the original 12AX7s in it and have a very gently used set of JJ KT77s in it for now. I just want to see it make voltages where they're supposed to be. But damn these voltages are high! What do yall think?

[Chuck H]

Check current through the power tubes. A very cold bias condition could present a less loaded voltage. Which would be high.

[Helmholtz]

Does the amp have a mains voltage selector by any chance? (All Marshalls in Europe do.)

[Greg_L]

Check current through the power tubes. A very cold bias condition could present a less loaded voltage. Which would be high.

I've set the bias medium/hot...around 62-65%.

[Greg_L]

Does the amp have a mains voltage selector by any chance? (All Marshalls in Europe do.)

No. Canada runs 120 wall voltage like the US.

[Helmholtz]

What is your actual wall voltage?

[pdf64]

A long time ago I had a JMP plexi Super Lead, that had a HT up around that kinda level.

[Helmholtz]

Same with my '67 plexi 100W ST(laydown PT) - if I set the selector to 220V.
But I use the 240V setting (typical wall voltage is 228V), so B+ is about 10% lower.

[Greg_L]

What is your actual wall voltage?

It's usually around 123-124.

[Mick Bailey]

All the ones I've had for repair had a B+ around 530V. Picking durable EL34 tubes is tricky and the most reliable have been TAD when they were made by Shuguang a few years back. I'm not sure which manufacturer TAD rebrands these days, or whether the new production is as good. I've also changed these amps over to use 6550 tubes at the owners' request but they came back to revert them to EL34 after a while.

[Greg_L]

All the ones I've had for repair had a B+ around 530V. Picking durable EL34 tubes is tricky and the most reliable have been TAD when they were made by Shuguang a few years back. I'm not sure which manufacturer TAD rebrands these days, or whether the new production is as good. I've also changed these amps over to use 6550 tubes at the owners' request but they came back to revert them to EL34 after a while.

Okay well I'm just gonna keep restoring it the best I can.

The JJ KT77 data sheet says 800v on the plate and 600v on the screens are max....unless I'm reading that wrong.
https://www.jj-electronic.com/en/kt77

If that's true my B+ should be okay? I might experiment with more B+ voltage drop to the phase inverter and preamp tubes. Probably elevate the heaters too.

[Helmholtz]

What's wrong with the amp?
How does it sound?
Why not just play it instead of redesigning?

[Greg_L]

What's wrong with the amp?

It came to me completely dead. The power switch was crudded up, four of the original 1976-era filter caps were bulging and spewing their guts, a screen resistor was blown apart, a power tube was wasted, and it had incorrect fuses. It was covered in a layer of what I can only assume is Canadian snow dust? The head shell has unfortunately been hacked to accommodate a cooling fan, which is gone, but the hole is there. A friend of a friend got it in Canada as a widow was selling off the contents of her deceased husband's storage locker. But...it's a mostly original fawn tolex 76 JMP 2203. It's a little beat up but I want it. I've fixed everything that I've found so far. I'm just curious about this high B+.

How does it sound?

Don't know yet. Haven't gotten to that stage. So far I've only done the things that make it safe to power on and reliable and I'm going from there.

Why not just play it instead of redesigning?

Oh I will. I'm not redesigning anything. But I might experiment.

[Helmholtz]

The high B+ seems normal. There's no fault that would increase B+.

I would restore it to original before considering heater elevation.

A scope would be nice to check output power and signal quality.

[Greg_L]

The high B+ seems normal. There's no fault that would increase B+.

I would restore it to original before considering heater elevation.

A scope would be nice to check output power and signal quality.

Yes, agreed. I will scope it for sure. I'm about to button it up and see what kind of sound it makes and go from there.