Fuzz pedal (blocking distortion) makes one EL34 shows early signs of red plating in 50 watt

[Helmholtz]

Will it drop voltage down stream the entire line?

Yes.

Some schematics show 2x 10k dropping resistors to the PI B+, some do not. And I saw at least 1 that were 2x 8.2k.
This amp build utilises one single 10k resistor in this area.

My JTM 50 has a single 8.2k. My '68 Super Lead 100 has 8.2k in series with 10k.

I think they increased dropping resistance when they switched from tube to SS rectification.
As the difference between 18.2k and 20k is insignificant, they might just have used up left over 8.2k resistors before they used 2x10k from 1970.
Some later master vol. models used a single 10k.
But I'm sure there are variations.

[pdf64]

So I have one, final, question related to this topic.

Some schematics show 2x 10k dropping resistors to the PI B+, some do not. And I saw at least 1 that were 2x 8.2k.

This amp build utilises one single 10k resistor in this area.

Any thoughts on that? Will it drop voltage down stream the entire line or just the PI supply? ...

A higher resistance in the HT dropper to the LTP V3 will also reduce the HT voltage to V2 and V1.
It will reduce the max available Vac signal swing from the LTP, which may move things in the right direction a bit.

I prefer the idea in post #63

wiring a 560R (>3W) in series with the existing choke

which, as well as the above, will (by reducing g2 voltage a little) compress the power tubes' gain at peak signals and also reduce the max plate current a bit; so 3 small hits for 1 small change.
Somewhat along the lines of a Trainwreck Express, which uses a 1k HT dropper to the g2 node to sag its HT and results in the LTP clipping earlier than the EL34.

[Helmholtz]

Here are my vastest voltages... in detail...

Your tube voltages all look rather high compared to Marshall charts. But you seem to like the amp as it is, so..

V2 plate voltages seem to be interchanged. Pin 6 (no plate resistor) voltage should be higher than at pin 1.

[Gtr0]

Your tube voltages all look rather high compared to Marshall charts. But you seem to like the amp as it is, so..

V2 plate voltages seem to be interchanged. Pin 6 (no plate resistor) voltage should be higher than at pin 1.

Yes, I seemed to have gotten those two backwards translating from notes to a reply. So Pin 1 would read the lower voltage because it has a 100k between it and pin 6. Sorry about that.

I've never thought to look if single resistor versions were powered by a tube rectifier (makes sense since there is some voltage drop by the tube itself) - I will have to look, because as an example, Marshall's initial reissue of the 1987x (in the '90's) shows a single 10k and the link, where as their revision as of the early 2000's shows 2x 10k.

[Gtr0]

A higher resistance in the HT dropper to the LTP V3 will also reduce the HT voltage to V2 and V1.
It will reduce the max available Vac signal swing from the LTP, which may move things in the right direction a bit.

I prefer the idea in post #63 which, as well as the above, will (by reducing g2 voltage a little) compress the power tubes' gain at peak signals and also reduce the max plate current a bit; so 3 small hits for 1 small change.
Somewhat along the lines of a Trainwreck Express, which uses HT sag at the g2 node to clip the LTP earlier than the EL34.

I figured it would drop the remaining nodes as well, but I figured I would ask so I can have the definitive answer :-) Sometimes things seem to go against my logic.

As for when, or under which circumstances, 2x vs 1x 10k or 8k2 were used, there really seems to be little rhyme or reason... looking at amp archive pics, I find little evidence to find a sure fire way to know looking at "transition years" - like '67 or '68. They sure seemed to have a knack to put those HT diodes in the most dangerous places... having off of the PT in many cases.

But looking at amps from '66, for sure when there was a tube rectifier, the majority seems to show there is one single resistor. Then at '69 and '70, there seems to always be 2 (whether they are 2x8k2 or 2x10k or even 1x8k2 and 1x10k) resistors in line here, obviously with ss rectifiers.

This applies to 50 watt only as I think many 100 watts come with the 2x dropping resistors here.

Sorry if you guys already knew this, I am just discovering the correlation for the first time - and I look at A LOT of vintage marshall photos.

I do have a high watt 470Ω resistor for a circuit that wouldn't have a choke (a la 5150 type amps), but I think I will first try adding another in line to see how that effects the overall "behavior" of the amp. At least this last part of the thread leads me to be curious enough to pull it out of the head case and fire up the iron.

Thanks everyone!

[Gtr0]

In the interest of being thorough I will end this thread with my final results and thoughts.

First, thank you ALL for helping me with this. I have not tested the amp using the fuzz, and so as long as it does not red plate without it, I am happy to consider it fixed. But I do have a few small lingering problems that I may or may not open threads for in the near future. So, thank you guys again!

Latest changes:
It was always my assumption that when Marshall used two inline 10kΩ resistors for dropping voltage to the PI supply, they did so (instead of using a single 20kΩ) for cost - they already used two other 10kΩ in the HT supply and coupled with the fact that some amps has a single... well, I think they were just trying to keep it simple and cost effective - of course I could be wrong.

But because of that, and what I had available in my little workshop, I decided to replace the single 10kΩ with a single 18kΩ/10 watt mini stick of dynamite. I didn't have quite the room to place two of the brick 10kΩ resistors in place that I was using. As everyone said, the voltages did drop, here are the results (took major points only)... all VDC

HT NODES:
HT1 (OP Tx CT) - 472
HT2 Screens - 471
HT3 PI - 342
HT4 v2 - 295
HT5 v1 - 276

OUTPUT TUBES:
both tubes were close enough to add it once...
pin 3 - 471
pin 4 - 469
pin 5 - -42.3

v3 (PI):
pin 1 - 229.5
pin 2 - 22.3
pin 3/8 - 36.3
pin 6 - 221.5
pin 7 - 23.4

v2 (PRE/EQ/CF):
pin 1 - 177
pin 6 - 294
pin 8 - 178

v1 (PRE):
pin 1 - 162
pin 6 - 203

The power at my home is all over the place. I have about a dozen amps and it's always the same in every one... one minute the B+ is 500, the next it is 494. It seems to fluctuate a lot depending on the time of day. Anyway, I feel it is pretty stable and sounds good.

[Helmholtz]

If your PT has a 240V tap, you could use this to reduce overall voltages. That's what I do - especially when I see high heater voltage.
It should have more effect regarding redplating than just lowering preamp and PI voltages.