Marshall 6100 bias circuit

[Randall]

I have a 6100-63-00 issue 4 100 watt head that is running 5881s in the 60's mA with the bias pot cranked full up. PLates are at 470v. Schematic calls for -50v bias voltage, but all I can get is -41v without tubes in it. This seems to be a known issue with these. I can't find the exact schematic, but two later versions have different values for two key resistors in the bias circuit, and neither match what is in this amp.

My amp has R212=15K and R213=33K. I have -69v feeding R213. Two later versions of 6100-63-04 have R212 at 22K (iss. 4), and 33K (iss. 10), and both have R213 at 22K.

I have to pull this board to do another repair and I only want to pull it once, since it takes almost an hour to get it back in. Which values should I change these resistors to? I'm inclined to go with the later of the two. Also, would it be worth changing out those two caps while I'm in there in case one of them is pulling down voltage? Incidently, Mine does not have the zener,

https://drtube.com/schematics/marsha...04%20iss10.pdf

[g1]

Has it maybe been changed over from EL34's, or does it have the 5881 sticker on it?
Regardless, I would do as you suggest, 22K for R213, 33K for R212, replace C209 & 210. I know what you mean about that board, you don't want to have to pull it twice.
The newest values were included in the "5881 upgrade" for this model. (pg.7 & 8 of pdf linked)
https://music-electronics-forum.com/...3&d=1451760528

[Randall]

Thank you for that g1. Mine has the 5881 sticker on it, and interstingly. all the other upgrade values are in this amp except R212. Odd there is no mention of R213.

[g1]

Any chance someone changed R212 and R213 in your unit? Or have you seen a schematic with those values you found it with?

[Randall]

I don't think these resistors have been changed, they look like all the others, plus given how much soldering needs to be done at the sockets to get under these boards, I see no evidence. I have not seen a schematic with my resistor values on it.

Any chance I can get away with a 100v cap for C10 and sleep well? There is only -68v there with no tubes in it.

[drewl]

I know Marshall used 5881's there for a bit on these and some JCM900's.
I guess the impedance difference between them and EL34's doesn't matter?
I always wondered about that.

And 100v cap should be fine there.

[Randall]

Success! Turns out my R13 was actually 22K once I pulled it and could see it better, so that's why it doesn't show up in the conversion chart, it looks Like Marshall never did change that one. But boy, changing R212 from 15K to 33K really made a difference. I don't know if the caps were a problem or not, but I changed them while I was in there.

Now here is a question about the bias point. Schematic says set it to -50v, but at that setting the tubes are only at ~25mA/485v which is about 12 watts. That seems low for this amp, especially since it has been running at about 30 watts since the factory. There is a wild card here, these tubes. There is about a 10 point spread between the lowest and highest draw, with one tube rising slowly to 70mA. Plus, one had a bad grid wire connection, so I imagine it has taken quite a beating. i have a new quad of 5881 WRT on the way, so we'll see how they run, but what about this -50v bias business?

[g1]

It also says (lower left) to set bias for -42V 'for USA only'.
I would set it up how ever you usually set up Marshalls (idle dissipation). Somewhere between -42 and -50V. It will be different for each set of tubes anyway.

[Randall]

Oh, I missed that. But, then that's almost exactly where the amp was (-41v and R212=15k) and it was running so hot? And I have seen numerous threads complaining about the exact same thing, and being happy once R212 was bumped up to 33K. Oh wait, then they issued the "upgrade", so they must have realized the schematic was wrong?

I guess I'll wait to see what happens with the new quad.

[Jazz P Bass]

"Now here is a question about the bias point. Schematic says set it to -50v, but at that setting the tubes are only at ~25mA/485v which is about 12 watts"

25ma at 485 Vdc is not a bad figure.

Have you scoped it yet?

Turn down the bias until you get crossover distortion.
Use a sensible output value (1/2 power)
Turn up the bias until the distortion disappears & measure what you have.
(I'll bet it's close to 25ma.)

If you feel that you have to 'Marshall' it, goose it up to 30ma.

[Randall]

"25ma at 485 Vdc is not a bad fugure."

approx 50% max diss. for a 100 watt Marshall is a good thing? Why?

[Enzo]

Why not? A screaming PV 5150 idles in the 11ma to 20ma range. I expect about 16ma there stock. 70% is only an internet rule of thumb.

[alexradium]

if you don't get crossover distortion its fine,many people think if you dissipate just 50% and not the magic number of 70,which in fact is a theoretical maximum to avoid red plating at full power,you won't get a good sound or it sounds "cold",that's all BS,i bet 99% of them can't even detect crossover distortion in the PA in a blind test.
Other thing is russian 5881,real name 6P3S-e,are basically 23W tubes,other tubes are 25,or 30W,so any speculation about which current value is good is all over the place.
In Marshalls with half power switch,or triode/pentode,the bias point was chosen a bit "cold" to allow correct operation in triode mode,its a compromise.

[Randall]

"Why not? A screaming PV 5150 idles in the 11ma to 20ma range. I expect about 16ma there stock. 70% is only an internet rule of thumb."

Please correct me if I am wrong, as always, but my understanding regarding the 5150 is that all the gain comes from the front end, so it is intentionally biased quite cold so as the outputs can quickly track the preamp signal cleanly with no added, I dunno, smear? Is this the same case with my amp? Or am I all wet here?

And if so, why did Marshall send these out idling at 3 x times that value?

I caveat that by saying I haven't yet measured with a new quad. But given the numbers so far, still.

[Enzo]

why did Marshall send these out idling at 3 x times that value?

Because they wanted to. It is just another way to demonstrate there is no right and wrong here. There is nothing set in stone for any plan.


Power tubes aren't about the gain. But if you want to overdrive them, then running them hot gets you closer. The PV amp was not designed with overdriving the power tubes in mind. PV stated clearly the design goal was long tube life and reliable operation. Running power tubes screaming at their ragged edge is not the formula for either of those things.