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Re: Marshall bias voltage (?)

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9/15/1999 3:30 AM

Re: Marshall bias voltage (?)

Liam, thanks for the tips. I've been corresponding with Speed a little bit about some of this. I'd be curious about what Trace might have to say, since I believe he also has metal panel, PCB built 1987 amps. Yes, Trace?

As for mine, it also has the 1k/5w grid resistors from the factory and that makes me wonder if this amp originally had 6550's. Then again, the 82k bias splitters are a strange pair. They, too, look to be original, but the schem's I've seen call out for 220k or 150k.

The bias circuit has a 220k before the diode, then the 15k, then 47k. I don't know what else I can tell you about it. If you don't mind digging up more info after you're settled in I would appreciate it.

Thanks,

-Dave

9/16/1999 3:48 AM

quote:
"Liam, thanks for the tips. I've been corresponding with Speed a little bit about some of this. I'd be curious about what Trace might have to say, since I believe he also has metal panel, PCB built 1987 amps. Yes, Trace?"

Yes I do indeed have some metal faced Marshalls (ha, ha) I'm not sure if Speed's already emailed you with his suggestions or not. The schematics in Doyle's book are pretty darn accurate. There are only a few small differences in the 1987 "plexi" ciruit to the 1973 that you have.

quote:
"I have found a couple of weird values so far; i.e. 82k bias splitters, 1k catode resistor from V2A"

Underneith the resistors the board should tell you what the values should be. Most likely it will say 220K's but I personally favor the 82K's. It all depends on what tone you are going for though. With the 82K's I like to bias in and around 40ma or so. I also tend to favor NOS tubes and I havne't used the Svetlana's all that much. When I last tried them they didn't really have that "glassy top end" but that was a few years back.

quote:
"As for mine, it also has the 1k/5w grid resistors from the factory and that makes me wonder if this amp originally had 6550's."

Most of the amps with the polarity switch on the front panel (three switches as opposed to two on the front panel) usually came with 6550's. Just follow the schemaitc and convert the bias section to the EL34 specs. Again, for what it's worth, I'd leave the 82K's in there. They roll of the "harsher" highs, allow more lower mids to come through, and also make the preamp work a bit harder (nicer overdrive tones)

Let us know how it goes!

Trace

9/17/1999 1:19 AM

Trace, Speed and I have exchanged some e-mail. Primarily about the 220k vs. my 82k bias splitters.

As far as converting the bias section to EL34 spec's; what components are included? I'm guessing the 220k before the diode, then the 15k between the filters, the 47k, and the trim pot. Anything else, or something particular to those I mentioned? FWIW, those are the resistor values in my amp, and the EL34 schem's that I've seen. One possible noteworthy difference is that I recently replaced the filters; the new ones are 10uf/150 volt (the old ones were 10uf/160 volt).

-Dave

p.s. How do you guys break up the replies and show the replied-to text in italics? Did I miss a FAQ?

9/17/1999 3:11 AM

quote:
"p.s. How do you guys break up the replies and show the replied-to text in italics? Did I miss a FAQ?"

Just copy the text and paste it in the box. Then go the begining of the text and type this --->

quote:
"

after that you need to include the "

on the end of the text. I hav eno idea if this come out right. I think if you click reply and look at the box above it will show you how it's done.

Trace

I'll check the schematics as far as the 6550 to EL34 conversion.

9/18/1999 4:29 AM

OK, he he, let's see if *I* can make this work...

quote:
"q{"text to be quoted""

9/18/1999 4:32 AM
Carlo	That last } should be right after "the text to be qouted" I sure miss the preview function 8^)

9/18/1999 3:51 PM
Bruce	The new preview function works great now Steve (tboy)!! Thanks, I can't type for squat and I feel stupid seeing all my typos! Bruce

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