Metropoulos Forum

Hey guys, I found me a sweet JCM 800 2204 made 6/29/83 Vertical inputs and it rocks! It has been converted over to EL34's too

But here is the question, in my research for building one of these bad boys I came up with some info from Marshall on voltages. It has a comment about bias approx -44v on pin 5 (6550 and -33v on EL34's)

So do they bias them from the factory at a set voltage based on tube type at pin 5? I am curious how the factory does it.

I did the Shunt method and the Resistor method. Depending on which site your on determines what is the BEST way. But with everyone recomends playing it and adjusting by ear and use the bias as a guideline to not get them TOO hot. What difference does it make as what method you use long as long as the plates dont glow OR burn out every few months?

Also they claim that unless you have a Fluke you cant do the shunt method as it will be off. But there again, if you are adjusting it by hear what the hell does it matter if they arent glowing or dieing? Enquiring minds wanna know! lol
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They are biasing at the factory based on the assumption that:

A) all the power trans will deliver approx the same voltage and...

B) that all the tubes will have approx the same current draw.

In reality these variables are probably pretty close. Besides they only have to hit a specific range, not an exact number.

Any of the popular methods will get you in the right range. Which, as you mention, is a starting point to adjust by ear. You want to stay within the realistic guidelines for the tube. Normally, 70% of the rated maximum plate dissapation. 17 watts for an EL-34 running in class AB1 push pull (Marshall type circuit).

You can do the math, or you can get it where it sounds right and doesn't glow. Which ever provides some piece of mind.

George

Thanks George that is pretty much how I was figuring it myself.

When they talk about the plate glowing is it the lower plate? or any of the plates?

You shouldn't see any part of the plate glowing. In my experience it usually starts in the middle of the plate and expands from there.


George

Thanks Geroge,

The plates are the verticle rectangular multi angled peices correct?