Do Marshall amps need ....

[DesertRose]

Re: Do Marshall amps need ....

hey man .. thanks for the info.
My JCM900 runs on 6L6EH .. i'm intending to replace them with the original 5881s. So i would need info

As for the JCM800, its EL34s .. so no issue with that. Your point are really relevent. thanks

[ErikH]

Re: Do Marshall amps need ....

When my tech biased my amp last year with new tubes, he used a scope. Signal comes out the scope to the amp's input then out the speaker out and back in to the scope (I think that's right). I got to watch him adjust it. It was pretty neat. The sine wave was actually pretty good from the start so there was minimal adjustment. He adjusted at low volume and then gradually increased it and readjusted until things were steady. He can bias my amp that way all the time. It sounds incredible.

[Kommerzbassist]

Re: Do Marshall amps need ....

Marshall DSL/TSL's have the bias points on the back panel, so you only need a voltmeter.....you don't even have to remove the chassis from the headbox.

Uhm... I'm not so shure if that's correct... there IS a little whatever it is, but it's acover and doesn't seem to be meant to be taken off... dunno... you can pull the cover out a bit, but then it stops.... where is that spot supposed to be excactly?

[Lake Placid Blues]

Re: Do Marshall amps need ....

hey man .. thanks for the info.
My JCM900 runs on 6L6EH .. i'm intending to replace them with the original 5881s. So i would need info

As for the JCM800, its EL34s .. so no issue with that. Your point are really relevent. thanks

I don't know why I wrote "1 watt" resistor between cathode and bias. It should read a "1-ohm" resistor. This resistor needs to be a 3-5 watt presision resistor. What this resistor does is allow you to equate voltage to current, when using your multimeter. This way, you don't have to have current running (The circut) through your multimeter, and a more expensive multimeter that reads millamps. You can just read millivolts across the 1-ohm resistor instead.

If your not experianced with these things, then by all means find some one whom is, and have them show you how, and set it all up, and all the needed safty precautions. It's not really difficult, but these are high voltage machines.

Both 6L6 and 5881 have a plate dissapation of 20-22 watts. Even though they have the same rating, no two matched pairs behave, or bias, exactly the same way.

Some plate dissapation numbers for popular output tubes:

6L6 early glass case and metal case models from the 50's-20 watts
6L6-22 watts
5881-23 watts
KT66-25 watts (28 absolute max)

EL34-25 watts
6CA7-25 watts
KT77-25 watts (31 absolute max)
E34L-25 watts

6550-28 watts
KT88-35 watts (42 absolute max)

6V6-12 watts

[Lake Placid Blues]

Re: Do Marshall amps need ....

Uhm... I'm not so shure if that's correct... there IS a little whatever it is, but it's acover and doesn't seem to be meant to be taken off... dunno... you can pull the cover out a bit, but then it stops.... where is that spot supposed to be excactly?

It is essentially correct, and I have confirmed this in the past. However, on the JCM2000 amps, the ma number that should read on the meter, is actually about twice that of normal, because of the way the leads are wired up. For example, instead of about 40ma, it will be 80-90 ma on your meter, IIRC.

[DesertRose]

Re: Do Marshall amps need ....

then there may a risk of miscalculation ?

[Lake Placid Blues]

Re: Do Marshall amps need ....

They give exact numbers in the official JCM2000 service info. This probably gets you very close, but you could still fine tune to personal prefrence.

To get an "exact" calculation for each amp you must know that particular amps plate voltage. The plate voltage does vary some. It varies with wall votage, and it varies slightly with the bias setting. A cold bias gives higher plate voltage, and a hot bias lesser plate voltage. Once you get the bias in the ball park, it's a good idea to recalculate.