Biasing JCM 2000 DSL 100

alexmansman

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I have a 2009 JCM 2000 DSL 100 that a few of you might remember went down during tour a couple of weeks ago. The HT fuse was blown and is now replaced. No tubes have been swapped and it seems to be working fine. I do have a brand new quad of JJ EL34s to put in, but may hold off if the old tubes still seem fine, though it does make me question what could have blown the fuse since it seems the most common cause is a power tube failure.

Anyway, since all this happened I've been researching a few maintenance tips for these amps, one of them being biasing. The official Marshall value appears to be 90mV on each side, but I've read additional opinions that say the amp is more comfortable between 70-80mV and one source even got as specific as saying 72mV is somewhat of magic number here.

What say you experts? What are the benefits/trade offs between running lower than the factory 90? Is 70 really a good idea? What's the difference in sound? I've read conjecture that claims a lower bias gives the amp an overall more vintage feel, but I'm not sure I'm buying that one. Assume I know nothing (which is basically the truth) and please give me all of your best advice and experiences with setting bias on these amps, it would be much appreciated! Thank you and happy thanksgiving!
 

Jon Snell

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A superlead works best at 70mA per pair as it isn't a Fender.
At 70mA (35mA per valve) will reduce the likelyhood of premature failing EL34s.
For rhythm use I would set the heat a little higher at 80mA.
It really depends on what sort of sound you like.
 

Tatzmann

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I have Electro Harmonix EL34 in mine and there
is a very noticable difference in tone and feel
that starts at around 80mV(mA).

I have it set around 82. Higher than that there is not much more to gain.

The typical AC/DC thump gets bigger and the highs get more pleasant. It just sounds and feels more "right".
 

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