Marshall Class 5 - sadly gone.... (Page 2 of 2)

Nope its a fixed bias, for that matter just about all Class A amps to my understanding is fixed until you get into the higher end amps.  They bias them hotter then they need to be, maybe because Fender and Groove Tubes have worked out a deal.  Also another thing you need to keep in your mind when buying EL-84 or any pair of tubes is get a matched set.  This means someone tested the tubes and their output and they sold them in a matching pair.

I believe EL84s are self biasing whatever the amp.

The Cornell Plexi should be a completely different league.

Not do sure about EL84s being self biasing. Pretty sure it depends on the electrical circuit design of the amp not the tubes being used?

EL84s are not self biasing... the amps they are in that don't have bias adjustments are saving costs by not soldering them in. Vox AC tube amps, and also amps like the Blues JR don't have bias microadjusters, but can be easily installed. This is a HUGE help to those amps... you'll find without the bias mod, the tubes are running way too hot. Now, some people "like" having hot tubes but the amp cannot fully enjoy 'hot tubes' because it's past the edge of optimization, so it's not as good as you think... it can overheat the board in the amp. Plus, the tubes will not last as long as when they are biased into the optimal range.

Here's the logic behind installing a bias control on a Blues JR

Bias is a negative voltage that's applied to the grid of the tube that controls the amount of electrons--the plate current--that hit the plate. If too many hit it, the plate overheats. It can actually turn red and even melt. But long before the plates melt, excessive plate current shortens tube life, overheats your output transformer, and makes your amp sound bad. The maximum rated plate dissipation for an EL84 is 12W, so for class AB (the way the tubes interact with the output transformer) operation, it should dissipate approximately 70 percent of that, or 8.4W, at idle. The stock Blues Junior dissipates nearly 12W per tube at idle, meaning that as soon as you turn the volume up, you're exceeding the tube's design limits. I have no idea why Fender chose to set the amp up this way--they certainly know as much about tube circuits and operating parameters as anyone. If you fix their error, however, your amp will sound better and last longer. I've measured the temperature difference between a the output tubes of a stock-biased Blues Junior and one that's properly adjusted, and the difference can be as great as 100°F.

http://home.comcast.net/~machrone/bjr/bjbias.htm

- Nic from Detroit... posting on JB's Forum since 6-2-2006
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