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Discussion in 'Ampage Area' started by noelweller, Jan 9, 2009.
Why would someone tell me EL84 amps do not need to be biased?
No one on here did somewhere else.
I don't think it depends on the tube, but if it's 'fixed' bias or not (I believe this is also referred to as 'cathode' biased.
Ok Noel! The classic 30 is an AB fixed bias meaning you can't rebias the amp without changing the bias resistor. If you matchup the powertubes it can be a plug and play amp. For example I just bought a set of TAD 6L6 short bottles for my Tophat The pair tested at 28 whatever that means but that's the tube stores rating system. If I replace those tubes with the same thing and give them the rating number provided I don't have to rebias the amp.
Another thing you can do is go here
Tell him what you have and he will set you up with a matched set of JJ's for peavey specs
Chances are that amp came with Groove Tubes. All GT's are marked with a hardness rating. If you were to replace those tubes you would order the same hardness rating. I just replaced a pair of GT 5881's that had a hardness rating of 7. The bias on my amp was so far off that I would have had to rebias anyway. I'm now in great shape!
The link I have now is dead but google the Peavey forum. There's alot of info there about mods to this amp much in the vein of the BillM mods to the BJr including putting in an adjustable bias pot and who can do it for you.
fixed bias means that the cathodes of the power tubes are at ground, and the grids are fed a constant negative voltage. A fixed-bias amp can have an adjustment pot, or it can be adjusted by changing out a bias resistor.
On a cathode-biased amp the cathodes of the power tubes are connec through a resistor to ground, and the grids are NOT fed a negative voltage.
Usually cathode-bias amps do not have a bias adjustment. You just stick in a set of fairly matched tubes and you're good to go. But some guys still like to go in and mess around with the cathode resistor, just because they like to tweak...