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Optimum B+ for EL84s

3/31/1999 12:36 PM
Lee MacMillan
Optimum B+ for EL84s
I will soon (maybe eventually is more accurate) be modding my Traynor YGM-2 to reduce the 440 B+ I now have (via diode rectifier). I may convert to a tube rectifier (I'll have to add a filament transformer) or I could replace the power tranny with a lower output version and keep the diodes.  
Have those of you who have built or modded EL84 power amps found an optimum B+? I see the Matchless Spitfire uses 348 v. and the AX84 November uses 300 v. I traded emails with a guy recently who has built his own (and thinks it's good enough to sell commercially) who would only tell me that EL84s sound better with lower voltage (without defining what lower meant.)  
Any thoughts/comments/suggestions? Thanks.  
3/31/1999 2:27 PM
Benjamin Fargen

I would have to agree with your anonymous friend. The Spitfire clone I built had a B+ of close to 400vdc......way to much for my taste. I am in the process of rebuilding it right now. I think 315-350vdc is what I will be shooting for on this one. I think the lower voltage brings out that nice VOX EL-84 breakup that we all know and love. I think the Spitfire design is cool but its way to stiff for my taste.......I'm looking for the earlier breakup type of sound. Lowering the 12Ax7 supply voltages and lowering the B+ will be just the ticket.  
4/1/1999 6:27 AM

Ken Lewellen's Lark (schematic at this site) kept his under 350V, and his sounds great.  
4/1/1999 9:40 AM
Mark Buckingham

My mesa boogie dc-2 puts 395vdc on the plates, 311 vdc on the screens of its pair of el84's.
4/1/1999 12:09 PM
Lee MacMillan

I think they have to be up near 400v to claim 20 watts. The Pro Jr schematic says 319v. and it sounds good.
4/1/1999 7:36 PM

I like 320V, and honestly can't think of a good reason to go above it.. it sounds great, and is well within the specs of the tube.  
Good enough for an AC30, good enough for me.  
4/1/1999 11:22 PM
Steve A.

Re: cathode bias vs. fixed bias?
    Just wondering if the "optimum" B+ voltage would depend on whether the amp was using cathode bias or fixed bias?  
    I understand that with cathode bias you would subtract the voltage drop across the cathode resistor to come up with the actual plate-to-cathode voltage (which would be lower than the B+ voltage measured to ground).  
Steve Ahola

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