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Question on Single-ended EL84s in parallel

5/5/2003 6:24 PM
Question on Single-ended EL84s in parallel
Hey there. I have built me a nice little homebrew -- a copy of a VOX AC4 (but with no tone control) which uses a 12ax7 for the tremelo/vibrato, an EF86 pentode for the input stage, and *two* single ended EL84s wired up in parallel. I.e., not push-pull. I use an 125ESE output transformer, and I think it puts out about 10-12 watts.  
Anyhow, I need a little help. I have the primary impedance of the OT set to 2,500 ohms. I tried it with a 5k impedance, but I can't make up my mind. The original schematic (with one EL84) called for a primary impedance of 3k. With two tubes in parallel, shouldn't I ideally have the primary impedance be half the setting for one tube? Which setting would be most efficient? I'm confused.  
Also, I want to boost the gain of the EF86 input pentode a little bit. To do that, don't I just boost the voltage of the screen #2? Right? An couldn't I just put a 1/2 watt pot to vary that voltage?  
Any help and advice would be greatly appreciated. Thanks!
5/5/2003 6:55 PM
Bruce /Mission Amps

Ya know what? I have the real, original VOX tube amp service manuals, issued to authorized VOX repair shops in the 60's and my AC4 schem does not show the OT zed like the Domino Jennings one's I've seen on the net.  
However, looking carefully at the Domino schem, showing the OT zed, are you sure that blurry number doesn't actually read 5K?  
I can't tell from my copy.  
I can tell you a single ended El84 should preform better at 5K then 2k5.  
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5/5/2003 9:57 PM

Hmm, I think you're right about the schematic. (I'm looking at the one linked from But I still wonder if the impedance should be set to something different now that I'm using two EL84s in parallel.
5/5/2003 11:26 PM
Bruce /Mission Amps

"I still wonder if the impedance should be set to something different now that I'm using two EL84s in parallel."
Yes, about half of what you were before.  
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