• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL84 fixed bias PP Triode

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Well heck folks, finally got my EL84 6CG7 project to a listenable state.

(PSU is on a separate panel currently with a plastic laundry tub over it to keep pets away)

LED biasing for 6CG7 input gain stage and for concertina phase splitter.

.47uf Jantzen Superior Z caps as couplers between 6CG7 stages and output tubes.

Approx 350v B+ silicon rectifiers feeding CLC pi filter.

Output tubes baised with LED array. No cathode caps anywhere in the cct, no local or global feedback. EL84's in triode mode.

No background hum or is to be reported.

Obviously with 5w or thereabouts on tap she clips pretty easily if you crank it up, resulting in a "snap" sound much like a record pop.

Need more efficient speakers now...

And to finish building proper housing so it's not a deathtrap...
 
Input stages haven't changed, still LED biased 6CG7 input and concertina. I've done away with the LED cathode biasing on the output tubes though. Now running user adjustable fixed bias. Approx 35 mA per tube. Sounds wonderful. Still need to finish building enclosure...
 
Drew are you using real fixed bias with a transformer supplied voltage or is it adjustable off the cathode bias?

This Tronola Electronics site has Dave Gillespie's fixed bias EFB mod which is low cost to implement and everyone really likes it. It's supposed to work better sonically than any other fixed bias.

Dave's Lab
 
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