• 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.

Max voltages for EL84?

Rod Coleman

Member
2004-07-25 6:55 pm
UK
I think 300V Va and 600V supply will be OK.

It would be best to include a "standby" switch (like a guitar amp) - Then you could fire the heaters before the EL84s see any of your 600V!

For NOS EL84s, the old spec (Va0) for cold anode voltage is 550V max, so you're a little high, and for this reason I would make use of the standby facility.

For comparison, the Peavey Classic 50 runs more than 400V at 25mA, and they don't cause too much trouble with their Sovtek-branded pentodes. The JJ EL84 works good is this amp, too.

In triode mode, stick to 300V in operation, for the sake of the g2 (screen) rating - screens seem to be more fragile than anodes (plates).
 

Rod Coleman

Member
2004-07-25 6:55 pm
UK
What kind of anode (plate) load do you have in mind?
Varistors act like a short circuit when they're triggered. Your plate load would then see the whole of the 600V across it - can it take that?

Also, varistors possess capacitance that may behave unexpectedly and degrade sound.

I think your circuit will be OK without protection, provided the cathodes are heated before applying the 600V. Occasional short-duration exposure to 600V is very unlikely to do serious damage to an endpentode like EL84.
 
I've run Reflector 6pi14pi and 6pi14pi-ev tubes at ~410 volts on the plate, cathode biased, ultralinear, 30mA. No problems so far. I don't have a supply that will put 600 volts on them. Your comment about running them at 300 volts plate, 10mA seems to be well within their dissipation limits, but I wonder where on the loadline they'll be at those specs.

How are you dropping your 600V supply down to the 300V you expect to see at the plate? I'm sure it's obvious, but I'm missing it...