VOLTAGE vs CURRENT ???

[Chuck H]

I have a new build using a pair of EL84s. Using a 160 ohm shared cathode resistor I have 362 Vp but at 95% dissapation. Using a 180 ohm I get 370 VP at 91% dissapation. I could keep raising the resistor value but this is obviously going to keep raising the voltage too. I know it's a common trick to underbias Deluxe Reverbs to get an acceptable Vp, but does this same mentality apply to EL84s??? What if I tank the screen voltage???

Thats my concearns from a safety standpoint. Then there's the tonal considerations...
Will the tubes sound better at 360 volts biased hot or am I better off tonally at a higher voltage and lower current. And what affect does a high plate and lower screen have on the tone of EL84s???

Just hoping for some past experience input to help guide my next move.

Thanks

Chuck

[MWJB]

Which sounds best?

Biasing DRs up to 30mA+ was recommended to keep plate voltage to <425v with NOS tubes which could handle the current better than voltage. Current production 6V6s with higher voltage handling mean that this is not always necessary..unless, of course, you like the tone at 30mA+.

As far as you're concerned you need to establish what is going to damage the tubes the most/fastest. This will depend on the brand of tubes you are using. In cathode bias I don't see any problem in running at 95% plate dissipation, I've seen Pro Jrs (fixed bias) that run at 12-15W per tube.

Even 360v is relatively high for EL84 (though Traynor & Mesa Boogie have run them at around 400v), I'd see what voltage you get around 100% plate dissipation, then watch for redplating or any signs of signal fade after prolonged playing.

There is always the military version at 13.5W plate dissipation, see Watford Valves Harma E84L for instance.

[Chuck H]

Thanks
"What sounds best" is how I've been aproaching this. But alot of the amps that run 400Vp on EL84s were built when different tubes were available. As for currently available tubes, I read alot of posts on the AX84 site that say you cant trust them above about 350Vp. I am using a pair of the old Sovteks right now. So no worries there. But I'm on my last pair and this amp is for someone else who will eventually be putting in whatever he can buy.

FWIW the amp sounds best with the 160 ohm at 95% diss for a Vp of 362.

Thanks again

Chuck

[Dan T.]

I wouldn't worry about the plate volts a long as your screens are lower.I have run my Traynor Studiomate Dimed into an attenuator with many different brands of EL84 tubes biased up to 70% at idle with around 420vdc on the plates and had no redplating issues whatsoever.
I realize that you are using cathode biasing, but If it was me ,I would try to get the idle current down and not worry so much about the plate volts.
BTW, do NOT use any "unused" socket connections as a tie point with EL84s

[stevesamps]

You can run pretty high with the JJ tubes
I use about 3k and then 250 ohm 5W screens

(overkill) but the plates are way up there and
so far so good about 13 watts or so..

[hasserl]

I run EL84's at high 390's to low 400's all the time, 70% of max dissipation, screens at just a few volts less than the plates, Sovtek and JJ's EL84's will run like this for many, many hours of use. Depending on how the amp is used they've lasted for years like this. No problemo. They sound fan-freakin-tastic too. Well, the JJ's do, the Sovteks, yuck, awful sounding tubes.

I"m running right now in my personal amp (test mule) some Russian 6N14-EP tubes that sound real good, and are pretty stout. I highly recommend these. You can get them for a good price, and I prefer them to JJ's.