Too much for JJEL84?

[Dingleberry]

Hello you tubeheads.
I'm a newbie on this forum from Finland and I have some questions about "real life" maximum values for JJEL84. Just finished my second amp, 18W EL84 P-P power amp with EF86 Matchless style preamp. Very, very nice sounding one. I got the transformers and choke from old big tube radio which had EL84 P-P poweramp also. They are in very good condition. From the sixties probably, paper bobbins naturally. The question is about the power tanny. It puts out +358 on the anodes. I increased the screen resistor value up to 4k7, so there is now about 350 on the screens. I have separate screen resistors and separate bias resistors. I have biased it just little under 70% of maximum dissipation. Am I still running the amp too hot? Are my voltages too high? What I have seen nearly every amp is pushing EL84:s far beyond their maximum ratings. Should the screen voltage be lower? Of course I could increase the screen resistor value more up but that would add also compression, right? Any Ideas how long myt tubes will last in that configuration. All input welcome. Tanks.

[markh]

The EL84s can stand some voltage on the plates, but the screens are a little more sensitive -- I prefer to keep them around 340v or lower.

You could always add a "side chain" RC before the screen resistors to drop it down more. The C (even 20uF) would alleviate the compression effect. That way you could use, e.g., 1k screen resistors and use the additional series R to adjust the voltage.

--mark h

[KT77]

I use 4.7k feeding separate 1k screen resistors, w/ a 16-20uF 450V cap at the junction of the 4.7k and 1ks.....I'm running very close to same '84 plate volts as you mention, but the screens are down at 310 or so......
I've used this configurarion on 50watters running high 400's on EL34 plates, as well, and it works great to keep tubes from nuking when the morons playing them are running everything on '11' LOL!

the '4.7k' can be adjusted to dial in a safe screen voltage

if you look at a Matchless 'Lightning' circuit, you'll see a 22k feeding 100ohm screen res w/ a 22uF cap.......that works good too

to my ears, there is a slight tonal change when you do this.....don't know how many have heard a DR103 roaring in your face full tilt......but it has that kind of character leanings....I personally love it

[markh]

...to keep tubes from nuking when the morons playing them are running everything on '11' LOL!

Hey!! You dissin' my buddy Nigel Tufnel??

--mark h

[Dingleberry]

Thanks a lot!

I should've figured that out myself... I should have looked the matchless power amp and PSU section also... This is what happens when you design your own amp and take preamp and power amp and transformers, all from different sources. But on the other hand it's pretty educational. You learn something every time you make a mistake...
The side chain R-C conjunction fixed my problem. I used 56uf/450V cap because I just last weekend purchased 10 of them from local electronic shop season sales 0,50€/each. 10-15Kohm was a good range for the side chaind voltage drop resistor letting me easily tailor the screen resistor value for different tastes. Once more: THANKS!

[KT77]

...to keep tubes from nuking when the morons playing them are running everything on '11' LOL!

Hey!! You dissin' my buddy Nigel Tufnel??

--mark h

Nig..if you're reading this...I apologize profusly for plagiarizing your ideas.....but when I see a good idea, I like to use it...where do I mail the royalties