Ideal 2xEL84 plate voltage

[Matt T.]

I have a 1x10", 2xEL84 and 2xECC83, Gibson GA15 (Trace Elliot Velocette in different clothing). It runs the plates at too high of a voltage so in order to get any decent current you have to over-dissipate the power tubes (stock it was @ ~17W per tube). If you could choose any plate voltage for this amp...what would it be?

Also, this amp has a slight metallic sort of 'rattle' (sort of ringing...but not really) but it's not the speaker or the cab, it's actually coming from the amp. It rides on top of notes, mostly noticable on lower notes. It happens even at lowish volumes, regardless of cabinet. Could it be the OT? It's always done it (it's pretty subtle but you guys know how it is once you 'zero in' on a noise) and I've done some extensive modding since then, no change.

[Bruce / Mission Amps]

320vdc-350vdc is my target voltages.
Have you tried an RC conjunctive filter across the primary of the OT?
Try something like a 10K@10W resistor in series with a .005uF@630v cap.

[Enzo]

COuld your noise be the EL84s themselves? EL84s are nototious for rattling inside the glass. I even make and sell a tube retainer for the PV Classic 30 that quiets them. It is that prevalent an issue. WHen you hear your noise, have someone grasp the power tubes and hold them. If it stops the noise, it is tube rattle. Use a rag or gloves if they are too hot.

Otherwise, to locate the noise, roll up a magazine or use a paper towel roll tube. Hold it to your ear and use it as a stethoscope. Aim it at various things on the amp until you hear the noise coming in.

[Matt T.]

Thanks Bruce and Enzo. No Enzo, it's not the tubes. I went through that when I first got the amp and almost every brand I tried rattled. I ended up with those Sovtek EL84M (not the regular Sovtek EL84) and they rattled the least. I've been wanting to try a conjunctive filter so I guess this would be the time to try it.

BTW- when I dug into this amp I found that several cap values were not per the hand drawn Velocette schematic I have. There were 3 places (IIRC) where the schematic called a .022uF and my amp had a .1uF. EDIT: the original statement here was wrong, it was other places that the caps were incorrect per schem. No wonder the amp was so blurry and bassy (through anything but the stock speaker) and sounded like a bad fuzz when cranked. I replaced them with .022uF caps and then installed a trimpot for the fixed 470K 'bass knob' resistor in the tone stack, and then altered the gain structure to slam the PI a bit harder and the power tubes less hard and it sounded much better. But it's still a bit out of control (I assume due to the high-ish voltages)...and that noise.

BTW again- I also replaced that Celestion Vintage 10 (I hate that speaker) with a Fane Axiom and that helped a lot too.

[Enzo]

Oh well. My stethoscope trick will still help find your cabinet/chassis buzz.

[Matt T.]

Thanks Enzo. I do have a cabinet buzz in a Marshall 4x10 cab, I'll try it for that. That cab doesn't have a center post...been meaning to try adding one.

[Enzo]

On any speaker cap, there is the whole usualy list of stuff, but don't overlook that the wires might be touching the rear of one of the cones. When looking for loose bolts one can overlook the flopping wires.

[Matt T.]

What timing! I just ruined a garden hose when I was cutting my lawn. I think I may have a use for it.

[Merlinb]

I wouldn't like to run EL84's any higher than 350V, not unless you have a very high impedance OT. I'd aim for the lower end of 250V - 300V. (Ignore the datasheet at your peril!)

El84's are notiorious for buzzy distortion due to reverse grid current. This is easily fixed using Paul Ruby's mod:
http://www.paulamps.com/18watterbuzz.html
or a similar one here:
http://www.nyquist-plot.com/displayg...ner_bias_trick