85W out of two EL34s??

[Zozobra]

There was a thread over on AX84 many yeas ago about how insane the load line for a super lead is when you plot it out. It's worth hunting it down and having a read. The load line gets pretty close or may even exceed 2x max plate dissipation in places iirc! Without the power supply sag the plates would melt pretty quickly.

[pdf64]

There was a thread over on AX84 many yeas ago about how insane the load line for a super lead is when you plot it out. It's worth hunting it down and having a read. The load line gets pretty close or may even exceed 2x max plate dissipation in places iirc! Without the power supply sag the plates would melt pretty quickly.

Aiken demonstrates that here Idle Current Biasing - Why 70 percent?

[catalin gramada]

http://http://www.freeinfosociety.com/electronics/schemview.php?id=360

[Mike Sulzer]

The best you could do if tubes were ideal devices (class B, 78.5% efficient, push pull, two tubes 25W max each):

25*2/(1 -.785) = 232.5 watts

However, the EL34 cannot pull down to zero volts with high current; you get about .275 amp dropping 100V, and that means you dissipate 27.5W when ideally at that point you would have zero volts drop and dissipate nothing. So, very roughly, leaving out the additional analysis you should do, you would expect to do less than half as well as ideal, or maybe 100W. And if you want some idle current to move in the AB direction, that would lower it still further. So maybe 85 W is reasonable for some applications. For a guitar amp, I would say that 50W is a good number, and the old fashioned conservatives might say 40. (Try it, "40", with your best Bill Buckley fake accent.)

[Dave H]

So, even though a Music-Man 130 is biased so cold, it still puts out it's rated power easily (if all else is running as designed), correct? SVT, same thing - they get measured putting out 280W, no problem, when the max dissipation of 6x6550 would be 252W?

I don't understand HOW they do it, exactly

They can do it because the power dissipated in the tubes is just waste heat. It is not the power going to the load. If it's a class A amplifier then in an ideal world the max efficiency is 50%. To get 50W out you'd have to put 100W in (efficiency = 50W/100W = 50%). Of that 100W 50W is dissipated in the tubes and 50W in the load but if it's a class B amp the max efficiency is 80% so we can have 250W input because only 50W of that is be dissipated in the tubes and the output power would be 80% of 250W or 200W. Of course in the real world the power output would be as in Mike's post above.

[bea]

Maybe this is like the great "it's current, not voltage, that kills" fight...

It is not "like" that fight, it actually IS that "fight".
From the experience with a few of such amps - i have two Dynacord Eminent 2 running and a G-2000 with even 4 EL34 at 800 V, it seems as if even most current production tubes can deal with these conditions fairly well (and we do not see those golden era tubes that have failed, we just see the good ones...)

Just another question. The KT77. Specified for a screen voltage of up to 800 V in UL mode but only 300 V in normal PP, i.e. less than the EL34. Is there any physical reason for this? To my understanding the tube "sees" just the voltage and "feels" the grid current, and regardless of the external circuitry it should suffice to ensure that the maximum screen current is not exceeded.

A note to the complexity of the power supply. If the rail voltage significantly exceeds ~400 V, it is generally advisable to stack it from two voltages. The limiting factor is the max voltage of the electrolytic caps. The additional winding is almost no cost factor, and it is indeed often present in mass pproduction transformers (but it is a cost factor because of the two additional caps...)

A few remarks on these high voltage designs: these 700-800 (i am aware of amps using 850 or even 900 V at quiescence, which will probably decrease below 800 V at full load). The layout is a lot more critical than it is for "only" 400 V.

Then - safety: 800 V are definitely deadly. At 400 V You have a chance to survive a shock if the current is low enough.

And measuring equipment. Most cheaper multimeters, but also many high quality instruments are just CAT II 600 V and cannot be used to measure the anode voltage.

BTW, and to those who are skeptical: are You aware that in many EL84 designs the anode voltages significantly exceed the specs and no one seems to mind?