The last word on "ultralinear" Fenders

I'm so amazingly lazy, I'm just going to copy/paste what I posted at MEF:

Finally took some measurements.

I pulled two tubes because my dummy load is only 100W.

Idle conditions:
Va = 487V
Ia = 29.9 mA (49% idle dissipation; a little cold, sure)

at full clean output: 24.1V into 10.1 ohm resistor for 57.5W

So for the screens we have, at full output:

27.3Vac across 1.991k screen resistor = It = .0137 A
DC ammeter in series with screen resistor = .00892 A

And the equation for screen dissipation in distributed load connection is:
P = EI - ei

where e = (1/2)(Ns/Np)sqrt(RlPo)
and i = sqrt(It^2 - I^2)

Sooooo:

e = (1/2)(.125)sqrt(10.1R * 57.5W) = 1.506 V
i = sqrt(.0137^2 - .00892^2) = .0104 A

P = (487 V)(.00912 A) - (1.506 V)(.0104 A) = 4.43 W

...which is a whole pants load different from (487V)(.00912A) = 4.44 W

But considering the maximum dissipation for a 6L6GC screen is 5W, I'd say with 2k screen stoppers we're in the clear. Unless, of course, I'm doing things horrendously wrong. It would be interesting to repeat this with different values of screen stopper; I kind of doubt that I lucked into such a good value of screen stopper.

In lieu of a thermocouple ammeter, I measured the AC voltage across the screen stopper. This is probably a significant source of error. The result does seem reasonable, when comparing how the screen voltage swings at a 12.5% tap instead of a 40% tap. I was expecting to see a minor reduction in screen dissipation due to power being dissipated in the load, and this is what I found, but I guess I was hoping it would be a little more.

On top of that, my plate voltage is lower than what I've measured previously, and the bias is on the cold side.

Now, what dawned on me after I posted this is that these calculations are independent of the value of the screen stopper. That is, without a screen stopper at all, the maximum screen dissipation at full output is roughly 4.5W. At least a hundred ohms should be used as a screen stopper anyway, to prevent oscillation.

I'm not sure what'd going to happen during overdrive, so a hefty screen stopper is probably still a good idea. Even with 2k stoppers, this bad boy'll put out 115W of clean power before clipping. Even if it did put out a clean 135W (mfg ratings are always dubious), it would be virtually indistinguishable from the 85W an earlier BF/SF Twin would put out.

Tl;dr - don't worry about it. 6L6s are just fine. That's not to say a pair of KT88s wouldn't be fun too, but there's no need to worry.

I'd like to get measurements under overdrive, but that'll have to wait until I get a true RMS meter.

Here's a link to the thread where I originally posted: Music Electronics Forum