JJ 6V6S idling at 15.5W but not redplating, leave it alone?

Cathode biased power section, into a Hammond 1760H (6.6K Deluxe Reverb OT).
Cathode bias resistor is a 360 ohms.

The amp has a "dual voltage switch", basically choosing between the 115V and the 125V primary taps on the 272FX PT, and it's diode-rectified. When using the 125V tap I get about 375V on the plates, with 28V dropped on the resistor (347V plate to cathode, 77.7mA). Assuming roughly 5% screen current, that gives us roughly 12.8W per tube of anode dissipation.

However, when switching to the 115V tap, HV jumps to 405V and 31V is now dropped across the resistor (374V plate to cathode, 86.1mA), doing the same math yields 15.3W per tube.

Officially the 6V6S is a 14W tube, but I've read that it'll do 16W, even 18W without breaking a sweat (bigger plate structure than a classic 6V6GT). It sounds glorious and is not redplating no matter how loud I'm playing it.

The higher voltage mode has a distinct quality that makes it very desirable (more headroom and tighter, useful for loud clean tones or higher gain stuff). The lower voltage mode is more "brown sounding" if you will. Good for bluesy or tweedy tones.

Should I leave it alone?

My main worry is that this amp is for a friend, and years from now when he's on the road and needs new power tubes, maybe he won't be able to find JJs. I could always remind him to never use the high-voltage mode without JJs...

 

I had a few 10W resistors on hand so I figured why not try it.

Tried a 560 ohm first, that brought it down to 76% dissipation at idle on high-voltage mode, but low-voltage was a lowly 68%. No noticeable crossover distortion, actually sounded very nice, but it lost some of the "chewiness" of cathode bias, pretty much sounded like fixed bias at that point. There was a bigger tone difference between both modes though, but the low-voltage mode was lacking punch.

So I settled on a 470 ohm... for now. 92% in high-voltage, 76% in low-voltage. Sounds good, rounder than the 560ohm, but a bit cleaner and tighter than the 360ohm.

I have a band practice tonight, we'll see how it fares in a band setting.

I also changed the bypass cap from a 220uF to a 33uF, mostly because the old one was rated for 50V and I like to be at least 2X the bias voltage. It didn't really change anything tone wise, 33uF with a 470R still has a -3dB point of 10Hz (that's 3 octaves below the low-E!).

 

Filters are 500V (cap cans cause I like how they look ).

Filament voltage is actually one of the reasons I put the dual voltage option. At least in my house, the 115V tap had the filaments at over 6.5V, while the 125V tap brought it down to about 6.1V. So I figured: leave it on the 125V tap, but you have the 115V tap option when you're recording and you don't care about punishing the tubes a little more. We're talking about a 3% difference both ways though. The 272FX is rated for 5A and I'm not even pulling 2A, so it's under very light load.
(I chose this PT to have the option of running EL34s or 6L6s)

 

As I understand, that's the theory behind biasing cathode-biased amplifiers much higher, because they can't "maintain" the higher bias point under load. Another reason I lowered the bypass cap (with a high enough bypass cap this point is made moot).

As for the anode sagging, since mine is SS-rectified and has a higher rated PT (173mA) I can't rely on that too much.

Plotting the load line on JJ's datasheet made me realize I was over 100% dissipation pretty much the whole time until just before one of the 2 tubes shuts off and shifts into class B. The lack of any redplating perhaps confirms how solid the JJs are (they are said to be more like a 6L6 than a 6V6)

 

My math did include 5% of screen current, which works out to roughly 2mA, which is confirmed by measuring the voltage drop across the screen resistor.

Either way, used it with the band yesterday with the 470 ohm resistor that biases it at 12.75W (91%) per tube and it sounded just as good, perhaps a little less "round" but only I would notice it. Don't think it's worth making the amp "JJ-only" for such a small difference. Especially that I've had a few JJs go bad in recent weeks (mostly preamp tubes though, I'm switching to Tung-Sol)