OT ratio too low?

[Amp Kat]

Good point Mark and being that this is a test bed a variable voltage regulator would be an awesome tool to use as you could test both 6V6's and 6L6's under load with different rectifiers. A MOSFET or Kevin O'Connors power scaling kit would make an excellent regulator to swing plate voltage from 0 to 600 or more if desired.

[DaveZ]

"The PT voltage is a bit high for 6V6s, I'd have to have a very large rK, probably 400 ohm so I will stick with my original plan & build with the 5U4G & 6L6s." Not necessarily. Depending on the effect of the 6L6 current draw pulling down B+, it's only when you fit the 5AR4 that you start exceeding 6V6 voltage with a 250/270ohm rK. At 395v on the 6V6 plate, a 330ohm Rk would probably be OK (B+ may jump 15v or so, but cathode voltage will rise also, marginally bringing down plate dissipation).

Yeah but the plate dissipation on 6V6s is already over 12 watts, per tube, with even the softest rectifier I tried. That is a little high, I think. When I was building my Cal Dreamer (AX84) I had about 395 on the plate and tried cathode resistors up to 330 ohm and it was still running hot.

that said, the testbed has not preamp current draw, so voltages might well be a bit lower in the actual build.

I'm not sure why you are referring to 6L6 current draw there, btw.

[MWJB]

With a 270ohm Rk you might be seeing 385v on the plates before you are exceeding 14W plate dissipation. This would be typical of many repro 5E3's. Don't forget that you need to subtract cathode voltage to ascertain corrected plate voltage (say 362v in my example) and that simply dividing cathode current by Rk value also includes screen current (so you can lose a W/tube straight off when using that method).

With a 330ohm Rk you might get 425vdc on the plates (uncorrected) before exceeding 14W.

A pair of 6L6 with a 270ohm Rk will draw substantially more current than a pair of 6V6's, this additional curent drawn by the 6L6s 'may' pull down the B+ compared to the 6V6's.

[DaveZ]

With a 270ohm Rk you might be seeing 385v on the plates before you are exceeding 14W plate dissipation. This would be typical of many repro 5E3's.

14Watts on a 6V6?! Wow...I was thinking 12W was max for a 6V6. And some refs (Torres, Lord Valve) suggest around 10 W. I hadn't found a schemo of a 5E3 with voltages on it so I never knew what they were running.

"Don't forget that you need to subtract cathode voltage to ascertain corrected plate voltage (say 362v in my example) and that simply dividing cathode current by Rk value also includes screen current (so you can lose a W/tube straight off when using that method)."

OK, I am using proper Plate-cathode to measure voltage but was not subtracting screen current from the cathode mA. So it sounds like it'd be OK to run 6V6s in this with the 5Y3 (12.3W but that includes screen current in the cathode measure)

[MWJB]

"And some refs (Torres, Lord Valve) suggest around 10 W" - I would assume that they are referring to fixed bias applications at 10W plate dissipation. I doubt ANY Fender made 5E3 runs its tubes at 10W.

[DaveZ]

"And some refs (Torres, Lord Valve) suggest around 10 W" - I would assume that they are referring to fixed bias applications at 10W plate dissipation. I doubt ANY Fender made 5E3 runs its tubes at 10W.

OK, then, that's is good to know. This should be very versatile then, 6L6s with the stiffer recto & 6V6s with the softer ones.

Thanks VERY much for all the help here everyone.