Metropoulos Forum

Just got a set of NOS GECs for my JTM45 kit.

George uses a value of 27 watts as the maximum plate dissipation for KT66s, but most folks use a value of 25 watts.

What do you guys recommend? Right now, I've got them just under 70% for 27 watt (45.5 mA and 44.3 mA at 407v), but that's around 74% if I should be counting 25 watts as the maximum dissipation.

I could drop the maximum draw to around 42.5, which, with the accompanying voltage increase, would be around 65% at 27 watts and close to 70% at 25 watts.

It sounds good where I've got it, but I don't want to overstress the tubes; they're expensive!

They are real rugged tubes though, and 74% probably isn't excessive.

Question is...
At what point do they sound best to you?

Thanks.

They sound incredible right now!

reswot wrote:Thanks.

They sound incredible right now!

Then LEAVE 'EM.

Is that 45.5 and 44.3 mA the current calculated by measuring the voltage across the 1-ohm cathode resistors ? If so, did you account for 1-2 mA loss of cathode current due to screen grid 'theft'. If you didn't, your 45.5 and 44.3 mA bias current calculations may be closer to 44.5 and 43.3 mA. Measure the voltage drop across the screen grid resistors, calculate current through each, and subtract from calculated cathode current to get a better approximation of plate current. I know it's minor, but since you are citing current to 1 decimal place you are obviously focused for accuracy.

I'm measuring across the resistor. I'm not really that worried about being precise to the decimal point, that's just what my meter said (and it's probably off a bit, too!).

What I'm really asking, though -- and what no one has yet answered -- is whether most folks use 25 watts or 27 watts as the maximum plate dissipation for KT66s -- and why, if folks do use 25 watts, does George use 27?