Help with tube specs

I don't know the answer, Harry, but I would caution that the voltages and current marked on the person's schematic may vary significantly depending on the characteristics of the particular type of 300B that is being used. Unless the person was known to be using EML mesh 300B's, I would not count on those numbers as being applicable, especially given how close you would be coming to the 28W absolute maximum rating.

Best regards,
-- Al

P.S: I note, though, that the 28W absolute maximum rating is for "plate dissipation." That would NOT include the 7W that is dissipated as a result of the filament current, which of course greatly increases the likelihood that the tube will be suitable.

Best regards,
-- Al

Al, you are my wish come true. I hoped you would see this thread and respond.

Good call on that "one" builders notes.

If I find the correct numbers for this amp, and let's say they are close to to 500 plate and 190 cathode, is the tube seeing the 500 (as the dealer warns) or do I subtract the cathode to arrive at the volts the tube is receiving?

Also, if I feel like gambling without known specs, do you think I would be safe using them if:

I turn the amp on in a dark room...

Look for any spark or flash at start up...

If so far so good, continue to watch the meshplates as they warm up in the first minutes and beyond to see if there are any signs that they are beginning to red plate and turn it off immediately.

These mono blocks use a 5ar4 for rectifying so I get a little bit of a slow start.

Hi Harry,

I wouldn't worry about the 500+ volts. The tube just sees the difference between the plate and cathode voltages. I suppose it is possible that there might be a small fraction of a second somewhere during the turn-on process where that difference is significantly greater than the much lower value it subsequently stabilizes at, but I'd be very surprised if that could cause any problems. And I see that the tube is rated to withstand up to 450 volts, provided that the 450V does not persist for a long enough period of time, in combination with high current levels, for the resulting power dissipation to be a problem.

The 450V rating, btw, is the same as the rating of the original Western Electric tube. I can't see any reason why exceeding that number by 10% or 20% or so for a small fraction of a second, if it were to happen at all, might cause a problem.

As far as the second part of your question is concerned, given the precautions you would be taking, and the substantial margin between the 28W absolute maximum rating and the 20.5W someone measured with the same model amplifier but probably with a different type of 300B, and also the margin between the 65 ma he measured and the tube's 80 ma maximum current rating, intuitively it strikes me as a reasonable gamble. But hopefully someone with knowledge and experience that is more directly relevant than mine will chime in.

Best regards,
-- Al