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Help with tube specs

I will be getting a pair of EML 300b "mesh pate" tubes in a few days.

EML warns that the meshes should be run between 22-28 watts of dissipation with the closer to 22 the better and 28 MAX.

The amp I will be using them in is fine but I have another amp I'm not sure of. It's a Welborne 300b DRD. I was able to find this much:

">>>I have a Welborne manual for the 300B DRD and there are no voltages given in the manual. However, the original owner wrote his voltages on the schematic after he built the amps. He shows 505V on the plate and 190V on the cathode for a plate voltage of 315V. He also shows a cathode current of 65mA. So that means you have a plate dissipation of 20.5W.<<<

I also got this:

" The 300b mesh will not tolerate 500v on the plate. Even with a reduced dissipation on the tube, the tube won't last!"

I also read that with the tube drawing 1.4 amps at 5 volts (7 watts) you would add this to the 20.5W for a total of 27.5W. At least that keeps me in the 22-28 realm.

Can anyone help me sift thru this info? Opinions as to "how safe" it would be would be appreciated.

In the meantime, I will only run it in the safe amp for warranty purposes but would eventually like to try it in the DRD and would hate to eat the money if it's a mismatch.

Thanks.
onemug
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almarg
9,670 posts
 
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
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almarg
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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
onemug OP
784 posts
 
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.
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