Favorite 300B: Sophia vs KR

80 mA.

Pdrehr- Got a response back from Kevin saying

We have one customer running the Treasure tubes in a pair of Signature 140's. If I remember correctly, the tubes labeled in the 70's will occasionally trip the 300B Sentry protection circuit. If that is correct, then perhaps the "48" tubes would be the better bet.

Comments anyone, before I order? Not trying to start a war, I am truly ignorant on these matters.

I`d follow Kevin`s advice he knows his product`s characteristics best.

I will heed Kevin's advice, unless there is a strong reason not to, but Kevin was a little equivocal saying that the measurements needed to be put into context for him to be sure. The vendor has an impeccable reputation for honesty (even against his financial interests) and knows way more than I do about these matters. He said that in his experience w Shuguang and Psvane products:

cathode bias amps do well with the higher plate current tubes, and fixed (manual) bias amps generally like the lower plate current tubes. I've sold across those recommendations a ton of times, but if I saw any trend in regards to plate current, that was the one.

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So I thought I'd put it out there to get comments from people who understand the concepts behind this issue, which I most emphatically do not.

Michael, I would definitely avoid the 80 ma tubes, on the basis of Kevin's indication that they might trigger the amp's self-protection mechanisms. I'm a little uncertain about the 48's as well, especially given that we don't know the measurement conditions. I would certainly expect that they will function, but the number is low enough to create at least a little bit of uncertainty in my mind as to whether or not they would be optimal sonically.

I suspect, btw, that at least part of the thinking behind the vendor's statement is that cathode self-biased amps can be safely run closer to "the max" than manually biased amps. That is because the cathode biased amps will self-adjust as the tubes age, while if a manually biased tube is set near its maximum ratings, it may drift too close to, or even beyond, those ratings as the tube ages.

As I say, I'm not certain that there would be anything wrong with using the 48 ma tubes, but I'm not certain that they would be optimal either.

FWIW, btw, which is probably not much, the relatively inexpensive Chinese tubes that were supplied by VAC for my Renaissance 70/70 Mk III are all indicated (marked on the tube bases) as being around 60.

Best regards,
-- Al