Favorite 300B: Sophia vs KR

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

Hi Markus,
With additional burn-in hours do you have updates for both the Frank/Takatsuki and VAC 30/30 with BT tubes?

Swampwalker - I am far from technically savvy when it comes to tubes... but I assumed that "mA" refers to milliamperes. If that's the case, my VAC REN 70/70 Signature amp has a milliamperes dial on it and it measures milliamperes for each tube. In my amp, my VAC sourced Chinese tubes measure between 75 & 78 milliamperes, and my latest KR tubes are measuring in the 80 to 85 range. I talked to Brent about this yesterday, and I could have sworn that he said healthy tubes should measure in the 85 to 90 range. At this point, I have too much conflicting info to know which end is up. FWIW, the WE 300B spec sheet (which REN amps are based on) call out a max. plate current of 100 milliamperes, which is a far cry from the 48 mA option your dealer is offering you (assumes mA and milliampere are synonymous).

Paul, yes "ma" stands for milliamperes, and in this case refers to the plate current under some given test conditions (plate voltage and grid-to-cathode bias voltage particularly; perhaps also filament voltage if that is varied from its nominal 5 volt value).

I talked to Brent about this yesterday, and I could have sworn that he said healthy tubes should measure in the 85 to 90 range.

Paul, I would feel pretty certain that what Brent was referring to was the plate current as measured in the amp, when biased by the amp.

That has no necessary relation to the 48 and 80 ma numbers the vendor quoted to Michael, because it is very likely that the test conditions (plate voltage and bias voltage especially) were different for those measurements than they are in the amp. The plate current numbers will vary dramatically depending on those test conditions. I would expect the 48 ma tube to measure much higher when placed in the amp.

The uncertainty I have, as I indicated in my previous post, is whether or not a tube whose plate current is significantly lower than average when measured under a given set of conditions (as seems likely to be the case with the 48 ma tubes), will provide optimal sonic results when biased by the Renaissance amp. I don't know the answer to that question.

BTW, do your VAC sourced Chinese tubes have any measurements marked on their bases, as mine do?

Best regards,
-- Al

Al - yes, the readings on the bases of the VAC / Chinese tubes read 45 & 5.4.