Favorite 300B: Sophia vs KR

03-10-12: Pdreher
In talking to Brent @ VAC, I've found a way to get the sound of my old 30/30 out of my 70/70 Sig. by simply taking out a row (quad) of 300B's. In doing so, I essentially now have a 30/30 Signature in the body of a 70/70 Sig.

Hey, Paul, that's good to know! Another reason being that if a tube ever fails, it would therefore be safe to operate the amp with a row of tubes removed, while a replacement is being obtained.

To be sure I am understanding properly, what could be removed would be either the four 300B's that are closest to the rear of the amp, or the four 300B's that are closest to the front of the amp. Is that correct?

Very glad, also, that your Ulysses ultimately worked out well.

Best regards,
-- Al

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

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

03-28-12: Charles1dad
The VAC REN amplifiers have auto bias, would`nt that compensate(at least somewhat) to the various ma levels?

Yes, to a considerable degree. Also, the bias conditions that the amp is designed to automatically establish are most likely different than the conditions under which the 48 and 80 ma numbers were obtained.

03-28-12: Maxmad
I believe there are differ auto-bias circuits around, also how and what exactly they do, is not the same.

The Renaissance amps are cathode biased. This paper provides a good explanation of that.

03-28-12: Pdreher
Al - yes, the readings on the bases of the VAC / Chinese tubes read 45 & 5.4.

I guess the fact that VAC supplied those tubes provides at least a little bit of confidence that the 48 ma tubes would perform well, as the 45 ma number is significantly less than the 60 or so that is marked on the tubes they supplied for my amp. Of course, chances are that the test conditions VAC uses are not the same as the test conditions the vendor of the 48 ma tubes uses.

BTW, the other number on my tubes ranges from 5.7 to 6.1. I suspect that number is the tube's transconductance, aka mutual conductance, in "millimhos" (ma per volt, referring to the change in plate current that results from a change in grid voltage, relative to the bias point). 5.7 millimhos corresponds to 5700 micromhos. The original Western Electric 300B spec indicates 5500 micromhos under typical conditions.

Best regards,
-- Al

I second all of Onemug's comments.

The 5 hour recommendation has to be somewhat arbitary, even for the specific Sophia tube, because to the extent that an optimal figure may be definable for any specific tube, the figure will vary depending on its operating conditions in the particular amplifier.

FWIW my own instinct, based purely on instinct, would be to not hesitate to have listening sessions now and then that are significantly longer than that, but on the other hand to not run any tube power amplifier continuously 24/7, or overnight, or when unattended. The concerns being not only the likelihood of shortened lifespan of expensive tubes, but the possibility of secondary amplifier or speaker damage, or even fire, if a short or other fault were to develop.

Best regards,
-- Al

Dave, thanks for your excellent and very informative post. One small correction, though. I think that when you referred a few times to "grid voltage" you probably meant to refer to "plate voltage."

Regards,
-- Al