Good discussion by all.
09-03-14: Charles1dad
Are you familiar with amplifiers that maximize the 300b power curve and sound good doing so?
As you'll most likely recall, Charles, the VAC Renaissance series is one example. Member "Raquel" (Drake), who is quite knowledgeable about these amps, posted as follows in
this older thread:
What the above poster states about the Renaissance amps running 300B's hard is true -- 95% of the WeCo spec maximum voltage. I've owned VAC Renaissance amps for eight years and they require carefully vetted output tubes. In fact, Sophia used to market a special set of the Sophia tubes for VAC Renaissance amps, which were the basic Sophia tube, but carefully tested at plate voltages that resemble the voltage that 300B's see in the Renaissance circuit. Generally, as for transconductance, 300B's for the Renaissance amps should test in the 3,000-5,000 range. In addition, the plate-to-cathode voltage for 300B's in the Renaissance amps is approximately 430 volts dc, with idle current approximately 85 to 90 milliamperes in a self-bias (cathode bias) circuit. Again, this is approximately 5% below the maximum rating for the WeCo spec 300B. The milliamp and transconductance testing for purposes of matching must be done at these voltage levels, or you will find out the hard way, when your amp starts imitating a popcorn popper, that your 300B's didn't make the cut.
Looking at a 1939 datasheet I have for the original Western Electric 300B, the absolute maximum plate voltage it is rated to be able to handle is 450V, and its absolute maximum rated plate current is 100 ma (70 ma for fixed grid bias, which is not used in the Renaissance amps). And a cautionary note indicates that the 450 volt and 100 ma figures are not simultaneous ratings. Elsewhere in the datasheet it appears that at 450 volts a plate current of 80 ma would be the absolute maximum. Under those extreme worst case conditions (which are explicitly NOT recommended) maximum rated output power is spec'd at between 11.5 and 17.8 watts depending on load resistance.
The highest rated output power under any of the many RECOMMENDED operating conditions (i.e., recommended combinations of plate voltage, plate current, and load resistance) that are indicated in the datasheet is 12.5 watts, btw. The corresponding set of operating conditions for that rating is 400V, 80 ma, and 2500 ohms.
Designer Kevin Hayes has indicated to me and others that a truly WE-spec compliant 300B will have no trouble in these amps. As he has indicated, though, and as "Raquel" has emphasized in a number of past threads, a number of 300B reissues from contemporary sources do not meet those specs, and can turn this particular series of amps into popcorn poppers. However, I have not read of or experienced (after 3 years of moderate use) any indication that 300B's which are truly WE-spec compliant would have their longevity unduly compromised in these amps.
Regarding the discussion of paralleling multiple output tubes, based on my (limited) understanding of their architecture I would think that the Atmasphere OTL's are an example illustrating that under some circumstances far more than two triode output tubes can be successfully paralleled. Although, of course, as differentially balanced amps they are not SETs.
Best regards,
-- Al
