07-08-15: Tmmvinyl
I know very little about multimeters; mine is the Commercial Electric, Digital Multimeter, Model # MAS830B purchased at Home Depot.
Using the 200V measuring range, accuracy is listed as +/- 1.2% rdg +/- 10dgt.
Given that the meter displays four digits on that scale (for example, 120.1 volts), the accuracy of readings that are in the immediate vicinity of 120VAC would therefore be:
+/- ((0.012 x 120) + (10 x 0.1)) = +/- 2.44 volts
For your worst case reading of 123.1 the accuracy would be +/- 2.48 volts
So the actual voltage when you measured 123.1 could have been anywhere from 120.6 to 125.6 volts.
This information comes from the Takatsuki Denki website for the Takatsuki 300B tube.
Type: directly heated triode vacuum tube
Filament voltage: 5V/1.2A
Maximum voltage: 450V
Maximum current: 100mA
Maximum consumption: 40W
These numbers, representing maximum ratings, are identical to those which appear in the 1939 datasheet I cited earlier for the original Western Electric 300B. My impression is that the 1930's WE 300Bs are among the most robust 300Bs ever produced. Kevin Hayes designed the VAC Renaissance amps based on the characteristics of those tubes. And as noted earlier those amps drive the 300Bs extremely hard, yet tube life is MANY thousands of hours for tubes which are truly WE-spec compliant.
I can share the test results for one of my matched pairs if that would be helpful.... The plate volts Ep vdc test result was 300 for each tube; DC plate current lp mAdc was 73 and 73.1 for each tube; AC plate current lp mAac was 6 for each tube, Bias volts G1 vdc was 60 for each tube; Grid #1 leakage uA was 0.29 and 0.27 for each tube; Tube gain Mu was 3.6 for each tube; Plate resistance RP ohms was 0.6K for each tube.
I don't see any concerns in those numbers, and the numbers for which corresponding figures appear in the 1939 WE datasheet are roughly similar. Although these are results of tests that were performed at a plate voltage of 300, and we don't know what the results might have been if the test would have been performed at 390 volts (474 - 84), which George measured across the tube in Joe's Franks.
Also, the plate dissipation in the Franks which can be calculated for the Takatsuki from George's measurements is 32.0 watts (rather than the 33W that was indicated), and the plate current can be calculated as 82 ma. Those are reasonable figures, although not on the gentle side of the spectrum. Although of course George measured a line voltage of 119.4, and I have no particular feel for how much those numbers might increase if your line voltage actually approaches the upper limit of your measurement +/- the meter inaccuracy.
Finally, we have to consider that we can't say with 100% certainty that your particular tubes, or other Takatsuki 300Bs for that matter, conform to Takatsuki's published maximum ratings, especially given that the indicated tests were performed at a plate voltage considerably less than the voltage the tubes see in the Franks, and much less than the maximum ratings.
So the bottom line seems to be that there are considerable grounds for encouragement, but there are no guarantees. Apart, perhaps, from the warranty :-)
Best regards,
-- Al
