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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: RE: VT 100 Mk II driver tubes posted by jea48 on January 1, 2016 at 12:22:33:
Unless the vendor matched the tubes for plate/cathode current the Gm readings are relatively meaningless in the bias scheme. The current can differ greatly between tubes of the same (or nearly the same) Gm. It was likely as much coincidence as anything else that the one pair worked and the others didn't.
Plate voltage in this circuit is a function of current through the plate load. More current = lower voltage at pin 1 or pin 6 (these are the plate pins of the two triode sections) due to the higher voltage drop across the plate load. So while the Gm match is important, the CURRENT match is critical!!
The tubes for these amps need to be very low noise, and have matched transconductance AND current to successfully bias up in the VT100. Matched transconductance alone will not get it done!!
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Follow Ups
- RE: VT 100 Mk II driver tubes - Jim McShane 10:04:47 01/02/16 (1)
- RE: VT 100 Mk II driver tubes - dgaapc7 15:44:53 01/05/16 (0)
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