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In Reply to: If they are new tubes, posted by sser2 on December 9, 2006 at 16:08:34:
"at a given bias plate current goes prety much with Gm, so matching for equal plate current at the same biasing voltage is sufficient."I'm afraid I can't agree. I have tubes with a wide range of Gm that have very similar bias currents and vice-versa. I have NOS GE 6550A that vary as much as 1250-1500 umhos but are within a few ma. on current, for instance.
New production tubes often show the same result.
That's why I match cathode current first (at 465 volts plate and screen and -50 on the control grid for 6550A, for instance), then while the tube is blazing hot I check Gm. I can't check just current or Gm and get good matches.
Follow Ups:
My limited experience is definitely no match to yours. BTW, if the PP stage is not operated outside of Class A, is Gm matching important?
Class of operation has nothing to do with it, sser. Gm is essentially a measurement of a tube's ability to amplify, so it always matters if you want more than one tube to amplify a given signal equally (or as close to equally as possible).
that because in Class A each tube reproduces the full wave, the summed signal will not be distorted even with gross mismatch of tubes' GM. Just one half will contribute more to the overall signal than the other half. In Class B, it is totally different: because each tube deals with only half-wave, mismatched Gm will cause a lot of distortion.
But in a push-pull amp operating in class A the signal has been inverted in 1/2 of the output section, just like it would be in any push-pull amp.So while both sections pass signal full time, one output section is "pushing" while the other half is "pulling". Clearly if the amount of "push" and "pull" isn't equal or nearly so, there will be distortion. And the amount of "push" and "pull" is at least partially a function of the tube Gm.
Did I make that clear with my explanation? If not I'll try again.
Oh, as an aside that may help, while we often lapse into calling the phase inverter a "phase splitter" that's not the case at all. The signal is not split, rather it is inverted 180 degrees. So one of the phase inverter's two outputs is not inverted and is applied to one side of the output, the other is inverted and is applied to the remaining side of the output. So when the output stage gets signal applied one side "pushes" the signal, the other "pulls".
You mean inherent tube nonlinearity - more current on upswing than on downswing with equal driving voltages of different polarity. So, with different Gm, less cancellation of the second order harmonic than with equal Gm?
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