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Re: Bear with me

Posted by dave slagle (M) on February 3, 2007 at 13:52:44

In Reply to: Bear with me posted by Russ57 on February 3, 2007 at 09:26:14:

with perfect tubes, DC should require unequal resistances and AC should require perfectly matched.

For the DC situation consider two 2A3's with the grids and plates in parallel and the filaments in series fed wiht a 5V filament, The bias of one tube will be 2.5V away from the bias of the other so one tube will draw more current shifting the "actual" current center point away from the "geometric center" of the structure. Once you veer away form the geometric center the impedances are no longer matched making unmatched external resistors required to keep identical currents at each filament pin.

For AC since the bias reverses, this should fix the imbalance and a pair of matched resistors should give you a perfect null, but for some reason that doesn't always hapen. My WAG is what if you take the above situation of the parallel 2A3's with the filaments in series and feed it with ac. If the tubes are matched, matched resistors should be ideal, but if the tubes are not matched, then suddenly an external offset of the hum balancing network would be required for the best null. Now imagine a single tube like a 300B with one side of the filament perfectly centered in the plate grid structure and the other side slightly offset. bisecting the tube down the middle and viewing it as two discrete tube structures will show two distinctly different sets of characteristics so even if heated with AC it will not behave as expected.

I think these geometric variations are why some tubes just seem to null out AC better than others.

dave

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