A couple of thoughts, first are you sure that your scope probes are properly compensated? If they aren't they will exhibit slightly different phase shifts which might account for the error. Your scope may or may not have a probe calibrator, in any case a good quality 1kHz squarewave at a volt or so will do the job nicely. Secondly are you using chopped or alternate mode on your scope, alternate is what you would want to use at these frequencies on most scopes.
Finally if the load capacitances are significantly different across the windings you might see some phase shift. How big are your grid stopper resistors? Probe capacitance imbalances with large grid stopper resistances might cause additional phase shift as would significantly different miller capacitances and strays between the electrodes in those GM70's. I would measure the phase at the splitter transformer directly.
Check with a sine source that your scope probes and scope don't introduce different levels of phase shift before doing the measurement. I think this problem is most likely a probe/scope induced artifact.
Hope this helps..
Kevin
Finally if the load capacitances are significantly different across the windings you might see some phase shift. How big are your grid stopper resistors? Probe capacitance imbalances with large grid stopper resistances might cause additional phase shift as would significantly different miller capacitances and strays between the electrodes in those GM70's. I would measure the phase at the splitter transformer directly.
Check with a sine source that your scope probes and scope don't introduce different levels of phase shift before doing the measurement. I think this problem is most likely a probe/scope induced artifact.
Hope this helps..
Kevin
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