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That's interesting

Are you sure you were driving only one grid of the diff amp? If you were driving both (maybe looking at a down stream stage) then I would expect to see little or no signal at the connected cathodes.

Anyway, since you called me out on it, I performed some measurements of my own. The only PP amps I have around are guitar amps. The one I looked at uses a 12AX7 LTP with a comparatively 'short' tail resistance of about 11k along with unequal plate resistors to compensate for the short tail (82k and 100k.) With 2Vpp applied to the grid I saw almost exactly 1Vpp at the cathodes, maybe a tiny bit more. That surprised me; considering the small tail resistance I expected to see less than 1/2 the input voltage there. Perhaps the unequal plate resistors are a little too unequal... (In case anyone is wondering, I did disconnect the NFB that is applied to the tail.)

While I was at it, I looked at the output voltage: roughly 32Vpp at each plate giving a circuit gain of about 16. This is a little lower than I expected. I would expect a 12AX7 at this operating point and with this plate load to have a gain around 40 in an ordinary grounded cathode circuit, so I was expecting a circuit gain of around 20 in the LTP.

I also put together a circuit in Spice with 6SN7 whose behavior was dead-nuts on what I expect, but at this point I see no reason to share the details.

I'll try to help make sense of the signal at the cathodes. Consider a simple cathode follower with a CCS under the cathode. How would you measure the output impedance? The usual way is to load it down until the output signal is 1/2 the unloaded output. Whatever load was needed to accomplish that is the output impedance of the cathode follower.

Well, the LTP (when used as a phase splitter) can be thought of as a cathode follower driving a grounded grid amplifier. If you think about it you'll realize that the input impedance of the grounded grid amp is equal to the output impedance of the cathode follower. Given what was said above, we should expect the signal at the cathodes to be 1/2 the signal at the input grid (ignoring the fact that a CF actually has a gain slightly less than 1.)

Ralph, I would really like to put an end to this discussion. I remain completely confident in the facts as I have been presenting them. You've learned something; that should make you happy.

-- Dave

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