Ryan regulators not regulating?

[bobschneider]

I recently built a pair of Kit Ryan's power supply regualtors for the Adcom GFA-555, using the Old Colony PCBs. I've installed the regulators into my GFA-555 (not GFA-555 II).

When the amp is at idle, I can adjust the regulators to the specified 62VDC output, and everything appears fine - the adjustment trimpots end up in about the middle of their range. But when I apply a signal to the amp, the output of the regulators begins drooping - at a constant 500mV 1000Hz input, which gives about a 10V output into an 8 ohm resistor, the regulated rails are down to about 58VDC. This is about the same drop in the power being fed into the regulators (from about 79V to 75V).

Obviously, the regulators aren't regualting anything in my setup. Does anyone have any ideas as to what could be wrong here? The amp otherwise sounds fine, and the output at these levels looks fine on the 'scope, and has measured distortion around 0.025%, which I think is about right for this design.

Thanks!