hey all, Thanks for any help anyone might have with this wacky issue. I have had this 1965 Gemini I (G-12) for about 10 years. Recently it started "throbbing" - sort of a motor-boating sound that quickly grew in volume. One of the power tubes glows red on the plates - throbbing in unison with the throbbing sound. I tried new tubes as one did test weak, but the problem is still there. I also replaced the phase-inverter tube without luck. The amp works fine when first turned on - takes about a minute 'til problem starts. It has had all electrolytics replaced a few years back. I replaced the tube sockets, since I thought arcing might be an issue - didn't help. I have replaced or checked about all the resistors in the power section, and replaced some - no help. Do diodes go bad often? Any other ideas? Nothing looks burnt. Proper fuse does not blow. No smoke. I always shut it down quick when problem starts, so maybe it would blow fuses or smoke if I let it go. Thanks!