I assume that auto bias in your amp means they have cathode resistors
in place, no negative voltage supply for the grids of the output tubes,
and no adjustment for bias.

(I have seen old wirewound cathode resistors develope breaks from the
weld points to their leads, and all four of them developing this. It
was a very old amp, however, and not likely here.)

When you say the tubes glow on and off, do you mean the filaments in
the center of the tube, or the plate structure or one of the grid
wires are glowing?

Do all the output tubes glow at once, or are a couple of them glowing?
Do the signal tubes do this as well?

If it's not the filament, but the plates/wires on both channels which
are glowing at an interval, then current through the tubes is greatly
changing, and is common to both channels.

If bias is not a common point to both channels, maybe its a low freq
feedback problem through one of the power supply caps, such as internal
arcing, cold solder joint, dried electrolytic causing a
charge/discharge or ground/unground at low frequency. You could get a
bench cap somewhat close to the cap values, and substitue each ps cap
one at a time to see if the problem stops.

Also, maybe a power supply resistor is arcing internally. I use a
plastic cheap stethescope to listen for this as it usually is audible,
but make sure it is NON CONDUCTIVE. I would imaging a drinking straw
would also work for listening for arcing in resistor bodies.

Also, you can power the amp up on it's side, turn out the lights and
watch it from a safe distance, and watch the underside for any arcing
to ground, etc.

Hope this helps
Bob H.



Dave xxxx wrote:
Bob H. wrote:
Does this amp have fixed bias, coming off of a single bias supply?

Bob H.

its got "auto bias"