So I think this is “motorboating”, but damned if I know what that means technically. Fortunately, there is the internet…
from a 2008 post on ampgarage.com:
from a 2008 post on ampgarage.com:
This is from the Tube Amplifier Debugging Page: http://www.geofex.com/ampdbug/ampdebug.htm
"In an amplifier that has ever worked correctly once, motorboating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance, not decoupling properly. Replace the B+ decoupling capacitors at least for the preamp. Since the other capacitors are old, also, consider replacing ALL of the electrolytic capacitors in the amp (doing a cap job - see the Tube Amp FAQ at http://www.eden.com/~keen for info on the what and why of cap jobs.)"