My biggest concern about the components used to protect against instability is what happens when the instability still occurs despite these components. eg I would expect that the anode suppressors could suffer rapid and severe thermal damage if the amp hooted strongly up around 120-130MHz. If the resistor in the suppressor blew open circuit then this would mean the loop gain would go UP a lot and also the zero phase frequency of the loop would drop. So The hoot would then change down in frequency. The impedance at the anode would rise a lot at the new (lower?) hoot frequency because the suppressor is no longer effective and I'd assume large peak voltages could occur at the anode especially if the hoot happened during a high power tune up into a load. This lower VHF frequency hoot could put great stress on the 200p caps and also the 1mH choke due to high voltages occuring within the grid grounding network in the feedback path. Do these parts ever overheat or fail? Can anyone advise if suppressors have been tried at the cathode? A simple system model shows a marked reduction in loop gain and also a useful shift in phase if something like 75R in parallel with 120nH is used the cathode. However, I don't know enough about the large signal behaviour of the tube to know if the suppressor would upset the driver stage performance so maybe someone can comment on this. Note: the suppressor won't be effective if there's any shunt components from cathode to chassis AFTER the suppressor.