I have not been able to decouple the amplifier stages because its difficult to get high voltage non-polarised caps locally.
Maplins!
I want non-polarised electrolytics, yes?
I note that putting a 1000uf capacitor as cathode bypass slows the motorboating down, but even 2000ufwon't stop it.
Going the other way, removing all cathode bypass capacitors makes the thing into some sort of siren!
Nowt else I can do untill I get the capacitors.
Watch this space.
Next one I use a seperate triode & pentode!