[Radio Wrangler]

If the output bottle is creating high frequency oscillation, you might not be aware of it directly unless you've had a prod around with a scope.... or unless you happened to notice an increase in the bias current, perhaps?

Quite often, stability issues in entertainment equipment is done on a suck-it-and-see empirical basis, sometimes abbreviated to the word 'bodging'.

That capacitor is in a place which provides HF loading on the anode of the preceding valve. This creates an added zero, rolling off the gain at higher frequencies, but also rolling in phase lag. The game is to bring the gain down to les than unity before the phase lag has built up enough to reverse feedback. On top of this, valves and transistors can go unstable on their own without needing loops or much of the other stages. This is what parasitic stopper reistors are used for.

Just because a box has a famous name on it isn't a guarantee that stability has been properly sorted.

So the next step is to go looking for any hooting.

David