Fleetie wrote:

Does it contain any carbon composition resistors ? Or ancient leaky
coupling caps ?

It's only 4 years old so caps should be fine, I think.

This is one of the ultra-cheap Chinese jobs, so manufactured on
the serious cheap.

Don't rule out any 'iffy' passive components then. The quality of Chinese
parts varies from excellent to abominable.

At only 4 years I'm very doubtful about any ideas relating to corroded
valve bases unless you stored it in a basement for ages.

Of course, valve swapping should sort out any possibilities there.
Remember to only swap one at a time.

Graham