So... here's a quick update. I had the cover off this afternoon and took some pics inside. It all looks quite a mess to me but I don't really know about these things, or what constitutes reasonable condition in an amp of this vintage? Certainly a lot of grime and fluff in there. Maybe someone here could take a look at the pics and tell me what they reckon as to the state of the important bits.
The loose part rattling around was just a screw. It may have been part of fixing the 4 blue capacitors (I assume) in place, as that whole assembly has come loose and is wobbling about quite freely. The 4 blue tubes were all blue-tacked together presumably to hold them in place. In fact there's quite a lot of blue-tac in evidence inside.
The chap who sold it to me told me he tried it out by putting a modern plug on it, then replaced the vintage one to 'keep it original'. When I told him I was getting it checked out by a qualified tech to make sure it was working as advertised, he started getting a bit upset and implied I was trying to make trouble (!) and maybe I should just return it to him.
It's booked in for a visit with David next week. What do you think, should I stick or twist?
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