Damn' blast and buggeration happened on Sunday evening!
Sitting there, with me cup oh tea, after the missus had gone to bed, I was enveloped in aural ecstasy, when all of a sudden, a lot of hum appeared behind the music, which was disconcerting, as the amp is normally a smooth and quiet operator. The mains transformer was also buzzing like a banshee. All heaters and filaments were glowing, so, 'feck it!' I thought. Whatever was going on could be left until the morning. The land of nod beckoned, so it was off to bed. I stayed in the present, realised that resistance was futile, messing about at that time of night would be unproductive, and so I turned in.
Monday I got the amp on the bench (yes I've actually got a bench now. Imagine that
) and it was clear there was something odd going on with the HT. It was there, after a fashion, but there was a lot of mechanical noise coming from the mains transformer and I was only getting 210V at the first cap.
I suspected the rectifier, so I removed the AC from it and tested the diodes, nothing wrong with either of them, at least according to the diode test facility on the meter. Testing across the transformer secondary, there was no continuity end-to-end, so for a second or two, I was puzzled about how I was getting any high voltage at all. It was then that I remembered the secondary was centre-tapped. Removing the centre tap from the earth busbar and testing to there, revealed that there was continuity from one end to the centre, but not from the other to the centre. IOW, half the winding had gone open circuit somewhere. I was only getting half wave rectification, which explained the noise, both mechanical, electrical and the higher voltage sag at the cap when all the valves were in because of the resulting poor regulation....Sh*t!
This was not good. Although I had a 200VA/230V toroidal isolation transformer I could substitute in, it had no 6V heater winding, so no heaters for the ECC88. Worse, the filament transformer for the 45s lived under the rear cover with the now busted mains transformer, which meant no room for the toroidal replacement, unless said filament transformer was relocated onto the underside of the top plate, but there was a pair of glow tubes right where I needed to put it. There was also the question of a 6V heater transformer needing to also go under the top plate. The chassis is tiny and I'm in serious trouble here because of that.
It has taken me three days of head-scratching, standing there looking, measuring with a ruler, moving stuff about under the top plate, fitting the parts needed, wiring and finally testing it all, just to get back to a working amplifier. Sodding thing!
The problem is I think, that I've built so many amps over the years, that the bits I've got are getting extremely long in the tooth and didn't ought to be being used in new builds; not if I want things to be reliable at any rate. The mains toroid is new, so hopefully I won't have any more problems. I had to convert the rectifier to a bridge of course.