A few days ago I spotted a Marconi KT66 on ebay being sold as "not working". I made an offer and obtained it for £15. Now, a casual glance will reveal that good original MOV KT66's are at a premium price as I'm sure you know.
However, I thought "what the hell" and got this valve just to see in what way it was "not working". Upon arrival it could be seen that the base had been re-glued at some previous time, rather neatly in fact. I tried the valve in one of my Quad II's and was surprised to see it warm up and appear to work perfectly. Presently, however, the heater went out. Moving the valve slightly made it work again. I removed the valve and de-soldered the pins to examine the base connections. It could be seen that a heater lead was not making a good solder join in the base pin. I re-soldered it and the remaining pins and the valve has been operating for an hour or so without issue.
My guess is that when the base was loose it had weakened the wire to pin solder connections. Anyway, a pure stroke of luck as it is of course not always possible to repair a faulty valve.
Here is the valve installed in the amplifier in the right hand socket, next to an existing Marconi.....
? ?
I've recovered a few octals in this way. Well done!
On a slightly similar note, I managed some time ago to repair a short-circuit Quad II smoothing block. I simply blew the short away by discharging another electrolytic across the terminals of the shorted section. It was a case of kill or cure, but I was lucky. Obviously it could have opened it completely. However, that capacitor block is still in use to this day without any further problems.
Will certainly dig out and re-check an earlier grey glass Marconi KT66 purchased second hand at an amateur radio rally many years ago.
Heaters on this one OK but absolutely zero emission. A visual inspection has limited success in identifying obvious faults such as a disconnected cathode, but will have a go at re-soldering the pins when I find it.
Brimar 6U4GT used to suffer badly with unsoldered pins (as did the Shure V15 III !)
Rich