[Techman]

He doesn't need to worry about why the HT is low at this stage as the whole thing is being under run due to the 100 watt bulb (lamp limiter) being in series with the supply, hence the low HT.

Once he's established the cause of the hum, and that nothing else such as the mains transformer has any shorted turns, which looks unlikely at this stage, then he intends to replace various capacitors as we discussed earlier in the thread.

However, I don't know if he's done this check yet (I think I advised it earlier in the thread) and that is to remove all the valves, or at least the HT rectifier, and run the unit on full mains for a while to make sure there's no overheating of the mains transformer. In fact, what I would advise is to leave the 100 watt bulb in circuit at first and see if this now remains quite dim with all the valves removed, then go on to the second part of the test with full mains, but with valves still removed. This will make a definitive test of the mains transformer.

So order of tests:-

1) Lamp limiter in series and no valves in.

2) Lamp limiter out of circuit (full mains) no valves in.

3) Lamp limiter out of circuit (full mains) all valves in EXCEPT the rectifier.

He may already have done this test, but due to the lamp limiter and all valves being in place as he described in his last posting, any possible fault in the mains transformer may not show up until it's too late. I think he'll be alright and the transformer will be fine - I'm just crossing all the 'Ts' and dotting all the 'Is' (or the other way round), if that makes sense?