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I've checked what the transformer is putting out, which is +/- 343V with no load, and +/-330V with everything running normally. Pretty much in line with the manufacturers specs. I have a manufacturer drawing for the transformer and its rated for 240V on the primary side, with no options for other voltages. They state the secondary voltage as +/- 330V at 138mA load.
So I think everything is ok on the tranny side and I still think its an overperforming rectifier. Apparently the Sovtek 5Y3 is an 'improved' design, with an indirectly heated cathode (presumably the original 5Y3 had the cathode and filament all-in-one), with the result being that it produces more volts than it should (ie doesn't behave like a 5Y3 should).
The higher voltages aren't too much of a worry at the moment. One thing I've learnt from this is that even with the amp running at nominal levels the quiescent anode power dissipation on the 6V6s would be way above the 12W 'safe' limit that is stated for these valves. This amp design is well-known for this. With the voltages on my amp, the power dissipation looks like it is about 20W! So I'm a bit concerned about red-plating, but haven't see any signs of it yet. Apparently 6V6s are known for being able to take quite a bit of abuse........
So next steps:
1. I've ordered the bits to do a resistive quarter-power reduction mod as per the Rob Robinette site.
2. Try a different 5Y3.
3. Get off my backside and start on a cabinet.
4. Fork out for a speaker.