Yes it will be very obvious in a darkened room, watch the crease area of the plate under the side getter, at 90 degrees to the part of the plate that has holes in it. That's where the heat is concentraited.

I am trialling a couple of USB fans which seem to be helping - they are silent, and I guess extra airflow over the tubes and tranformers can't do any harm. I'm up to 95ma with no apparent overheating.

In order to replace the KT120's with EL34's or 6550's (leftovers from my Willsenton R8 days), do I need to do anything other than change the bias to the appropriate setting? And how much influence on the sound do the driver and rectifier tubes have? Is it worth trying alternatives?

Cheers, Stephen

I honestly haven't tried EL34 tubes yet in mine, I would just caluculate the cathode current/voltage to stay under their 25W limit and go for it :)

Thanks Stephe - would it be ok to use a 470k 1w bleeder resistor, and a 10R 1w 5% test point resistor just until the correct ones arrive?

The resistor will have 0.6W across it so should be OK at 1W for temp use but for long term reliability 2W would be safer. And 5% resistors are OK, I would just try to get them as close to 10ohms as possible.