[Amps] Switching off Drake L-4B occasionally trips mains RCD

[Alan Ibbetson]

As is common in the UK, the mains supply for my whole house is protected 
by an RCD (see http://en.wikipedia.org/wiki/Residual-current_device for 
a description). I think they are called GFCIs in the US.

Every once in a while, say 3 times this year, this RCD trips when I 
switch off my Drake L-4B amplifier. Murphy's Law dictates that this will 
always be at night, so the house is plunged into darkness, and in the 
middle of the XYL's favourite soap, so I am plunged dans la merde.

Tonight was one of those nights and I'm seeking help on finding the 
cause and curing it. Reproducing a fault that only occurs a few times a 
year is going to be problematic, so I wonder if anyone has any 
hypotheses as to what is happening?

For those unfamiliar with the L-4B, it only uses a single pole switch 
for the mains power, can be wired for 115v or 230v AC with jumpers on 
the transformers, and uses the tapped primary of the filament 
transformer as an auto transformer to supply 115v to the blower when 
operating from 230v mains. I had problems with heat with the stock 
blower so I fitted a Dayton 4C761 as advised on this list. I always have 
the mode switch on the lower HT CW/Tune position when I switch off. I 
don't know whether any of this is significant, I'm just trying to 
provide you all with data.

Ideas anyone?
-- 

Thanks,

Alan G3XAQ
alan at g3xaq.demon.co.uk