Hi,
New Vintage Modern 100W and already need advice, so first post!
I recently acquired a second hand 2466 head in excellent condition. It's from 2010 so has twin bias adjustment pots, and the valves appear to be stock branded Marshall's. I've had a few hours playing time and as a matter of course decided to check the bias. I have a JVM 410 head and have successfully biased the amp several times, so have a reasonable idea of what to do but have no experience with KT66's.
I have read a few posts and the FAQ on the Vintage Modern so knew the plate voltage for the KT66 is high on these amps. However, I have a few questions regarding plate voltage:
1) As a matter of interest does anyone know why the plate voltage is so high on the 2466?
2) I live in the UK, and in my local area, the mains voltage is high with a norm of around 245v and have measured voltages in excess of 250v at certain times. I checked the bias when the mains input was at 244v and measured a plate voltage of 543v and a bias at the test points of 86mv for the left pair and 88mv for the right. Now if my calculations are correct, this works out at 96% of max plate dissipation (25W for the KT66). My only experience is with EL34 valves, where 70% is considered the max of plate dissipation and I'm assuming this value is considered the max for KT66's. I use 65% for the EL34's in the JVM to allow for the possible high mains voltage.
Now to my problem. If I bias to the worst case scenario of 250v input voltage and for 65% of max plate dissipation, then to achieve the correct bias of 0.058A pushes the plate voltage up to 565v which seems a bit high to me.
So, in my case is it better to run at a high plate voltage to keep the max dissipation to 65%, or, run at a higher bias current giving a higher max dissipation (75%+) but a lower plate voltage? In either case I can't see it being possible to get the plate voltage much below 560v.
Any advice, comments will be much appreciated.