Pete Farrington
Well-Known Member
Maybe consider the 290MAEX PT https://www.hammfg.com/files/parts/pdf/290MAEX.pdf
That gives a lot of flexibility, HT voltage can be tweaked via the use of alternative taps on the HT winding / the use of alternative rectifier types (including the 3A heater of the 5U4).
Perhaps aim for an idle HT in the range 400-450V with KT66, 350-400V with 6V6.
If using matched valve pairs, then a shared 270-300ohm cathode bias resistor should be a reasonable starting point for either.
I suggest to include a bias supply and a cathode-fixed bias switch, just so you can be sure that cathode bias is the best option (lots of ideas I obsess over end up being a bit naff when I actually try them out
).
Just reviewed post #1, and it’s worth noting that regular fixed bias JTM45 are lucky to manage 30W at clipping with Chinese KT66.
That gives a lot of flexibility, HT voltage can be tweaked via the use of alternative taps on the HT winding / the use of alternative rectifier types (including the 3A heater of the 5U4).
Perhaps aim for an idle HT in the range 400-450V with KT66, 350-400V with 6V6.
If using matched valve pairs, then a shared 270-300ohm cathode bias resistor should be a reasonable starting point for either.
I suggest to include a bias supply and a cathode-fixed bias switch, just so you can be sure that cathode bias is the best option (lots of ideas I obsess over end up being a bit naff when I actually try them out
).Just reviewed post #1, and it’s worth noting that regular fixed bias JTM45 are lucky to manage 30W at clipping with Chinese KT66.
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They are indeed, as mentioned by 
