I have found problems when using high impedance supplies with 6146s,
especially as they get older. Especially bad is the Yaesu 'anti flash
over diode' in series with the screen grid - any negative screen current
pushes up the screen volts, and the anode current runs away, with
disastrous results for the valves.
Grid bias and screen supplies should either both be stabilised, or
neither. If neither supply is stabilised, variations in supply volts tend
to cancel. I prefer to stabilise them both, and keep the supply impedance
(at low frequencies, including zero frequency) low. I'm still using the
original 6146Bs in my FT102, and after 20 years of hard work, they're still
fine.
73
Peter G3RZP
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