Tom says:
>It affects the anode and grid current sourced by the filament, by
>changing the BIAS voltage from each area of the filament.
OK, agreed.
>IMO, it's a poor idea to have one end of a directly heated filament
>string at -5 volts below the reference bias, while the other end is
>at +5 volts, and reverse that bias with the power line with a tube
>that requires nearly zero bias.
I have seen it claimed that 6A3s were worse on hum than 2A3s in audio
amplifiers for exactly the same reason. Which is why it's better to
ground the centre tap rather than one side of the filament.
I guess the bottom line is that it's another thing to consider in zero
bias, but isn't so important when running a tetrode with 40 or so volts
of bias on. In any case, there's other things to consider - like
minimising inrush current.
73
Peter G3RZP
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