>One idea I had was to make an 'artificial'
>centre-tap by connecting a low-value wirewound resistor to
>each side of the filament, and take the HV- to where they join.
>(checking that not too much filament AC current flows through
>these 2 resistors)
The problem with that is that the bias will vary with anode current, unless
the resistors are very low value. What's the tube?
Another possibility is to use a centre tapped choke across the filament: it
could be the secondary of a fairly small filament transformer, with the
primary left open - or used for a bias supply. The secondary would need to
be rated for the current. If the primary is left open, then the secondary
current rating would need to be (for safety) the same as the plate current.
If you draw current off the transformer, then it needs to be higher. As the
dc would flow in opposing directions in the windings, core saturation
shouldn't be a problem. Then you can put the cathode current meter between
centre tap and ground. Or to measure anode current, put the meter between HV
negative and ground, with a silicon diode across the meter so that if the
meter goes open, the diode clamps the voltage. It shouldn't affect the meter
reading if it's a standard 100millivolt drop meter.
73
Peter G3RZP
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