[AMPS] PS question

[Peter Chadwick]

Bill N2BC says:

>The simple cure for this is to place it in the CT minus return. 

The capacity of the winding to frame can be effectively slightly different,
which can alter the resonance. I've been using tuned chokes in the negative
lead for years with no problem. In my present amp, I actually rectify the
volts across the tuned choke in the screen supply with a 6X5GT (there must
be some others old enough to remember them!) to get the negative bias
supply.

The argument against negative lead filtering was always that the capacity of
the plate xfmr to ground appeared across the choke, bypassing the choke's
effectiveness for higher frequencies. See the old ARRL handbooks - the 1970
edition comments that this only really applies to speech amplifiers, VFO's
and low level stages, where the low voltage allows positive lead filtering
anyway.

They add a caveat of providing plenty of insulation, since in the event of
the choke going open, the negative supply floats up to HV. In those days,
they thought mainly in terms of full wave bi phase rectifiers rather than
bridge, but the principal is the same.

	>There must be drawbacks to placing the choke in the minus side - or
>all HV PSs would be built that way

Old Spanish custom? If you have two chokes, and a capacitor from the
junction to HV, both sides of the capacitor are live. Maybe that's a
reason....

73

Peter G3RZP



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