On Jul 13, 2006, at 12:10 PM, Tom W8JI wrote:
> Another problem people commonly miss is voltage drop in the
> filament choke.
>
> A normal properly sized filament choke has a few tenths of a
> volt AC drop across the windings from filament current.
>
> This is meaningless in a center tapped filament transformer
> feeding a directly heated tube, but when one side of a
> heater is connected to the cathode you either need a
> hum-compensating pot at the DC return for the cathode, you
> need a third winding only for dc that ties to the filament
> pin common th the cathode (a trifilar choke), or an entirely
> separate dc return choke that does NOT carry any filament
> current.
>
> It's surprising how little ripple voltage in bias at the
> cathode can cause hum on the signal.
>
> By the way, I see quite a few Internet pages on constructing
> amps with Russian tubes that ignore this problem. It's also
> a problem with the 3CX5000, 3CPX5000 series of
> heater/cathode tubes that have the cathode tied to one side
> of the heater. You can't run the cathode DC back through the
> same choke winding as the heter uses without inducing some
> unwanted hum. The only exception is when the transformer end
> has a hum-balancing pot.
Another solution is to use a separate RFC to carry the DC HV negative
directly to the cathode so that there is no DC in the heater's
bifilar RFC. Since the cathode Z is 50-ohms, only c. 30uH is needed
to cover 160m - 10m.
>
> I know a fellow in Mississippi who worked on his 3CPX5000
> for weeks and never figured that out. His solution was to
> run a filtered DC supply!
>
> 73 Tom ...
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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