For what it`s worth,I had a filament hum/ac modulation problem several years
ago after constructing a GS-35b using conventional center tap negative.I
cured the problem using a seperate choke to the cathode side, and not using
the ct on the filament transformer.Since then I have constructed a YC156/
3CPX5000A7 and used half of the filament choke feeding the cathode side with
success and no reports of any hum/filament ac modulation. I did try a
seperate choke with the same results.Testing was done with a ham friend
close to me listing for any hum with no voice modulation.No diffrence in
signal or presence of hum was detected with amp in line or on standby,and on
air reports world wide have been very gratifying. With 6400vdc/36v bias the
zsac is 250ma and the tube behaves very well with close to 60% efficiency
20-80 meters. 73,Jim K7RDX..
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: <amps@contesting.com>; "John T. M. Lyles" <jtml@lanl.gov>
Sent: Thursday, July 13, 2006 12:10 PM
Subject: Re: [Amps] high frequency filament excitation TSPA
> 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.
>
> 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
>
>
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