The choke in the filament circuit is there so that the cathode can be tied
to one side of the filament. There is not much isolation between cathode and
filament voltage wise on this tube. An arc from plate circuit to ground
drives the negative side of the power supply high with respect to ground.
This can allow the cathode to arc to the filament and ruin the tube.
Floating the filament without the cathode tied to one side will help but you
still run the risk of the filament transformer arcing and then having an arc
to the cathode.
If you do separate the cathode from the filament be sure that you have large
protection diodes from the negative side of the HV supply to ground.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Roger D
> Johnson
> Sent: Saturday, October 05, 2019 8:54 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Cathode Choke
>
> I may be wrong but the old memory banks came up with two possible
> reasons
> for the choke:
>
> 1. To prevent heater to cathode voltage breakdown
>
> 2. To prevent the heater to cathode capacitance from shunting drive
> power to
> ground.
>
> The first is probably not a factor in a low drive tube like the 8877.
> The second
> might only be at problem at VHF or above.
>
> 73, Roger
>
>
> On 10/3/2019 3:47 PM, Roger Parsons via Amps wrote:
> > I am attempting to repair a PT3000 amplifier for a friend. (This is a
> prototype of a design that never went into production.) The amp uses an
> 8877 in grounded grid.
> > I am puzzled about the cathode choke, as the cathode and heater are
> connected together for RF, all three connections go through the choke,
> and the heater is fed from DC. This has the undesirable effect that
> because of the voltage drop in the choke something over 6V is necessary
> to give the proper heater voltage. And the DC supply has broken and I
> want to replace it with a transformer....
> >
> > I can't see any advantage in doing things this way, but I may be
> missing something. FWIW some other amps I have worked on have kept
> heater and cathode separate and have worked fine (MLA2500 as original,
> and as modified by me to use a 3CX800, an ex CBC 1kW FM broadcast
> transmitter (8877), and as modified by me to be an HF Class AB1 amp.)
> I'm sure that other examples can be found for both techniques.
> > Is there a good reason?
> > Thanks
> > RogerVE3ZI
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|