Re: [Amps] Cathode Choke

[Jim <jimw7ry@gmail.com>]

Spark gaps on the filament and cathode to ground also work well. 
Something rated to conduct at 50 volts will do the trick.

Thanks
73
Jim W7RY

On 10/5/2019 6:34 PM, Gary Schafer wrote:
> 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
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