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
|