One point I have not seen mentioned about this as yet....When you bypass
the heater to ground on the 8877 the cathode and heater will be at a
different potential by the amount of RF voltage driving the cathode.
Over the years I have seen several 8877's develope a heater to cathode
short when over driven or a bad load is presented to the amplifier with
normal drive applied. By using a tri-filer choke on the heater and
cathode you can keep them at the same RF potential by coupling the rf
drive to both. By doing this AND having a grid over-current trip on my
8877 amp I have never lost a tube through a lot of years of DXing and
contesting with guest ops.
Good Luck with your 8877 amp project Jon...it is a GREAT tube!
Jon Ogden wrote:
> Was looking at my 89 handbook at some of the amplifier designs to look at
> what folks were using for filament chokes. Starting on page 30-50, there
> is a design for an 8877 amp. In looking at the schematic, the guy uses
> NO filament choke. The two filament lines are both bypasssed to ground
> by .01 uF caps. From what I can tell the filaments are on pins 1 & 5 of
> the tube and the schematic says that pins 2,3,4,6, & 7 are the cathode
> input. Yes, it's grounded grid.
> Is the 8877 set up this way so that the filament pins are isolated from
> the cathode? Is that why he's able to get away with it? I'm ignorant of
> how the 8877 is set up.
> Just curious.
> Jon Ogden
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