I've done it both ways and either seems to work fine. There are 2 schools of
thought on connecting the filament to the cathode to keep arcing from
happening between the filament and the cathode. I've used spark gaps from
filament to ground to prevent this.
And I've not used them. Since the 8877 has no issues with parasitics, any
of the methods have worked well for me.
I run 8877 amplifiers at 1500 watts output on RTTY contests on all bands.
73
Jim W7RY
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
Sent: Friday, August 17, 2018 4:27 PM
To: amps@contesting.com
Subject: [Amps] 8877 HF amp cathode wiring
Helping a buddy with a HF hb 8877 amp.
It appears there is 2 ways to wire the cathode assy.
What I suggested.. is fil leads to sec of fil xfmr. CT of fil xfmr not
used.
.01 uf cap from each fil to chassis..right at the socket. Drive applied to
cathode terminal on socket..via a coupling cap.
Cathode goes to one end of a 28 uh inductor. Other end of same inductor
goes to series bias diodes.
Bottom end of series diodes goes to resistor used for cut off bias on RX.
Shorted out on TX.
Bottom end of cut off resistor goes to the positive terminal of BOTH
the plate and grid meters.
Negative terminal of plate meter goes to the B-. Negative of the grid
meter goes to chassis.
Does this sound ok ? This is one way it is done in orrs last handbook.
2nd method is as follows. Bifilar used on fil leads. .01 uf caps from
cold sides of bifilar to chassis.
Drive applied to cathode..via a coupling cap. .01 uf cap from hot side of
each fil back to cathode.
28 uh choke wired to cathode. cold side of same 28 uh choke wired to
series bias diodes.
Cold end of bias diodes to RX cut off resistor. Cold end of cut off
resistor wired to the positive terminal
of both plate and grid current meters. Negative of grid meter to chassis.
Negative of plate current meter to B-.
There is no grid over current protection in this amp..... only a fast 3
agc small current value fuse between negative term
of grid meter to chassis. Some real grid over current protection may
be added at a later date.
Right now, we used the 1st method. ( amp has not been tested...yet). On
paper, either will work. The 1st method does not require a bifilar.
And neither method requires the CT of the fil xfmr. B+ supply is being
rebuilt. But the RF deck can be tested with the usual plate load
resistor temp wired from anode to chassis, and MFJ on output side..and all
vac relays operated.
Tuned input will also be tested with drive applied... but shows flat
swr, after tweaking..into a 50 ohm dummy load.
Bypass SWR on RX is also flat.
Jim VE7RF
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