[Amps] 8877 HF amp cathode wiring

Jim jimw7ry at gmail.com
Sun Aug 19 22:53:58 EDT 2018

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.


Jim W7RY

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Thomson
Sent: Friday, August 17, 2018 4:27 PM
To: amps at 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
.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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