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Re: [Amps] 8877 HF amp cathode wiring

To: Jim Thomson <>
Subject: Re: [Amps] 8877 HF amp cathode wiring
From: MU 4CX250B <>
Date: Sun, 19 Aug 2018 17:20:26 -0700
List-post: <>
I prefer method 1, except I'd ground the CT of the film xfmr, while
also bypassing the fil leads. For an 8877, I drive don't see any need
to couple the cathode to the fil. The lack of bifilar fil chokes is a
plus, of course.

I've used both methods, however, and don't see any difference
performance-wise. The high-value resistor for self-biasing the tube
(shorted out on xmit) is a bit dated design, since it's not
well-suited to EBS switching (or QSK), which I prefer.
Jim W8ZR

Sent from my iPhone

> On Aug 17, 2018, at 4:27 PM, Jim Thomson <> wrote:
> 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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