[Amps] 8877 HF amp cathode wiring

Jim Thomson jim.thom at telus.net
Tue Aug 21 03:03:54 EDT 2018

##  Fil leads are bypassed..right at the socket.  Method number 1 was used, 
even though a bifilar was
already installed.   Buddy is going to ground the CT.

## I have tried EBS in the past, but gave up on it in the end.   It appears 
to be almost quick enough
between  words.....but not quick enough between syllables.  On ESSB mode,  a 
tiny bit of reverb is used,
and the tails  will get cut off with most ebs schemes.   About the only time 
any heat savings  results is
between  when you stop talking..and when the VOX drops out..which amounts to 
very little.
I used a small  SPST  vac relay, like a K41,  to shunt out the cut off 
resistor...on TX.   I had a bunch of them, so used them.

##  The original EBS schemes  back in the 70s, used a simple  bi-state 
arrangement, and tube cut off with no rf drive
applied.  It was used  for  break in cw.   Later versions  were  tri-state. 
Cut off on RX, then a small amount of idle current
with amp keyed, but no drive applied, then normal, more  idle current, when 
drive applied.  Where it does work is when using
PTT modes.   But I seldom use  PTT, nor QSK.  The amps will work on qsk 
mode,  but I am not a qsk operator.  Listening to
band noise  between  dots and dashes drives me nuts.   I seldom TX anymore 
on CW... but listen  on cw now and again.

##   So to keep things simple,   I use a vac relay on the  input side, 
output side, and the 3rd one for the bias.   The amps in question
handle full power..when on bypass mode.      I can vary the bias voltage on 
the fly, so just set the idle current lower on CW mode,
and higher on SSB.

Jim   VE7RF

-----Original Message----- 
From: MU 4CX250B
Sent: Sunday, August 19, 2018 5:20 PM
To: Jim Thomson
Cc: amps at contesting.com
Subject: Re: [Amps] 8877 HF amp cathode wiring

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 <jim.thom at telus.net> 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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