[Amps] 8877 HF amp cathode wiring

[MU 4CX250B]

I agree, Jim. Either way works fine. Re arcing from cathode to
filament: I don’t see why the cathode voltage would ever soar above
the tube’s  cutoff bias, which in my 8877 amps is about 28V. An
internal arc in the tube would probably cause a current pulse into the
grid, not the cathode. In effect, the grounded grid would shield the
cathode.
73,
Jim w8ZR

Sent from my iPad

> On Aug 19, 2018, at 9:53 PM, Jim <jimw7ry at gmail.com> wrote:
>
> 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 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
> 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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