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@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@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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