[Amps] 8877 HF amp cathode wiring
4cx250b at miamioh.edu
Sun Aug 19 20:20:26 EDT 2018
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.
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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