[Amps] 8877 HF amp cathode wiring
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
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.
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.
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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> Amps at contesting.com
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