Date: Sun, 10 Nov 2013 07:58:21 -0700
From: "Jim Garland" <4cx250b@miamioh.edu>
To: "'Larry Benko'" <xxw0qe@comcast.net>, <amps@contesting.com>
Subject: Re: [Amps] Best source for Gigavac GH-1 ?
## the correct device to open off B+ is a vac contactor..... but alas, they
are way too slow anyway. The gigavac GH-1 is a superb relay..and way better
built than either the jennings RJ1A or kilovac versions. Most GH-1s will
easily
hi pot test to 7500 vdc and higher. Note, you can get em in either 12 vdc OR
26.5 vdc coils. So if you are voltage limited...and want to use the typ speed
up circuit....like a RC hotshot setup... which is just an overvoltage on the
coil,with
a drop resistor plus cap across the drop resistor, then its easy to do.
## I changed the simple half wave rectifier ..which is 27 vdc, on my 4 x drake
L4B
amps to a FW doubler..and ended up with 63-67 vdc no load. That’s plenty
for even a 26.5 vdc vac relay coil. Even alpha used only 35 vdc + a RC hot
shot
circuit on their older amps....using a 26.5 vdc coil. But the 12 vdc coil on
the
gigavac GH-1 works superb in qsk mode..with as little as 18 vdc, drop resistor,
and cap across the drop resistor.
## Yrs ago, Kilovac informed me that their results of using the typ speed up
using an
overvoltage + drop resistor on some of their relays. They also informed they
had also
tested the 12 vdc coil versions of their relays in the lab....and mentioned the
12 vdc
coils were a good option for folks who are stuck with only 26 vdc..and still
want QSK/.
## BTW, the gigavac G2 and G2 ham relays will also run full qsk with the RC
hot shot circuit.
They are rated at 15 msecs operate..and 9 msecs rls. When sped up with 130
vdc on their
26.5 vdc coils..and using a .5 uf cap across the drop resistor, BOTH the
operate and rls times
are 2.5 msecs. And as high as 2.7 msecs for the 2nd relay we had tested.
And that includes contact bounce
too. The G2 is rated for 10A CCS at 32 mhz. Parallel 2 x G2s....and then
its good for 20A at 32 mhz.
That’s 20 kw CCS at 32 mhz...into a 50 ohm load......AND qsk too! The
paralleled vac relay concept
works very good. 3 x relays in parallel was also tested..with superb
results..and a 30A at 32 mhz rating.
Of course the V rating wont change, but the peak V rating at 32 mhz and lower
is already sky high vs
the smaller GH-1 .
## Just sequence the entire mess correctly. I also use high swr protection on
the amps, via
the array solutions wattmeters..set for 2:1 swr. I also installed an
adjustable spark gap directly across the vac
LOAD cap in my hb amps. Its set to fire at a voltage that is higher than a 4:1
swr.... but well
below the vac load caps rating..and also well below the peak V rating of the
vac relays.
## How many G41 relays have you tested?? Something is amiss if you are
seeing 20 contact bounces.
Jim VE7RF
Interesting discussion, re vacuum relay failure modes. I've zapped two RJ-1a
relays over the years, but both times it was my fault. The first time was in
the 1980s, when I used one as an HV interrupter in a power supply, not
realizing they're not designed for DC load switching. The second time was in
the 1990s, when I used one to switch an extra capacitor in parallel with
the tune cap on an 8877 amplifier, exceeding the relay's voltage rating.
The failure mode in each case was to weld the NO contacts together.
I've used RJ1a relalys in QSK circuits for many years, including a half
dozen contests each year, but nothing like the contest involvement some
others have reported. During the past year, I've gotten interested in QSK
circuits for a homebrew project and made many tests on a variety of vacuum
relays. Most types, I've found are not well-suited for QSK use, either
because they're too noisy, too slow or, more commonly, have excessive
contact bounce. For example, the Gigavac G41, used in the Alpha 9500, can
take 7-10 mS to settle down and can have twenty or more contact bounces when
the NO contacts make. The problem, I believe, is that the coil produces too
much magnetic force on the movable contact and really slams it into its
mating contact. If the bouncing happens while hot-switching RF, the relay
will have a very short life. So-called "speedup" circuits, that goose the
relay coil with a voltage pulse to accelarate the switching time, can really
exacerbate the contact bounce problem. As I mentioned before, the Jennings
RJ1a appears to be the best relay for QSK use at amateur power levels
(though I've not checked its Kilovac or Gigavac clones). The RJ1a is not
only very fast (1-2 mS closing, and 500uS opening times), but has absolutely
no contract bounce.
In researching the subject during the past year, I've looked at the relay
circuits of a number of commercial amplifiers. Most seem to treat T/R
switching pretty cavaliearly, without adequate protection for the relays. In
my own design (the circuit mounts on a 3"x5" PCB), I've tried to prevent any
situation that can damage the contacts. This involves several precautions:
(1) sequencing the amp so the output relay, bias switch, and input relay
operate sequentially in that order, after allowing for the relay closing
times and contact bounce. (2) Having an RF "lockout" sensor that prevents
the amplifier switching sequence from beginning if RF is present on the
input line. (3) Having an RF "lockon" sensor that keeps the relays closed so
long as RF is present. My new amplifier completes its T/R switching in 4 mS,
which is suitable for QSK speeds up to about 60 wpm, far beyond it's
builder's capabilities.
73,
Jim W8ZR
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