[Amps] QSK

[W2RU - Bud Hippisley]

Although I've used QSK from the very beginning of my on-air activities a 
half-century ago, and feel like I'm blindfolded if I don't have it, I've 
known many excellent operators who didn't use it.  A prime example is 
W1AX (ex-W1JYH), who is one of the best operators I've ever had the 
privilege of meeting, and he never had QSK during the years that he and 
I operated the same contests.  What he _did_ have, instead, was a foot 
switch-driven PTT line, a nimble foot, and the "smarts" to lift his foot 
frequently to make sure he wasn't "stepping on somebody" (so to speak).

The technology of QSK is an interesting issue.  As a young Novice I used 
a separate antenna for my receiver.  It was vacuum tube equipment, of 
course, and my transmitter only ran 15 watts output, so I never had to 
worry about blowing out the front end of my receiver.   In college I 
built up one of those 6AH6 units that went inside my transmitter, where 
it hung off the high impedance end of the plate tank circuit with a 
coaxial cable capacitive divider.  One bonus with that setup is that you 
could (and still can) quickly tune your final by listening to the band 
noise in your receiver.

When finances allowed, I bought a "pre-owned" Johnson TR Switch, and 
I've used those ever since.  In fact, I have four of them here.  About 
the only improvements I'd make to them today is to add a dB or two of 
gain on 10 meters, extend their "official" operating range down to 160 
meters, and redesign the power transformer to run a lot cooler.  The 
6BL7 inside is a pretty rugged tube.  My antenna tuners and the natural 
selectivity of my dipoles and Yagis keep the out-of-band signal levels 
down low enough that I don't suffer from cross-mod or blocking problems.

Early on, I had to modify all my HF gear to be able to run QSK.  Whether 
Hallicrafters or Collins, the exciters needed to have an additional 
mixer keyed so that there was absolutely no feedthrough of mixed local 
oscillators during key up periods.  In contrast, today's transceivers 
all handle QSK pretty well -- up to the 150 or 200-watt level, at least. 

Amplifiers are another story.  Over the years I've modified every 
amplifier I've ever owned (Collins, Heath, Hallicrafters, Amp Supply, 
homebrew, etc.) with Zener-diode biasing schemes (sometimes keyed) to 
cut off the plate current during key up periods.  This eliminates any 
shot noise or partition noise at the output of the amp where the vacuum 
tube TR switch input is connected, and has the side benefit of reducing 
heat generation when operating CW.

I currently own an Amp Supply LK-550 with a factory vacuum relay QSK 
system installed but I've never used this built-in QSK hardware because:
     a.  It would probably make too much mechanical noise for me; and
     b.  The key line that has to run through the QSK circuit (to 
provide proper time sequencing of the amplifier vs. exciter) doesn't 
function when the key down voltage on the key line is a Vsat above 
ground.  I haven't had time to rebuild the solid state outputs of my 
various el-keyers to offset their outputs to 0.0 volts or below; and
     c.  There's no (easy) way to get the output of the el-keyer in my 
transceiver to the QSK system in the LK-550.

Another factor for me is that I am totally opposed to keying any 
unnecessary relays at a CW rate except for the high speed relays in my 
transceivers that I can do nothing about.  I know there are PIN-diode 
QSK boxes out there, but I've heard of too many failures in them to be 
interested -- yet.

I think many folks avoid QSK today because of the economics.  I have in 
the back of my head that adding a manufactured QSK option to most 
commercially available amplifiers is around $400  -- provided you can 
afford an amplifier that _has_ a QSK option available in the first 
place.  My primary amplifier is a Heath SB-220, so I'd have to get an 
external QSK box that costs about as much as the amplifier itself is worth.

But the truth is -- if you feel competent making simple mods to the dc 
bias circuitry in your amplifier, you can have noiseless QSK for far 
less than the cost of a commercial QSK box or a new QSK-enabled amplifier.

That's why I keep on truckin' with my vacuum tube TR switches.  Johnson, 
B&W, Handbook 6AH6 -- doesn't matter, they all do a comparable job 
except I'm not real keen on having to separately bandswitch a TR switch, 
so there's no bandswitched B&W units here.

Bud, W2RU