[CW] CW Keyer Interface

[Donald Chester]

> Does anyone on here know of an available keyer interface (built or in kit
form) to interface 
> a modern ss keyer to a cathode keyed transmitter?  Something good for 800
volts @ around 
> 300 ma or more? I'd also use it with a semi-automatic bug to get the high
voltage and current 
> off of the bug or straight key.
> All my modern keyers around here are OK with QRP rigs and the newer
transceivers, 

> but they can't handle cathode keyed transmitters. Right now I'd like to
use my Eldico TR-75 TV transmitter as I had one as a kid > back in 1953.  I
think I need an interface to use my keyer.

> Anyone have any suggestions.?

> Lee, w0vt

I homebrewed mine using a TV sweep transistor, a stripped-down version of
the circuit described in the 1981 ARRL handbook, page 11-2. You should be
able to find the same article in other editions, give or take a few years. A
detailed write-up of my circuit, including schematic, appears in August,
2014 Electric Radio, page 6.  I used an ECG 238 as the keying transistor,
driven by an ECG 129, purchased at the local electronics store. The ECG 238
was a little pricey when I bought it, but it will handle up to 7 AMPERES of
cathode current at 1500 volts, leaving plenty of safety margin for about
anything I would ever want to run. I built it in the mid-1980s, so I don't
know if those transistors are still available following the demise of
picture-tube TVs, but if not, suitable substitutes may be available or the
sweep transistor could be salvaged from a junked TV.

Mine works very  well with my bug or the electronic keyer I built up around
one of those little Curtis IC chips.  I use it with both my homebrew
transmitters, in which the keyed stage runs about 40 ma at 600 volts, and on
160m with my converted Gates broadcast transmitter that uses a pair of 807s
in the driver stage, 650 volts at 200 ma.

I didn't find the R-C key click filter described in the Handbook article
very effective, so I added the standard key click filter for cathode keyed
tube type transmitters, using a choke, capacitor and resistor as described
in all the Handbooks from the 1930s through probably the 90s or early 2000s
(I don't find much useful information of any kind in the later editions).
The filter is inserted between the sweep transistor and the cathode of the
keyed stage.   At first, I was a little worried about inductive kick-back
from the choke destroying the keying transistor, but that hasn't been a
problem.  The R and C components in the filter circuit appear to take care
of that, plus the voltage rating of the transistor is probably high enough
to withstand any spike that  would ever occur. 

One problem I have encountered with cathode keying an intermediate  stage in
a couple of my transmitters is back-wave, when only one stage was keyed. In
one of my homebrew transmitters I key a single type 802 tube, and got rid of
the backwave by neutralising the 802.  In the Gates, with a pair of 807s
driving a pair of 833As in the final, I key the 807s along with the 12BY7
stage that drives them and the backwave disappeared.

Don k4kyv




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