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High Speed Switching (QSK) For the Drake L4B Amplifier

Written By: Garry Drummond – K4OR


I became interested in Full Break-In keying while operating CW during Field Day with the local radio club where an IC-7800 was at my disposal.    After a short period of time, I fell in love with how smooth the 7800 operated in this mode.   A while later, I ran across an article while doing an internet search, written by Rich Measures, AG6K, called High Speed Switching for Grounded Grid amplifiers.   This immediately caught my attention and seemed like a great modification for the L-4B.   Even if I didn’t continue to operate QSK, it seemed like a great way to silence and speed up the keying of a very fine amplifier.   

I searched the internet for any articles where an L-4B had been modified with this circuit.   I came up zilch! I put out queries on the various amp reflectors and still got no response though I did find a few SB-220’s that had been modified with this circuit.

Here is my adaptation of Rich’s QSK circuit.   I also incorporated an Electronic Bias Switching circuit which uses a zener diode and pass transistor to lower ZSAC.   Rich sells the individual parts which includes a reed relay but not the vacuum relay.   The parts kit does not include a circuit board or instructions, but does include a schematic diagram with good notes.   I cut a piece of perf board to fit the space and mounted the parts as shown.   I used two pairs from a length of CAT5 Ethernet to run from the board to the reed and vacuum relays.   I bent the component leads over and solder them together under the circuit board.

I had modified my L-4B previously to accommodate a Harbach Electronics step-start relay so I no longer had the +120 VDC line into the RF deck from the power supply.    So I used the transformerless power supply using the supplied parts to power the circuit.   This gave a voltage of 170 VDC at 80 ma across the key line which was higher in voltage than my IC-746PRO could key.   So I built a circuit designed by my good friend, Tom - W4NGW, using two transistors and four resistors as a buffer stage between the accessory jack on rig and amp and it works perfectly.


Anyone interested in this circuit should check out Rich’s website, www.somis.org and reference Figure #7. Any questions for me, feel free to contact me at k4or@arrl.net.


Garry Drummond – K4OR