Glitch protection diodes in the Drake L-7

As part of my first foray into the world of 3-500Z amplifiers, I have been reading everything I can find on the internet about them. This includes articles and discussions of various mods, some good and some not so much so.

One article in particular caught my attention. It can be found on Rich Measures', AG6K, web site. The article details adding "glitch protection" diodes to the metering circuits of high power amplifiers to protect the meters and other components from high voltage damage during fault conditions. You can read Rich's article by clicking HERE. This protection scheme is directly applicable to the Drake amplifiers. It seems like a good idea to add this protection to the Drake L-7, as this amplifier has been out of production for years and replacement meters are difficult or impossible to find.

It turned out to be relatively simple to add the diodes to the L-7. Since a picture is worth a thousand words, here is how I did it (click on the picture for a larger view):

WARNING: The voltages present in this amplifier are LETHAL, and can cause serious injury or death. Strict adherence to the safety precautions printed in the operator's manual are necessary for YOUR protection. If you do not feel comfortable making this modification yourself, take the amplifier to a technician who is qualified to service high voltage equipment.

The two series diodes were attached directly across the plate current meter by soldering them to the solder lugs that connect the wires and bypass capacitor. Be sure to at least loosen the meter nuts, or remove the lugs entirely from the meter terminals, to prevent melting of the meter case from soldering heat. The black wire on the right connects the third diode that protects the grid current metering circuitry to the string. The anode of the grid meter diode was soldered to the existing ground lug under the left-hand screw that attaches the pushbutton PCB assembly to the front panel. I used 1N5400 series diodes, as recommended by Mr. Measures and others.

That's all there is to it! For a couple of bucks worth of diodes, and a few minutes' work, the meters are protected from disaster. This protection scheme is also applicable to the L-4B. I will add that information to this page after I get mine modified. Stay tuned!

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