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):
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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