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Solving High Voltage Connector Problems with the Drake L-4B and L-7 Amplifiers

Written By:  Evan Rolek / K9SQG

When you own a Drake L4B or L7 linear amplifier, you may soon come to realize a shortcoming which has potential safety implications. What I'm referring to is the infamous Millen high voltage connector, specifically the male/cable assembly. While the connector design is basically good and has been used in many pieces of equipment over the decades, there is a weakness as used with the Drake amplifiers. But don't throw out that amp quite yet!   There is hope!

For those of us that own these fine amplifiers, we might experience that the attachment of the high voltage wire to the Millen plug is prone to breakage, especially if the amplifier, power supply, or cable is moved. It doesn't take a lot of movement depending on how much flexing occurred while the amp was treasured by current or previous owners. The angle with which the wire attaches to the plug puts a strain on the stranded inner conductor. Flexing of virtually any part of the cable can aggravate the strain. Removal and replacement of the cable for servicing, relocating, or shipment further add to the wear on this vulnerable connection. Too, when screwing the connector shell on or off, there is some slight binding with the insulation causing movement as well. Having redone the connection on several amplifiers that I have serviced, including my own, I realized that repairing the connection only made things worse! Why is that?

When you redo the connection after a failure (a broken wire), you have to remove the piece from the center pin of the plug, then strip, tin, and resolder the wire from the power supply. This results in an even shorter wire, by about a half to three quarters of an inch. This, in turn, puts even more stress on the HV wire, and increases the amount of binding and movement when the outer shell is turned during installation or removal. I thought to myself, "There has to be a better way!" And there is. It's relatively simple and the parts will cost under $15. Aren't YOUR SAFETY and the reliability of the amplifier worth it? All you'll need are a replacement plug, socket, about an inch of Teflon tubing, some RTV sealant, a small "Caplug", and a small length of 1/16 inch brass rod. Connectors are available from RF Parts, eBay, the Internet, and the brass rod and Caplug are readily available at your local hardware store or hobby shop. Do NOT begin any service on the amplifier unless it is unplugged and all high voltage capacitors are discharged!

The attached photo shows the final configuration for the assembled adaptor. To fabricate the adaptor, merely make a loop in one end of the brass rod so that it can be held to the socket by the screw; the solder lug is not needed. After attaching the brass rod, it is soldered to the screw and bushing. After it cools, you insert the brass rod into the plug, with a little room for spacing/movement, and then cut off the excess. Wear EYE PROTECTION! When you cut that brass rod it will shoot off who knows where. Then, remove the plug, put a small length of Teflon tubing (one or two layers) over the brass rod. Reinsert the plug, solder it, and wait for it to cool. Take the caplug and cut a notch in it so it will fit over the barrel part of the socket. Then, coat the exposed "non-connecting surfaces" metal with RTV. Put some more RTV inside the Caplug, and then put the Caplug over the barrel. Do not wipe off the excess RTV. Wait 2-3 days for the RTV to cure, then cut off the excess. And there you have it.

Insert the completed assembly into RF deck, but don't tighten the shell completely. Insert the plug from the high voltage cable into the socket of the adaptor, and snug it down. Then, snug down the shell of the adaptor to the socket on the RF deck. This configuration greatly reduces the stress on the high voltage connection wire. How long does the new design last? I don't have a clue, I've never had any failures since I started using these adaptors. I'm sure the creative Drake Enthusiasts can improve upon my design. It is presented here merely as a starting point for even better solutions. Please share your ideas with the Drake Community.

Figure 1.
Completed High Voltage Adaptor