3/15/2026: This page is a work in progress; suggestions are welcome. Components of the Tank Circuit are very expensive and these can be badly damaged by an RF arc caused by an improper tuning procedure. Included are:
1) The Tune Capacitor
2) The Load Capacitor
3) The Bandswitch or Roller Inductor
4) The HV Choke
Repairing or replacing any of these components is very laborious and expensive. Sometimes, the components are out of production and very hard to find. The best defense against this is a good offense: Install protective devices.
Spark Gaps protect the transmitter’s costly Tune capacitor and Band Switch. An arc in the expensive Tune capacitor can ruin it, so it is good practice to install a Spark Gap in parallel to protect both it and the band switch. In a recent Drake-Group exchange, VE7RF reminds us how to create one. Paraphrasing his remarks:
Home brew spark gaps can be made from 6-32, 8-32, 10-32 or 1/4-20 hardware (or European metric equivalents). Be sure to use locknuts to lock the gap once it is set. Where the pair of machine screws face each other, install brass or stainless steel acorn nuts with their domed or semi-domed surfaces facing each other.
Sometimes the chassis, an aluminum standoff, or angle aluminum, etc., can be used as one of the surfaces.
Set the gap using a feeler gage or other insert and make sure the gap is slightly smaller than the gap between the plates of the Tune capacitor.
This way, the spark gap will arc first and protect the Tune capacitor, the Bandswitch, and in some cases, the power supply (your power supply does have a glitch resistor and high voltage fuse installed, yes? Yes?).
N0CU has more helpful info to say on this subject; quoting:
“Arc suppressors are a great concept but the problem with them is knowing how to get the gap set properly. As Jim pointed out, the Ameritron factory had trouble doing this. Every one of their legal limit amps I have worked on had a damaged plate tuning cap due to arcing. One of these amps had the gap set at over 1/2 inch which made it useless. Setting the gap based upon theory is complicated by the fact that breakdown voltage is affected by the shape of the surfaces- sharp surfaces breakdown before flat surfaces. The best way to set the gap is with a current limited, variable high voltage source, however, very few hams have access to one of these. Whenever arcing is experienced an effort should be made to at least verify that the arcing is occurring across the suppressor and not the plate tuning capacitor.”
Another Tank Circuit safety measure is to install a properly-sized Gas Discharge Tube in parallel with the Load capacitor. But that’s a subject for another day. Of course, if you pose a carefully worded question to ChatGPT, it will tell you all about GDT’s and their sizing.