tag:blogger.com,1999:blog-4184250009182241972.post1759250521377610896..comments2023-09-16T13:13:38.942-05:00Comments on CQ CQ CQ de W0VLZ: Cathode Keying SafetyNiel - W0VLZhttp://www.blogger.com/profile/09860694874162459879noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-4184250009182241972.post-50133131853394564132023-09-16T13:13:38.942-05:002023-09-16T13:13:38.942-05:00I use a 1944 US Navy CLT-26012-B key that is fully...I use a 1944 US Navy CLT-26012-B key that is fully insulated. The contacts are insulated from the trunion/arm, the connection posts are fully insulated, and there is a ground connection on a trunion stanchion for grounding the stanchions, and trunion/arm totally separate from the contact circuits. Of all the vintage military keys I've used, this is the safest of them for cathode and grid-block keying.Jimhttps://www.blogger.com/profile/01441158198838077714noreply@blogger.comtag:blogger.com,1999:blog-4184250009182241972.post-51099068043606526642020-02-29T18:34:54.192-06:002020-02-29T18:34:54.192-06:00You're right that the lack of screen regulatio...You're right that the lack of screen regulation makes KEY UP cathode-keying voltage soar, because in effect the tube "chases" its cutoff voltage because the screen dropper doesn't drop anything when the tube doesn't conduct.<br /><br />And yes, triodes, especial low- and medium-mu power types, would have high cutoff voltages (and therefore KEY UP voltages for cathode keying) as well.<br /><br />In true cathode keying, what you see at the key terminals in KEY UP is the highest cutoff voltage of whatever stage(s) are keyed. But when your "cathode keyed" stage(s) are beam power tubes and/or video pentodes (like 6AQ5, 6CL6, 12BY7, 6DQ6, 807/1625 and so on, and you're seeing over 100 V in the KEY UP state, one or more stages is using not cathode keying, but B-minus keying.<br /><br />Spotting the difference between cathode keying and B-minus keying is easy: If a stage's grid resistor goes to common and you key only the cathode, that's cathode keying. If a stage's grid resistor goes to the cathode and you key the cathode _and grid-resistor_ ground connections at the same time, that's B-minus keying.<br /><br />The difference between the two keying types can (literally) be shocking: When cathode-keying, say, a 6L6/807/1625, the KEY-UP voltage is just the stage's cutoff voltage -- around 45 V if the tube's screen voltage is pegged, maybe up to 65 V or so if a series screen dropping resistor is used. If one or more stages has its grid resistor connected between grid and cathode, you'll _that stage's _full plate supply voltage_ across the key terminals in the KEY UP state!<br /><br />Many, many authors and equipment designers didn't make this distinction. For instance, the ARRL Handbook 12BY7-1625 transmitter (article PDF freely available at http://www.arrl.org/files/file/protected/Group/Members/Technology/tis/info/pdf/64hb172.pdf) connects its oscillator's 47-kilohm grid resistor between grid and cathode, and keys both. Key up, there'll be 340 V across the key!<br /><br />And B-minus keying isn't hazardous just for people. Among tubes' many specs is their maximum allowable heater-cathode voltage differential, exceeding which can cause an arc and short-circuit between the tube's cathode and heater. _No_ tube we're likely to use in a vintage ham transmitter is rated to handle several hundred volts from heater to cathode. If B-minus keying doesn't punch the H-C insulation through (thereby putting the keying line KEY DOWN, BTW, if the chassis is used for the heater-voltage return), we're just lucky.<br /><br />All of my rigs at W9BRD are cathode-keyed, BTW -- in multistage rigs, driver and final, each via a separate keying line that ultimately is keyed by its own high-voltage power MOSFET in a separate keying/control box. I use the system across all of my multi-stage transmitters -- just a straight key on its own suffices for the my AC-1 clone and its 5763-based sibling -- for control across multiple transmitters, so all of their keying sounds the same. And the KEY UP voltage is no more than 13 or so.<br /><br />Best regards,<br /><br />Dave<br />amateur radio W9BRDDavid Newkirkhttps://www.blogger.com/profile/01881478550092617259noreply@blogger.com