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Monday, April 23, 2018

Cathode Keying Safety

As I participated in various radio events this past winter I thought about a safety issue common in budget and homebrew transmitters prior to about 1960. Many, if not most, of these transmitters used cathode keying with no regulation of the screen voltage. In these transmitters it is common to find over 100 volts across the key terminals. I measure 125 VDC at my Johnson Adventurer key terminals. Some of my friends checked their 1929 homebrew transmitters. They found as much as 191 VDC at the key terminals. All of these are basic one or two tube cathode keyed transmitters, the sort of transmitters used by many today to experience vintage tube gear.

What can be done about the high voltage at the key terminals?

- Keep your fingers off the key terminals: I've been doing this up to now. It usually works but my wife worries that it does not always work. It also follows the "If it hurts don't do it" rule.

- Covered Key Terminals: Several military keys and even some bugs are available that hide the terminals. This is a workable low tech solution but you have to find a suitable key or bug. One option today is the Bencher RJ-1 sold by Vibroplex ( http://www.vibroplex.com/contents/en-us/p202.html ).

- Mechanical Relay: Use your key or bug to operate a relay that is actually keying the rig. I've never tried this but it appears that speed may be an issue. How well does the relay keep up?

- Electronic Isolator/relay: I used a "Keyall HV" from Jackson Harbor Press / WB9KZY. The Keyall HV includes an opto-isolator and MOSFETs rated for 1000V at 3 Amps. Besides low voltage at the key terminals it is also low current so it can be used to interface a modern keyer to the heaviest of our cathode keyed boat anchors. See http://wb9kzy.com/keyallhv.htm for details.


The Keyall HV kit includes five parts and a small printed circuit card. This kit is just about as simple as one can get. Instructions are on the Jackson Harbor Press website.
I used a small aluminum utility box for my keyall housing. I could have used a smaller box but the only two battery battery holder I had was for type C flashlight batteries. The batteries needed all of the available width. I used a barrier strip for the transmitter connection because some transmitters do not have one side of the key connection grounded.

The Keyall HV interface sits in-line between my key and the transmitter. The key terminals now have less than 3 volts between them. The two C cells should be good for over 500 hours of key down time.

1 comment:

David Newkirk said...

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.

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.

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.

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.

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!

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!

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.

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.

Best regards,

Dave
amateur radio W9BRD