Featured VT: The 7193

 

I picked up a half dozen of these at the Dayton Hamvention recently. This is an "industrial" type since it has a numerical type designation, as opposed to the alpha-numeric identifiers more commonly known as "consumer" types. It's a singleton, small signal triode. The configuration is decidedly odd. The octal base comes complete with all eight pins in place, though only three of them are connected to anything: two heater connections, and the cathode connection. It seems that this was designed as an avionics type, and the full compliment of octal pins included for extra mechanical stability.

The plain white boxes they come in:

 

The two top cap connections are by design, as the type originated in the early 1940s, as VHF tech was coming into its own. The top cap connections being to allow for short, low impedance, connections, as a VHF oscillator. Other than that, electronically it is very similar to the 6J5: an excellent small signal audio triode that is basically one section of a 6SN7 dual. It can be used as a direct substitute in any design that requires the 6J5, and they are dirt cheap and available. That's a good thing in case 6J5s become difficult to acquire and/or audiophool expensive. Since I use 6J5s, this was a good acquisition. Some like 'em for their funky appearance and/or as a substitute for the more expensive and harder to acquire glass versions of the 6J5. (There's a whole bunch of audiophoolery concerning the undesirability of metal types.)

If using the 7193, care must be taken with those top cap connections, as transposing the plate and grid connections will result in poofage. The 6J5, unlike the later, hardened, 6SN7GTA-B types intended for TV vertical deflection duty, was never intended to run into significant grid current. A hundred volts or so between the grid and cathode will be a disaster. The connections are clearly visible, so getting them right isn't a problem if you know what you're doing. This could pose problems if doing "consumer" designs for end users who don't know what's inside the glass and couldn't care less.

Otherwise, use 'em like any other type with top cap connections, as the caps fit connectors designed for the small top caps such as the ones fitted on 6BQ6's and other power finals.