>Some years ago, I acquired an HP-400D (AC VTVM) that had its 6CB6 tubes swapped with Fetrons.  Here's my old notes on it:

<https://people.ohio.edu/postr/bapix/HP400D.htm>

>Rich KB8TAD

Ah, yes. That’s exactly one of the Telco applications of Fetrons i failed to remember.

I have a Heathkit VTVM with a transistor adaptor for the (  i believe ) 12AU7 tube. The innards of the adaptor were not published. Somewhere on my

projects list is to destructively open the adaptor and see the circuit. ( I am aware ‘Ham Radio’ magazine had an article on ‘transistorizing VTVMs”. )

-Hue Miller