IG-102 FET Version – Tinkering with Tubes

I’d recently heard about a conversion of a tube Signal Generator to Solid State. I found the original article in a great old book called ’99 Test Equipment Projects You Can Build’, by 73 Magazine. I snagged the book from eBay for only $2.00 (plus shipping). My copy is a small hardback, red cover. Print’s kinda small.

Same article mentions adding a three-range (10khz, 1mhz, 10mhz) crystal calibrator on-board (since you now have boatloads of room inside without the tubes and the transformer).

Each ‘half’ of both tubes is replaced with a FET Pin numbers are mentioned, so you go underneath (unfortunately sticking the leads in the tube socket holes won’t work…) and solder a FET lead to a Socket Hole. I think there’s one socket hole (besides the filaments) that remains unconnected.

I’ve written on the schematic which FETs appear to work. I settled on MPF-102s, although I tested a couple of versions. The book project specifies four 2N5951’s.

IG-102 - FET
Schematic for the FET version of the Heathkit IG-102

There are two resistor changes (actually mods) due to the lowered voltages:
Solder a 75 ohm resistor across the existing 33k (see left of ‘BF Front View’ switch.
Solder a 90 ohm resistor across the existing 4.7k (see above V2A 1/2 6AN8). On some models the existing resistor may be a 10k.

There are no other changes other than what’s marked at bottom of the schematic (removing the cord and power supply. Replace w/9V battery. I also did the fancy LED thing. Nice to know if it’s ON so you don’t run down the battery.

I also added a ‘wall wart’ plug for outside power. Fancy.

4 thoughts on “IG-102 FET Version – Tinkering with Tubes”

    1. You’re welcome. The mod is from an old, old paperback book collection of test gear projects.
      Best of all, it’s not permanent. It doesn’t really change anything in the circuit. If you want, you can unsolder the FETs and go back to vacuum tubes.

    1. Thanks for finding that!
      I hadn’t thought to look up the original ’73’ article that they’d used to put into the book (“99 Test Equipment Projects You Can Build”), since it was a compilation.

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