ICS Signal Generator

I would guess that this was a kit of some type. It's definitely not a professional piece of equipment. And I found one dry joint which tends to reinforce the idea.

I am not as good a draftsman as Roberta, but here it is.

Basically it's an Armstrong oscillator built around a 6AQ5 a.k.a. EL90. I suspect the light bulb is acting as a monode noise generator, based on the capacitive coupling to the grid (a direct or resistive coupling would have suggested stabilisation of the Bill Hewlett variety. Comments will be appreciated. 2021-11-14: Comment received from Tom Lee at Stanford:

Hi,

I just came across your years-old blog entry about the ICS signal generator. I was recently given one, and a little googling led me to your post.

Your speculation about the bulb’s function was very interesting. But the bulb is actually a neon bulb, not incandescent, and actually forms a relaxation oscillator with the R’s and C’s around it. That’s the audio source that modulates the RF. Heathkit and other manufacturers used this same idea in their earliest, cheapest signal generators.

Thanks for such an interesting blog — I will be reading more of your entries, now that I’ve discovered your site. You’ve put in an impressive amount of effort. And thank you for tracing the schematic of that widowmaker-generator!

I hope all is well with you and your family as we approach Year 3 of COVID. Keep up the great work!

— Cheers Tom

I checked, and yes, it is a neon. Makes a lot more sense. Thanks, Tom.

2016-08-22: Received pictures of a different model.

So ICS is short for "International Correspondence School". I was right then.

This one uses a 117L7 -- The 117L7 is a combination pentode and rectifier with a 117V filament. The filament is connected straight to mains and HT is just that same mains, rectified. That's what we call a widowmaker.