Is possible I've seen a schematic with a lamp used in NFB circuit ?

[kagliostro]

Seems to me that some time ago I've seen a schematic on which a lamp (filament or neon ?) was used on the NFB circuit
may be the schematic was from one of the Radiotron Handbook's, but I don't remember well
Someone remember such a circuit ?
Thanks
K

[tubeswell]

I don't see why not,

Just off the top of my head, I do not believe that a standard neon light would work, (minimum voltage requirements).

I could see LED's usd in NFB loop, possibly with a selector to choose colors and to tune the loop

What a cool idea!

[printer2]

There was incandescent bulbs used across the output as a sort of powersoak (or maybe I was dreaming about it).

[HotBluePlates]

Seems to me that some time ago I've seen a schematic on which a lamp (filament or neon ?) was used on the NFB circuit ...

It was the basis of Hewlett-Packard's first product (an audio oscillator), which launched them into the test equipment market.

The oscillator used a lamp in the cathode, which was also a negative feedback return. If the oscillator signal tended to get bigger, the lamp turned on harder and was a larger resistance (it had a positive temperature coefficient; more current = more heat = higher resistance). Being in the cathode circuit, it created a larger bias voltage under these conditions, which tended to turn the oscillator off. If the oscillator signal dropped, the lamp became a smaller resistance, creating less bias voltage, which turned the oscillator more-on.

The lamp-stabilized negative feedback circuit created a nearly-constant output level with changes of frequency and over time, which made the H-P oscillator valuable among its competitors.

See the schematic below.

[darryl]

Seems to me that some time ago I've seen a schematic on which a lamp (filament or neon ?) was used on the NFB circuit
may be the schematic was from one of the Radiotron Handbook's, but I don't remember well

Radiotron Designer's Handbook Version 4. Chapter 16. Volume Expansion, Compression and Limiting.

[kagliostro]

@ Drgonzonm

I agree with Tubeswell, interesting idea, thanks, can you explain it a bit further ?

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@ Printer2

As far as I can remember there was a discussion about a filament lamp in series with a cap, this elements were placed in parallel with the output of the amp, PRR gave some explanation about

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@ HotBluePlates

That schematic was one I've find searching on the Radiotron 4 before asking here, thanks for the explanation, very interesting

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@ Darryl

Many Thanks you got it ! That is exactly what I remembered, my search was without result because Radiotron has more than 1500 pages

K

[PRR]

> Hewlett-Packard ... oscillator used a lamp...

FWIW, Meacham of BTL lamp-stabilized a crystal oscillator at about the same time.