Call me the incandescer!

[K8ERV]

That's the one. I couldn't remember the name cause I'm getting old.

I couldn't remember it either, so you didn't see it.

Think HP came up with it, their first product. Before that most/all audio oscs were the beat-freq type.

I think those were in a contest with Swan to see which drifted the most.

TOM K8ERV Montrose Colo

[WA6TKD]

I couldn't remember it either, so you didn't see it.

Think HP came up with it, their first product. Before that most/all audio oscs were the beat-freq type.

I think those were in a contest with Swan to see which drifted the most.

TOM K8ERV Montrose Colo

Of the many projects they worked on, their very first financially successful product was a precision audio oscillator, the Model HP200A. Their innovation was the use of a small incandescent light bulb (known as a "pilot light") as a temperature dependent resistor in a critical portion of the circuit, the negative feedback loop which stabilized the amplitude of the output sinusoidal waveform. This allowed them to sell the Model 200A for $54.40 when competitors were selling less stable oscillators for over $200. The Model 200 series of generators continued until at least 1972 as the 200AB, still tube-based but improved in design through the years.
One of the company's earliest customers was Walt Disney Productions, which bought eight Model 200B oscillators (at $71.50 each) for use in certifying the Fantasound surround sound systems installed in theaters for the movie Fantasia.

Yep, from Wikipedia

[AC0FP]

I have a HP 200CD, which is a later model but it still uses the tube circuit. This is the only tube powered piece of test equipment on my bench (sits on reenforced shelf), the others have been purged.

fp