Opamp frequency mixer

No cathode. Directly heated, the thermal TC of a cathode would respond far too slowly to volume hand movement.

piglet

The filament runs at the volume oscillator frequency (ca 400khz in the original 1929 RCA) via tuned circuit like a slope detector so that varying the volume oscillator frequency by hand movement varies filament emission and varies current drive to one of the AF stages. Its 90 year old VCA. Zany.

Schematics and descriptions here:

piglet

Farnell has several thousand MC1496 and LM3046 but not in DIP

If you mirror a second current output for an OTA, with opposite polarity (so it sinks when the 'main' output sources), and then connect that second output to one of the OTA differential inputs, you get a 'diamond transistor', which is what the OPA861 is.

A bipolar transistor has transconductance; an OTA has transconductance, too, but accepts input bias above AND below the "emitter" voltage (i.e. the "emitter" is actually the second of the differential inputs, but with nonneglligible current draw). IMHO, they should have used more connection pins and brought out all the functions.

Our sites are currently under maintenance.

Please try back later.

We apologize for any inconvenience.

"PENTON"

Some inventors are weird, but then again, so are some of the general population.

The inventors I've known - and I've known a few - have mostly been regular people.

The one bipolar inventor I know did quite well in business, but his particular venture capitalist seems to be a nice guy.

If it's using an oxide-coated or thoriated filament, the same issue applies. If it's plain tungsten, the emission will be much less, but the filament is far more rugged.

Cheers

Phil Hobbs

He doesn't need anything complicated for this little hobby app, an analog switch mixer is adequate, LO selects one of two signals 180o out.

Yes, the UX-120 was developed in 1924 and went on sale 1925 and did use the still very new thoriated filament. Important as it was intended for power work and battery powered so emission efficiency is important.

Still think it is a delightfully simple and crazy way of making a voltage vartiable volume control.

piglet

A 1B3 makes a nice 30 kilovolt amplifier. Input is filament voltage. Bandwidth is a bit limiting.

I used to charge oil caps this way when I was a kid, neon sign transformer and a 1B3. A flashlight battery and a wirewound pot, with a long insulated shaft and a knob, was the control mechanism.

Somehow I'm still alive.

None of those devices is any more complicated than the MC1496, and they all do much the same job. My memory of the MC1496 was that is was bit harder to get it to do the job than it is with more recent chips.

The output of the 1496 is the voltage difference between the two pairs of collectors, neither of which is near ground in general. For AC use, as in a superhet, you just AC-couple to the IF, but at DC it's a bit less convenient. Last time I used a 1496 for real, I used a flying capacitor output stage to fix that. It had a CD74HC4352 to switch the cap between three 1496es and the output. Worked great.

Cheers

Phil Hobbs

Thanks!

Arrow even has them in DIP but hurry, they are EOL'd and they've only got about 10,000 of them :-)

Of course, they may not be retro enough for this project because they don't come in a glass package and nothing glows.

I already bought four tubes of them, so Arrow's are safe for now. ;)

Cheers

Phil Hobbs

Check Richardson's stock, maybe they have some that glow! :^)

Tim