Tube oscillator modification

This is just a wild shot.

I found an old Heathkit ao-1 tube oscillator in a dumpster today.
It works, is stable, and has a really nice smooth sound.

Would it ever be possible to get some kind of Control Voltage like V/Hz as used by Korg into it?

Cheers

Mikkel

The frequency is changed by variations in capacitance. That is very similar to my Heathkit Function generator. I believe that is a hetrodyne oscillator similar to an AM radio only audible frequency oscillation.

I have seen somewhere on the web a schematic for a voltage controlled tube "relaxation oscillator". Unfortunately the control voltage is neither linear nor is remotely close to the good old standard analog synth 1 volt per octave. I unsuccessfully breadboarded this project. It did make a sound more along the lines of a poof rather than an oscillation.


An all tube synth would be awesome.

You could voltage-control the Heathkit's frequency over a limited range by using varactor diodes. V/Hz response would be too much to hope for :wink:

That's a Wien bridge oscillator, not a BFO (beat-frequency oscillator, aka heterodyne).

> some kind of Control Voltage like V/Hz

No. That's why the Good Lord and Shockley gave us transistors and $2 chips. Tubes make terrible volt-controlled oscillators.

> a really nice smooth sound.

{sigh} Yes, that's why we have Wein Bridge Oscillators. The simple volt-controlled multivibrator does not make SINE waves, and the several "sine converters" all suck. The Moog-users' trick is to run the VCO into a VCF, both fed the same control voltage, to filter-up the triangled-sine wave to something that does not scrape the eardrum. (An advanced technique is to use the VCF with positive feedback and an amplitude limiter, but I don't recall ever having enough modules on our ARP to do that properly.)

That Heath is a very fine test-bench oscillator. If you don't test-bench with ACVM and oscilloscope, trade it to someone who does. I have four oscillators already, and am about 5,000 miles over the ocean, but someone in your land needs it.

> using varactor diodes.

Huh. I see they even make hyper-abrupt varactors with almost enough pFd. But the signal levels here are higher than a radio, I suspect it would change frequency throughout an audio cycle, which amounts to gross distortion. And as you say, the control-law would be screwy.

Two 6SN7. Amazingly cheaper than the H-P 200AB, yet about 99% as useful on a typical audio test bench. Someone should mass-clone it (but where do you get that tuning cap today?). The alternative seems to be the Global 2001, a nice but expensive box with kinked "sines".

> An all tube synth would be awesome.

Once upon a time, when Goldilocks was still a young girl, I maintained an ARP modular (and solid-state) synth. It was roughly a half-time job just keeping up with the tarnish on the many thousands of contacts. An all-tube synth would be like that early computer: techs with shopping-carts full of tubes walking up and down the aisles, getting about 15 minutes between failures.

The schematic you linked has an error. The cathode resistor on the second triode says "470K", but is almost certainly "470". Since the box works, the resistor in it must be right, just the drawing is wrong. Thought you should know in case you wind up "fixing" it.

All tube synth, sorry for the long load time, don't know what Mr. Barbour has done to this site.

http://www.metasonix.com/

Actually the first Moogs did pretty much the same function gen thing as we do with bog standard function gens now---although Bob started with a sawtooth from a unijunction osc, ran it into a bipolar that had the collector run into the base and keep on going, to convert to a triangle, then did the usual sine wave shaping on the triwave.

There were VCFs, but they were separate modules.

> Actually the first Moogs did pretty much the same function gen thing as we do with bog standard function gens now

Not sure if you are objecting or what.

You can categorize the several VCOs by triangle or sawtooth base plan, but the difference is moot once you find a clean way to change one to the other. (Or a fun way like Paia's one-transistor saw-tri splitter; but they reduced the basic VCO to a unijunction.) All The Same, yes.

None of these make sines, and no matter how you trim the sine "converter" it comes out ugly to the ear.

The synth-trick I learned was to run the sine through a filter (assuming your patch had an idle filter). Since everything was V/Oct, it could track and hold a very clean sine over a wide range.

We did this mostly for student edification: a sine is really a lousy musical tone. We generally left the sine converters over-biased to get a ripely musical (the word "phatt" had not been invented) 3rd-harmonic instead of the thin peak-top wave that read lowest THD. (Another proof that THD is bogus.)

Using the VCF as a VCO, on-purpose or accidentally, was popular, but can't get a sine that way. You could with a VCA (never enough of those) and a level-detector. I guess you could dummy-up a semi level detector with the sample/hold, but I don't recall it being done.

> All tube synth

Advancing audio's frontiers.

[quote author="PRR"]> Actually the first Moogs did pretty much the same function gen thing as we do with bog standard function gens now

Not sure if you are objecting or what.

You can categorize the several VCOs by triangle or sawtooth base plan, but the difference is moot once you find a clean way to change one to the other. (Or a fun way like Paia's one-transistor saw-tri splitter; but they reduced the basic VCO to a unijunction.) All The Same, yes.

None of these make sines, and no matter how you trim the sine "converter" it comes out ugly to the ear.

The synth-trick I learned was to run the sine through a filter (assuming your patch had an idle filter). Since everything was V/Oct, it could track and hold a very clean sine over a wide range.

We did this mostly for student edification: a sine is really a lousy musical tone. We generally left the sine converters over-biased to get a ripely musical (the word "phatt" had not been invented) 3rd-harmonic instead of the thin peak-top wave that read lowest THD. (Another proof that THD is bogus.)

Using the VCF as a VCO, on-purpose or accidentally, was popular, but can't get a sine that way. You could with a VCA (never enough of those) and a level-detector. I guess you could dummy-up a semi level detector with the sample/hold, but I don't recall it being done.

> All tube synth

Advancing audio's frontiers.[/quote]

I just was pointing out that the basic oscillator modules did not include the VCF function, as I thought you were implying. I understand now that it was more of a patch technique described---my apologies.

I thought it was a long shot. Thanks guys!
I am going to look into if there is a 470k failure in the schematics, in a couple of days.

Cheers.

I am restoring a Hammond Novachord, which is an all tube synthesizer. It is fully polyphonic, and gives you control of the filtering and attack/decay. It also has a wonderful vibrato. Mine was built in 1940 I believe. Theres over 160 tubes in there. All the caps were factory matched, requiring you to test each cap and match it to what the original was. Only 1069 Novachords were built.

I made a web site about it:
www.paserv.com/novachord

[quote author="vintagedreams"]I am restoring a Hammond Novachord, which is an all tube synthesizer. It is fully polyphonic, and gives you control of the filtering and attack/decay. It also has a wonderful vibrato. Mine was built in 1940 I believe. Theres over 160 tubes in there. All the caps were factory matched, requiring you to test each cap and match it to what the original was. Only 1069 Novachords were built.

I made a web site about it:
www.paserv.com/novachord[/quote]

Heroic! Based on the heat generating capability, I suspect your nick might well be "winterdreams"...

insane in the membrane. good luck! does anyone remember that huge russian console with 6000 tubes in it?

I have just used the Heathkit Tonegenerator at a perfomance in Ridehuset in Aarhus.
Its faboulus for loud deep sine bassssssss..... Yeehaaa!!

hvad sker der i Ridehuset?

Der skete en Kulturama festival, hvor jeg optrådte med min perfomance StreamFlower. 2 dansere, 4 musikere med en bunke gamle synther og en Cocktailmikser, og 2 VJ's til scenografi.
:green: