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Futterman Class C oscillator?
Futterman Class C oscillator?
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Old 16th February 2017, 05:37 AM   #1
woodrough is offline woodrough  United States
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Default Futterman Class C oscillator?

Hi!
I dont know if anyone has seen or had experience with this idea before, but maybe someone can give me some thoughts on this.

For a good bit, I was interested in the Futterman OTL layout. Its a sturdy simple schematic that is drives low impedance loads without the need for an output transformer. A mysterious holly grail for audio applications...

Also for a good bit, I was building tube & igbt oscillators for tesla coils. Class C armstrong oscillators. Tube designs are quick to construct and are nice and robust but live at high voltages that cant handle low impedance primary coils that I want to use. I already built a 811A oscillator, but im having some difficulty with my low impedance coils.

IGBT designs can drive lower impedance coils but are delicate to user errors. I blew a few of them out a little too quickly :c c:

A 6c33c tube in futterman orientation looks like it could drive low impedance antenna coils in an economic & reasonably low voltage kind of way. The best of tubes & the best of mosfets? c:
is there a gotcha?

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Last edited by woodrough; 16th February 2017 at 05:40 AM.
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Old 16th February 2017, 05:45 AM   #2
jazbo8 is offline jazbo8
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Futterman Class C oscillator?
Moved to Everything Else - non-audio projects.
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Old 16th February 2017, 11:30 AM   #3
Koonw is offline Koonw
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Interesting, what normally you use it for (lighting etc)?

You need to add dc blocking capacitor one each for 2 coils at the grid and cathode of 6c33c OR you insert the bias supply (floating) at the end of each coil and cathode. What freq. are you working on and if you list also the inductance of each coil I maybe able to see if that is what is intended.
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Old 16th February 2017, 01:09 PM   #4
Koonw is offline Koonw
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This is the basic schematic of oscillator, 200kc, about Pout 100W.
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File Type: png 6c33c OTL osc.png (85.4 KB, 133 views)
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Old 16th February 2017, 03:14 PM   #5
Koonw is offline Koonw
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I think it's Class AB, Class C is biased cut off, this is not.
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File Type: png 6c33c OTL osc-2.png (75.7 KB, 40 views)

Last edited by Koonw; 16th February 2017 at 03:26 PM.
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Old 16th February 2017, 03:47 PM   #6
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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I can't understand you. Class C is not an option for audio purposes because the result will be extremely high distortion (in Class C the conduction is below 180), independently of its topology (SE or series).

And if the oscillator is to use a lower transformer size, you will also need a further isolation barrier.

I have experience in class C amplifiers, but sincerely I don't understand the main question.

Respectfully.
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Old 16th February 2017, 03:58 PM   #7
Koonw is offline Koonw
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This is an oscillator, usually Class C, not audio amp, as OP suggested to borrow OTL totem pole topology to drive low impedance type of tesla coil, i.e making it into an osc, which I believe can't be a Class C.

Google for "tesla coil" you will understand.
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Old 16th February 2017, 04:32 PM   #8
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Yes, although I never build a Tesla coil, I know what this is.

But for driving them I believe it is better to work nearest to class D over class C, at δ near 50%.

Some time ago I made this experiment:

http://www.diytube.com/phpBB2/viewtopic.php?f=4&t=6408

A SEPIC + Cuk converter, only with tubes.
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Last edited by Osvaldo de Banfield; 16th February 2017 at 04:47 PM.
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Old 16th February 2017, 05:15 PM   #9
woodrough is offline woodrough  United States
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Hey guys!
Thanks for the discussion,

I say class C because im interested in strong sinusoidal or impulse like oscillations for studying resonant antenna and ground currents. At the moment I dont need signal/audio conveying / amplifying. With class C, when ran at the appropriate frequency, the tubes would conduct briefly to continue the oscillation seen in the antenna (or secondary coil).
(like kicking a swinging object at the right intervals to produce strong momentum)
Class C is a all about searching for extreme harmonics to produce the strongest pressures.

With Tesla coils, the holly grail is to have the largest capacitance primary driving a smaller capacitance secondary coil to produce the strongest oscillations (aka magnification).
The only real thing though that can efficiently impulse large capacitance low inductance coils are spark gaps. Transistors & mosfets are next best suited for conveying fast current, and vacuum tubes unfortunately are high impedance objects However when tuned and triggered appropriately, vacuum tubes can be useful.

The Futterman OTL seen in audio applications continue to stun me the more I read about it. Being able to drive suspiciously low loads with plenty of Watts without the need of a matching transformer.

It is some sort of totem pole like push pull configuration, but im curious if it can be possible to push this configuration into briefly conducting short strong pulses (like seen in Class C scenarios)

Can 6c33c beast be used in transmitting applications? Mosfet like high current discharges into low impedance coils?

I have a
.08mH primary coil
.28mH Secondary or tickler coil (2Mhz) 21 turns
.72mH Secondary coil (1.26Mhz) aprox 100 turns

Im also debating about the tickler coil concept, it makes a simpler oscillator, but I kind of like the ability to tune the frequency around with a signal generator. [that would require some driving circuits I imagine :E]

Last edited by woodrough; 16th February 2017 at 05:23 PM.
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Old 16th February 2017, 05:41 PM   #10
Koonw is offline Koonw
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The output from "out" of last sch is 50kv( or up to 1Mv or more), is a carrier freq 220kc, far from audio, no difference from carrier generated by class c oscillator, apart from efficiency, I think Class AB is not very low efficient, as long as it solves your problem. There are many just use AC spark gap. 6c33c goes up to about 1Mhz at least. I try to figure up what freq your gears work on later.

Last edited by Koonw; 16th February 2017 at 05:52 PM.
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