A tube amp that's actually a tesla coil. - diyAudio
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Old 17th May 2012, 02:02 PM   #1
bigwill is offline bigwill  United Kingdom
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Default A tube amp that's actually a tesla coil.

I was thinking about making a tesla coil of sorts, and thinking over various possibilities. One of them being a design that's basically a push-pull tube amp.

For your tesla primary, you could perhaps have an untuned centre tapped coil, just like the primary of an output transformer. Each phase could be driven by the anode of something like an 807 or GU50.

Driving the grids of these valves could be a relatively fast phase splitter (something capable of around 500Khz), LTP pentodes would work nicely here. The input could be a sig get running at the resonant frequency of the tesla secondary, or even an antenna or pickup winding so it's self oscillating.

I have little RF experience so this may not work but it seems like a neat way to do it - a fair pit of power from push pull finals, and the flexibility of being fed with an arbitrary input signal (or feedback from itself).

I don't want to make a huge coil, just something with CW output that doesn't buzz horribly

Do any of these design ideas make sense for a tesla coil?
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Old 17th May 2012, 02:54 PM   #2
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These output RF tubes operates at Class C, meaning that in the most part of a cycle, the tube is cut off, and only conduct over a small % of the cycle. Usually, the bias is generated itself by simply rectifying the signal input of a couple thousand volt RF (200V or so peak), so the -200V at grid becomes it cut off. Yuo can try any low power pentode oscillator, classical is a 6V6 as a Hartley, and a 6L6 driving the 807s. At this low frequency, they will perform OK. Be careful with the high voltages inside the equipment, and with MW radio interferences.

Good luck.

Im ham radio from 1987.
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Old 17th May 2012, 03:00 PM   #3
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My favourite:

Tesla-Generator mit PL504

use Russian line pentodes when PL504s are out-of-stock...

..
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Old 17th May 2012, 04:48 PM   #4
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Google: vacuum tube tesla coil

They are switching amplifiers, some SE and some push-pull. Some very large projects...
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Old 17th May 2012, 05:05 PM   #5
TheGimp is offline TheGimp  United States
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Isn't this pretty much a horizontal sweep circuit from a VT TV set?
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Old 17th May 2012, 05:16 PM   #6
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Quote:
Originally Posted by TheGimp View Post
Isn't this pretty much a horizontal sweep circuit from a VT TV set?
It's a whole lot simpler than even that. Just rectangular pulses.
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Old 17th May 2012, 05:23 PM   #7
bigwill is offline bigwill  United Kingdom
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Thanks for the replies everyone.

Would the primary have to be resonant like on a spark driven coil? I'm not really sure how it would load the tubes, if it would be reactive enough so that excessive current isn't drawn, etc
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Old 17th May 2012, 05:34 PM   #8
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Class D ionophone driver?

Wait for guests from FCC...

When I experimented with class D tape recorder neighbors complained.
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Old 17th May 2012, 06:06 PM   #9
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Quote:
Originally Posted by bigwill View Post
Thanks for the replies everyone.

Would the primary have to be resonant like on a spark driven coil? I'm not really sure how it would load the tubes, if it would be reactive enough so that excessive current isn't drawn, etc
A tesla coil is a resonant transformer The resonant frequency is something like 1/4 of the electrical length of the secondary, which is 1/transmission line delay.

Whether the primary is a tuned circuit of uses an oscillator, it is driven at approximately the resonant frequency of the secondary. The classic tesla coil uses a spark gap as an impulse generator and the primary circuit is tuned to resonate. These operate oin a repetitive burst mode. Modern coils often use an oscillator that is tuned through feedback from the coil or manually tuned to peak the coil output. The load on the coil changes the resonant frequency a little. The driver should be quite efficient in that the tubes are either on or off, which will minimize the plate dissipation compared with a sine wave as used to characterize audio amps.

There is good power transfer from primary to secondary, so the load on the tubes depends on the load on the secondary. Typically a coil is sized for a particular VA or wattage, and there are a few websites that show how to calculate the variables.

Last edited by Michael Koster; 17th May 2012 at 06:09 PM.
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Old 18th May 2012, 05:40 AM   #10
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Hey Bigwill

I have built 7 Tesla coils and one of them was a VTTC, you can see it here Kaizer VTTC I | Kaizer Power Electronics and I will gladly answer any questions you have.
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