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Small dirty VTTC from scratch done in one hour

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LAN_403

Arcstarter

Sat Feb 21 2009, 05:19AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Weston wrote ...

Could I use a 12au7?

Sure, why not? :). Parallel the two triodes to make sure it doesn't heat too badly.

Weston

Sat Feb 21 2009, 05:29AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Yousure? I thought that the 12au7 was only for audio frequency

Proud Mary

Sat Feb 21 2009, 09:07AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Weston wrote ...

Yousure? I thought that the 12au7 was only for audio frequency

12AU7 is a general purpose low-mu double triode, primarily designed for use in television receivers in such roles as oscillator/mixer, phase inverter, multivibrator, and RF amplifier. It will, of course, work as an AF amplifier if configured in that way.

Steve Conner

Sat Feb 21 2009, 10:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Nice work RayeR. Unfortunately I had to crudely smush your oversized images.

Bottom feeding is traditionally used in the plasma tweeter circuit, and I've seen this circuit used with tubes up to 4-250 size, making a flame that looks like a welding torch.

The bottom feed circuit is equivalent to the pi-tank circuit used in old tube radio transmitters. The first capacitor in the "pi" is the tube's stray anode capacitance, and the second one is the Tesla resonator's self-capacitance. It follows that you may be able to adjust for optimum output by putting more capacitance between plate and ground. (In an old ham transmitter this would be the knob marked "Loading".)

Tweeters usually run in the 10s of MHz, where the tube's own capacitance is probably enough. But for lower frequencies, I think the loading cap would be needed. Rather than try to calculate it, I'd just make it variable and tweak for maximum sparks.

Finally, I seem to remember that someone (Marko?) really did make a VTTC with a 12AU7. It would light fluoro tubes, but not break out.

Proud Mary

Sat Feb 21 2009, 01:56PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I would suggest increasing the value of RayeR's anode RF choke from 470uH to 2.2mH, to maximize the voltage gain of the valve - especially if someone else intends to copy his circuit using a different valve.

Rayer's LD2 valve is a VHF/UHF power triode, originally optimised for use between wavelengths(sic) of 1 m and 0.5 m , which means that rather more power could be expected from it in the low HF where Rayer is using it i.e. 3,7MHz.

RayeR

Sat Feb 21 2009, 02:50PM

Registered Member #333 Joined: Mon Mar 20 2006, 06:02PM
Location: Czech Republic
Posts: 45

> Steve McConner

Thanks for explanation with plate capacitance, can you guess the needed range of additional capacitance? I have some varable tunning caps from old radios up to 500pF but I fear the arcing will occur between cap's electrodes - they are placed too tight. I wanted to know the bottom feeded TC resonator impedance to compare it with tube impedance to have some idea how well is it matched. Maybe I can rather measure it, but it will need to make some current transformer to plate circuit and it's calibration because it is floating from ground and cannot be scoped normal way. And then from current and plate voltage get impedance...
The choke may be larger to prevent RF power wasting in filtercap, I just used what was lying around.

BTW what dimensions have yours >10MHz coils? I think that mine has very few turns and it has still quite low f0. Do you use some spacing between turns?

Proud Mary

Sat Feb 21 2009, 03:18PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

RayeR, if you don't have any transmitting variables, you can reduce the chance of flash over by removing every other plate from a receiver type variable capacitor - so doubling the distance between them. (but reducing the capacitance of course) If you have a double gang receiver capacitor of 2 x 350pF or 2 x 500pF, you can wire both in parallel after taking the plates out.

I would suggest starting with a variable capacitor of about 150pF with your present circuit values and fo.

For safety purposes, don't forget to add an anode blocking capacitor of, roughly about, 3000pF/3kV with your present circuit values and fo. If your load variable capacitor should short across, the anode blocking capacitor will block a direct short circuit between anode and Earth. It will also block the anode supply (High Tension - HT) getting into the secondary.

Steve Conner

Sat Feb 21 2009, 04:14PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

A starting guess for the loading capacitor would be something that has a reactance at your operating frequency, of one-sixth to one-tenth of the plate load resistance recommended in the tube datasheet.

Even if you don't care about safety, the blocking capacitor recommended by Harry is worthwhile, because it lets you get away with a lower voltage rating on your loading capacitor. (Can anyone explain why?)

But once you've added the blocking cap, then you need an RF choke to ground too, to stop the safety feature being defeated if the blocking cap shorts out. Suddenly the circuit doesn't look so quick and dirty.

Proud Mary

Sat Feb 21 2009, 04:35PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve McConner wrote ...

Even if you don't care about safety, the blocking capacitor recommended by Harry is worthwhile, because it lets you get away with a lower voltage rating on your loading capacitor. (Can anyone explain why?)

The blocking capacitor in series with the load variable capacitor together form an HF potential divider where

Vout = Vin*C1/(C1+C2)

Vout in this case being the hot end of the VC

Steve jokes on: "But once you've added the blocking cap, then you need an RF choke to ground too, to stop the safety feature being defeated if the blocking cap shorts out. Suddenly the circuit doesn't look so quick and dirty."

The point is to make the anode blocking capacitor the best quality you can get, with as generous a voltage rating as you can get. Ideal HF blocking capacitors are of the high voltage 'door knob' type, and big silver mica sorts.

To help translation, here is a picture of typical 'door knob' capacitors:

Tonskulus

Sat Feb 21 2009, 07:37PM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133

There are many kinds of doorknob caps available, some of them cannot handle RF at all. Those caps seen in previous post seems to be capable to handle RF power. There are also POT style RF caps, like these:
http://www.elisanet.fi/tonskulus/tankcap.jpg

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