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New Mini VTTC

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LAN_403

EDY19

Tue Jul 18 2006, 05:33PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Alright, Now that i have the hang of VTTC's and have my dual 811A coilup and running without problems, I am going to go back to the mini approach. I want to use a power supply of tripled 120VAC for ~490V with a relatively large filter capacitor (~10uf at ~600V or so) so the discharge is very close to CW. However, I am looking for a suitable tube for the project, preferably one with a 6.3V filament since i have a transformer for that (1.2A)

The secondary should be around the same size as the tube, and I want it to resonate around ~3-4MHz, so the coil can be relatively small. Suggestions on tubes that could be good candidates?

Simon Barsinister

Tue Jul 18 2006, 10:40PM

Registered Member #116 Joined: Fri Feb 10 2006, 03:19AM
Location: Erie Pa, USA
Posts: 29

Use a 6146, it fits your filament transformer, has a 25W plate dissipation and is relatively common.
Rich

EDY19

Wed Jul 19 2006, 03:07AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Taht looks like a good tube, should work well with tripled 120VAC (490V) as the rating on it is 600-700V- I may even go up to quadrupled, for 660V, which is still under the 700 or 750V ICAS specs in the datasheets. Would I just tie the grid and screen together in this application? If not, what should the bias current or voltage be?

Steve Conner

Wed Jul 19 2006, 09:45AM

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

The 6146 is designed to have its control grid biased negative and drawing next to no current, and have a screen voltage of about 250V. So I think tying the grid and screen of a 6146 together would end up burning the grid out while the screen still didn't get enough for proper working.

I would drive the screen with 300V DC from the second stage of the tripler, maybe with one of those sugar lump power resistors in series (470 ohms or whatever). The extra voltage will help to abuse it more for longer sparks >.<

EDY19

Wed Jul 19 2006, 06:07PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

How exactly would I do this? I was planning on using a quadrupler- where would the feedback coil be conected, the screen or the grid? And which needs to be biased negatively? I am just a little confused

If i use the quadrupler, the cathode of the 6146 is connected to the negative side, and the plate/primary and caps are connected to the positive, the negative side would be the relative ground, so the feedback would also go through the grid leak network to "negative" ? And mains ground for the secondary, right? (super small coil anyway, mains should be OK-if not, the base could be metal as a counterpoise)

Steve Conner

Wed Jul 19 2006, 10:24PM

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

I was thinking about building one of these a while back since I have some 6146s lying around. This is the circuit I was going to use, adapted for your wimpy 120V mains. It uses self-bias so the tube will overheat terribly if it fails to oscillate. Adding a resistor in the cathode helps with that, instead of connecting it straight to "Ground".

Note mains neutral (ie the white wire in the USA) isn't ground. It's usually at almost the same potential as ground, but it's bad practice to count on that.

If I didn't indicate the value of a component, it's because I don't know what it should be. I'd have found out by experiment when I built the thing, but I never did build it. And I guess I forgot the RF bypass cap too (from the 600V node to the cathode) and other nice things like bleeder resistors on the electrolytics.

Marko

Wed Jul 19 2006, 10:38PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Go for it Steve.. VTTC's are definitely missing on your site

Maybe you could use remaining tubes for nice linear audio mod (since you love tube amplifiers anyway).

Regarding neutral wire, I always use it as ground

and treat it like that even if it isn't best practice.
With our poor electrical distribution, it is actually pretty common to have 'neutral' connected to ground cotacts on outlets, while real ground wire simply doesn't exist.

There is some HF noise in range of few volts on it (I was able to light a blue led by hooking it to ''ground'' and connecting big metal object, long wire or somethign on another end).

Tom540

Wed Jul 19 2006, 11:06PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Hey that reminds me. What wire do you guys use to ground the base of your secondarys? I used the green on mine. Should I be using white instead?

Marko

Wed Jul 19 2006, 11:14PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

'green' is right wire, since it is (is supposed to be) really grounded somewhere within the building.

Neutral wire also works as ground but it represents very high inductance, and voltage excursions develop on it once yourun a TC grounded to it. This causes havoc on devices sharing the mains line.

?green' ground is usually really grounded somewhere in the building, and represents much lower inductance altough it still isn't advisable to ground very big coils trough it.

Biggest TC's should have some metal pipes or plates dug deeply under ground and separated from everything else. Good surogates arelightning rods, water pipes or anything with good earth connection.

...

Wed Jul 19 2006, 11:56PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The green is generally used over the white for various reasons (puts noise on the line, if the outlet is backwards you don't kill yourself, it is there, etc) however in most places it doesn't matter as the green/white *should* be bonded to a big ole copper rod into the ground by your house. (well at least in the US, which is the only place I can think of that uses green and white wires for the mains).

I think I have a few old tubes laying around if you are interested... What kind of plate dissipation rating are you looking for?

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