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Kolas

Tue Jun 13 2006, 07:33AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

Hi guys.
I've found it rather intriguing that there has been so much intrest in VTTC's lately since it's been dead so long before. anyway I'll make the preface as short as possible. I've been reluctant to bring this project to discussion becasue of my general lack of experiance with tube operation. However, I've made it about 30% in my design process and it's time for a few questions.

I came across a free power triode a year ago, and since I've been planning on a coil. It's a 6696A. First I looked at the simple armstrong circuit, and quickly decided that the feedback coil would not provide enough gird current to switch the tube. So I went solid state. (Big no no, I know.)

I have developed a decently detailed schematic of what I think needs to be used to power this tube.

Note- plate current is 2A voltage is 8.8kV peak. and C3 has a bleed resistor, not in the schematic (please don't freak), also L1 is the primary.

This is class A amplification, will it be too wastefull?
Will square wave amplification reduce/increse efficiency?

Please let me know what you guy's feed back is. thanks

HV Enthusiast

Tue Jun 13 2006, 11:55AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Looks good. My only comment is that you can probably just get away with a simple grid leak bias circuit for the grid drive on this application. This will make the tube only conduct during the peaks, and provide for more efficient driving and i'm guessing longer arcs too.

Simon Barsinister

Tue Jun 13 2006, 02:23PM

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

As the 6696 is a 100KW water-cooled beast, this should be the bomb of vttc's. You'll need a special feed from the power company just to idle it. It has 2.5KW filament for cripes sake!

Rich

ragnar

Tue Jun 13 2006, 02:49PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
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Posts: 1425

My god....

This sounds.. fantastic. Can't wait to see it come together =)

Dr. Drone

Tue Jun 13 2006, 02:54PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

Steve Conner

Tue Jun 13 2006, 03:01PM

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

Yea, it took me a while before I realised the tube Kolas had was the mighty Machlett Labs ML-6696

I don't see any reason not to use the Armstrong oscillator circuit. The grid feedback coil can produce as much driving power as you like, since it's picking up from a tank circuit throbbing with 100kW of power >_< In fact, it'll probably be easier to get enough drive for this mighty tube with the feedback coil, than with a MOSFET circuit.

HV Enthusiast

Tue Jun 13 2006, 06:03PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Yeah, i thought this was just a small tube like an 833 or something . . .

Yipes.

Kolas

Tue Jun 13 2006, 10:11PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

well the problem simply is, i cannot give it 100kW drive power. my Plate supply is 15kW
causing me to have to bias the grid.

kolas

Hazmatt_(The Underdog)

Tue Jun 13 2006, 11:39PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

We've been going over this a lot with concerns and thoughts about its operation in what could be considered near cutoff or 'starving the tube'.

There are many problems with picking up the feedback, for one we don't know how much current will be needed to drive the grid. Remember from the data sheet it may need as much as 8 Amps to drive the grid!!! Think about it! And we've been looking at the swing on the nomongraph for a while, and may require as much as 400V pkpk swing!

This is a real extreme for using passive elements, and will likely require active driving circuitry.

I've been trying to consider a lot here, including using transmitter design from ARRL, this truely is a beast of beasts.

Steve Conner

Wed Jun 14 2006, 09:23AM

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

OK, consider this.

The tube must produce more RF power in its plate circuit than it consumes as grid drive. Otherwise it wouldn't be an amplifier, and nobody would buy it. What's more, this must hold at reduced power levels too, or it would be highly non-linear. It's not going to need that 8 amps of grid current unless you really are firing 100kW through the plate circuit. In my experience, any non-linearity is the other way around: the tube starts needing more grid drive at higher powers.

If the plate circuit always produces more power than the grid circuit consumes, it follows that the tube must work in an Armstrong oscillator. If the worst case in terms of grid drive is at high power, then it should work even better at reduced power. To maximise the chances of success I would use the highest plate voltage you have available.

One thing you may need is a variable negative DC bias to the grid, to stop the tube trying to suck more power than your plate supply can give.

If you can get it working with solid-state drive, though, that would be even more impressive :-o

OBTW, the water and forced air cooling systems probably ought to be working before you even turn the filament on >_<

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