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A few questions about base fed 'vttcs'.

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LAN_403

Arcstarter

Sun Jul 10 2011, 03:56AM

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

Hey Jan,

Thanks for the info on the plate, i couldn't find anywhere that had any info on plate material. I heard that the 833C (graphite version of the 833A) was not supposed to be dissipating enough to change color, so i just assumed that was true for the GU-81M.

As for the feedback, it is indeed phased right. If it's not connected right, it would do nothing but pull alot of power in my experiences. The grid drive is already -120v min, and i don't want to go a whole lot lower for the health of the grid. Also, if the spark length (and therefor resonator's throughput) doubles, so will the voltage on the grid, and i am expecting a lot more than double length.

I think the resistor is ok, isn't it to provide some positive bias to make the tube turn on hard? I never found any site that explained that, but that is what the waveforms suggest. However, i only have around 25v positive drive, so i guess i could use a bit more to turn the tube on harder. However, that would probably just make the plate dissipate more and not improve the spark much until i figure this out.

I think i am going to go ahead and go with a typical vttc (Use a primary with a resonant tank cap), but just use a hf secondary, at least for now. I love how the sparks look, they are still pretty long but they are thick, and seem to have a cool color (because of the inherent current density i figure).

(4.6MHz, from this page:

)

Dr. Dark Current

Sun Jul 10 2011, 10:21AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

How much voltage do you put into the screen grid, any screen resistor?

From my experience it easily takes -1kV peaks on the control grid for this tube to run efficiently.
(I have been using some 35 turns on the primary and 12 turns feedback, if there was 10 kVpp on the primary there could have been up to 3 kVpp on the feedback winding, but coupling decreases this voltage).

Sulaiman

Sun Jul 10 2011, 11:05AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I guess that the performance is, like TCs, a mix of modes;

with a supply of 800v, say 500 Vrms on the anode
a large voltage transformation is required to get an arc
so a high 'Q' resonator is required
with a low capacitance load
(so I doubt that a topload will help)
e.g. the schematic posted above by D. Kilovolt adds no extra capacitance to the eht for feedback purposes.

When the arc initiates the impedance of the resonator has to match the anode rf to the arc impedance.

Based on the little experience that I have, I would guess that
the resonator should be wound with magnet wire,
with about one wire diameter space between turns,
wound with a low H/D (0.5 to 1)
with as little stray capacitance as practical.

I believe that with a 'typical' TC the arc/spark is made of
a series of capacitive discharge events.
Each cycle consisting of the secondary 'ringing up' the topload voltage until the topload breaks out.
VTTC etc. are not quite the same as there is effectively a continuous discharge (CW in radio terms) into a load 'matched' to the source,
which is why the discharges look so different.
So I guess that the art lies in matching the impedance of the arc to the source.

Dr. Dark Current

Sun Jul 10 2011, 11:08AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Sulaiman, the topload increases the Q of the resonator. It also increases the reactive component of the current, but this does not flow through the anode. The higher Q of the resonator allows for a more efficient oscillation.

Sulaiman

Sun Jul 10 2011, 11:21AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Yes, the topload would increase the Q
thus increase voltage magnification
and give a more stable/defined resonant frequency
but I GUESS that it would make power transfer impedance matching worse.
Without a topload I can imagine the resonant frequency bouncing all over the place as the arc impedance varies, which would have to be allowed for in the circuit design.

Never 'played' with valves so I'll keep quiet and just 'lurk' on this topic.

Last guess - winding the anode inductor on the same former as the output resonator 'Oudin' style is probably better than two magnetically isolated coils. (again, power throughput)

Dr. Dark Current

Sun Jul 10 2011, 02:24PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

That's what the anode tap is for in my circuit, to tune the impedances (and in turn output power) right. You can not do this on a base fed resonator.

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