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How grid leak system work and how to design it?

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hsieh

Thu Apr 26 2012, 12:36AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

After searching the internet I still don't understand how grid leak system in a VTTC work and how to design it.

Steve's VTTC FAQ seems to says that grid leak system is used to turn off the tube for a portion of a cycle in order to limit power.But an old electronics textbook I found says that grid leak system is a class C oscillator is used to produce the necessary negative bias for the tube.Which one is correct? or is this the same thing?

And how can I determine the R and C value when building a VTTC.The old electronics book I found says that the RC time constant should not be to large or the oscillation may become intermittent.How much is to large?

I am now planning to rebuild my 811A VTTC. If the next time I am going to use a tube that has never been used by other people before, how can I design my grid feedback system?

Thanks

Steve Conner

Thu Apr 26 2012, 07:41AM

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

Both explanations are correct. The negative bias voltage is what causes the tube to turn off for a portion of the cycle. However, the circuit actually performs a third function, it helps to drive the control grid positive for more power.

If you use a triode that was designed for self-oscillating service, the tube datasheet should tell you what R and C to use.

For tetrodes and pentodes the situation is more complex. They can't take much grid current, so R ends up large. C then has to be tiny to avoid the intermittent oscillation, also known as "squegging".

hsieh

Thu Apr 26 2012, 01:03PM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

Steve Conner wrote ...

If you use a triode that was designed for self-oscillating service, the tube datasheet should tell you what R and C to use.

I read those datasheets of power tubes(811A,805,810 and some less common power tubes) again and again, but I cannot find where this information is.

If it is not given,how can I determine these values?

Steve Conner

Thu Apr 26 2012, 01:18PM

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

Well, look in the 811A datasheet:

in the section "RF Power amplifier and oscillator". It recommends a grid resistor of 1100 or 1750 ohms.

The datasheet also states values for DC grid voltage and DC grid current. You can divide the voltage by the current to get the grid leak resistance.

I don't know how to find the capacitor value, except that the RC time constant should be long compared to one cycle of RF, but not too long.

Dr. Dark Current

Thu Apr 26 2012, 02:39PM

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

I usually design the RC time constant around 5 times longer than one HF cycle. The higher the Q of your tank circuit, the larger time constant you can use and the more efficient the circuit becomes. But using large time constants with low Q tank circuit can result in the circuit losing the oscillation, which in classic VTTC circuits usually leads to huge overvoltages and destroyed doubler diode or arc over in the transformer, so beware.
The resistor should be small enough to drive the grid just right, but not too small, otherwise it would dissipate a lot, could overpower the grid and you would have problems tuning the coil to a low conduction angle (which is another aspect of the design which influences the efficiency). You could possibly use a large resistor and put a lot of turns on the grid coil and get good efficiency, but this can result in grid-cathode overvotlages and spark-overs.

hsieh

Thu Apr 26 2012, 03:13PM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

Another VTTC related question:
I have two mica capacitors that marked:
working voltage:2.5KV DC
test:5KV DC

if I connect them in serial,can I use them in a MOT powered VTTC?And will they be better than MKP capacitors(use as tank capacitor)?

Dr. Dark Current

Thu Apr 26 2012, 10:26PM

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

I've never seen mica caps used as a tank capacitor. Unless it's some special type designed for high current high frequency application, I think they'll fail very soon. Your best bet is on special ceramic caps which are made especially for this purpose and usually state the kVAr rating, the second best is a MMC out of MKP caps, if you go this route I suggest to overrate the MMC to maybe 3 times the peak voltage it will see.

Dr. Drone

Fri Apr 27 2012, 03:26PM

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

Dr. Dark Current

Fri Apr 27 2012, 10:15PM

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

Hmm, I've never managed to heat up a proper RF ceramic cap. Not even a little, they just seem to stay at room temp no matter what I do. I have a fist sized cap in my VTTC which shoots up to 24" sparks, and again the cap just stays cold.
I have some mica caps but never really used them in RF duty. I know of someone who put them in an SGTC and they did heat up a lot, a MMC out of MKP caps fixed this. I do use a HV mica cap, but only as a rf bypass.

Edit: There are HUGE differences between different types of ceramic caps. Some are just for low current or rf bypass duty, and these don't work as a tank capacitor. Most "doorknob" caps are useless. The "good" ones are those used in tube induction heaters and transmitters, and look like this

One more edit: It is possible there are differences between mica caps as well. It is just easier and cheaper here to get a good ceramic cap than good mica one.

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