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811A VTTC problems

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alf

Sun Jun 16 2013, 05:57AM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

Hi,

My 811A VTTC has been needing a new tank capacitor for a while, since the one I first used seems to vary in capacitance when in use.
So a few days ago I tried to use 2 homemade transparency sheet capacitors in series as the tank cap to give around 2.8NF, those capacitors seem to work a lot better and do not vary in capacitance.

However, the sparks are still rather small (<4") and the plates of the tubes glow red-orange at full voltage (240V to the MOT)
One tube runs much hotter than the other, I assume this could be corrected by using a tapped grid coil, although all attempts so far to do so have failed.

I have tried adjusting the topload, primary L, primary C, grid leak resistor (and the capacitor connected across it), grid coil turns, and grid coil position. And none of these things seems to have had much of an effect towards making the sparks any longer and the tubes run cooler.
The filiments are powered in series with a resistor across one to adjust its resistance to that of the other tube's filiment, so that 6.3V is across each.

The schematic (

) shows that the bottom of the secondary coil should be at the same potential at the cathodes of the tubes, which is ground potential. I have attempted this, and it seems to increase the size of the sparks, but results in sparks/corona between the secondary coil and the feedback coil.

I am using an earth rod to ground it, which seems to work fine.

The only difference between my coil and the more successful one at

is a rather large difference in the primary former diameter - 4.3" (110mm) instead of 6.3" (160mm) as I live in UK and the closest thing I could find was 110mm pipe, although I probably could have found pipe of the correct diameter if I had looked a bit harder XD

The secondary former is slightly different in size, but not much so... The one specified in the circuit is 2.3" (58mm) and mine is 60mm PVC drainage pipe if I remember correctly.

As far as I can tell, the primary resonance doesn't need to be critical, besides I have tried to change the capacitor and it doesn't seem to help

Should the bottom of the secondary be connected to the cathode of the tubes?
Could this large difference in diameter of the former be the cause of the small sparks and the tubes running too hot?
And in general, any ideas how I can make it work a bit more like its supposed to?

Thanks in advance, Alf

Dr. Dark Current

Sun Jun 16 2013, 10:39AM

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

Hi
1. Did you calculate resonances using JavaTC?
2. Are the tubes new/matched?
3. You need a proper RF capacitor, any homemade one will be lossy in a VTTC.

-Jan

alf

Mon Jun 17 2013, 06:50AM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

Dr. Dark Current wrote ...

Hi
1. Did you calculate resonances using JavaTC?
2. Are the tubes new/matched?
3. You need a proper RF capacitor, any homemade one will be lossy in a VTTC.

-Jan

Thanks for replying...

1-No, I'll do that
2-They are new, and I believe that they are matched aswell
3-Ohh, I will get one and perhaps this is the problem

Thanks again, Alf

Dr. Dark Current

Mon Jun 17 2013, 08:28AM

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

As for the calculations, tune the primary a bit lower than the secondary, and make taps on it. This is an important thing to do.

Then, tubes in parallel sometimes tend to oscillate in push-pull manner on a very high frequency. Few tens of Ohms resistors directly in series with the grids will fix this.

-Jan

alf

Mon Jun 17 2013, 01:07PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

Dr. Dark Current wrote ...

As for the calculations, tune the primary a bit lower than the secondary, and make taps on it. This is an important thing to do.

Then, tubes in parallel sometimes tend to oscillate in push-pull manner on a very high frequency. Few tens of Ohms resistors directly in series with the grids will fix this.

-Jan

Okay, I will remove the plastic insulation from a section of the primary windings, and solder on wires to each turn.

Right, will try this aswell

Thanks again.

PhilGood

Mon Jun 17 2013, 05:42PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

alf wrote ...

The schematic (

I built a 811A coil based on the same schematic, but one tube only

I connected the bottom of secondary to ground (mains earth) and had no problem with this. I dont think such coils really need a separate RF ground because of their low power and because they work in a non-disruptive way.

With the primary and secondary diameters you used, it is weird you are having sparks between secondary and feedback coils when connecting secondary to mains ground. I'd suggest you put your feedback coil under the primary (where secondary's voltage is lower)

As Dr DC said, a decent primary cap is mandatory. Russian K15Y-1 series

are great for this and are easy to fond on ebay.

Could you post some close pics of your whole assembly ? might help

alf

Sun Jun 23 2013, 09:07PM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

PhilGood wrote ...

alf wrote ...

The schematic (

I built a 811A coil based on the same schematic, but one tube only

are great for this and are easy to fond on ebay.

Could you post some close pics of your whole assembly ? might help

Wow. 15CM sparks with one tube

Okay... A decent cap. Will get one soon

Yeah I will take pics and post them when I get a chance.

I simulated the coil with JavaTC as recommended. Sorry it has taken me 5 days to get round to replying somehow

Here are the results:

Thanks for your help..

Alf.

PhilGood

Mon Jun 24 2013, 12:00AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Hi Alf,

I'm not very familiar with all JavaTC's outputs

One thing I noticed:

'JavaTC' wrote ...

426.21 kHz = Secondary Resonant Frequency
462.56 kHz = Primary Resonant Frequency

Your Primary Resonant Frequency should be at least 10% lower than your Secondary Resonant Frequency

For this you can either add more turns or slightly increase primary C

Then you will use taps to adjust primary coil to the best value

alf

Tue Jun 25 2013, 08:04AM

Registered Member #3925 Joined: Fri Jun 03 2011, 10:50AM
Location:
Posts: 121

PhilGood wrote ...

Hi Alf,

I'm not very familiar with all JavaTC's outputs

One thing I noticed:

'JavaTC' wrote ...

426.21 kHz = Secondary Resonant Frequency
462.56 kHz = Primary Resonant Frequency

10% lower... Right
I really need to get a proper cap anyway XD
And add taps. Okay will do that

Thanks again.

Alf.

PhilGood

Tue Jun 25 2013, 10:54PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Hi Alf,

Sorry I just realise 10% might not be enough

You should plan on a primary with a resonant frequency 40% lower than your secondary frequency
Then make taps that allow you to tune your primary from +20% to -40% secondary resonnant frequency

Just change the number of primary turns in JavaTC to find out what is the total number of turns you need, and at which turn you need to start making taps

It's always better to have more taps to play with when tuning your coil, rather then having to build a new primary because you don't have enough turns (this happened to me)

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