Re: [TCML] New 833C VTTC - poor performance

[Quarkster <quarkster@xxxxxxx>]

Kris -
   
  It sounds like there may be some confusion regarding filament voltage and grid drive current.
   
  DC Cox questioned your grid drive current, but you responded with some details about your filament transformer. These are totally different issues.
   
  Measure your filament voltage directly at the tube socket pins. If it's 10 volts, +/- 4 tenths of a volt, then the filament current will be automatically be OK at ~10A. Remember to pre-heat the filament for at least 30 seconds BEFORE you apply plate voltage, and also to measure filament voltage when the plate supply is turned on. Both filament and plate transformers in parallel may cause the AC line voltage to sag, especially if you are powering the coil through any sort of extension cord.
   
  It would also be interesting to measure the grid voltage and drive current.  Typical 833A grid voltage is -150 volts, typical grid current is 70-90ma.
   
  Regards,
  Herr Zapp
   
  
Kris Grillo <kristianisawesome@xxxxxxxxx> wrote:
  D.C., list,

"check to be sure the TDK tank cap is rated for RF current and is not a DC
filter type cap."

As Garry mentioned, it is not specifically stated in the spec sheet, but it seems implied. Besides, this is the same capacitor that Steve used on his 833 coil and had much better success than I am having.

"you may also have to experiment with the position of the grid leak coil --- sometimes it is necessary to have it 6-10" up the sec coil for best
results."

I have already done this. It is at about 9" up the secondary.

"measure your grid drive current to be sure you are driving the 833 properly."

This may be the problem. I am having nothing but problems with this "homebrew" filament transformer. I am without a means to test the current at the moment. I would assume it is at at least 10A. It is a 1200V MOT with the secondary and shunts removed. New secondary wound with 12 gauge for 10 volts out. It must be eating up some power, judging by its temperature after about 10 minutes, but could it be enough to bring the current down below 10A? 







--- On Fri, 8/1/08, DC Cox wrote:
From: DC Cox 
Subject: Re: [TCML] New 833C VTTC - poor performance
To: "Tesla Coil Mailing List" 
Date: Friday, August 1, 2008, 2:23 PM

check to be sure the TDK tank cap is rated for RF current and is not a DC
filter type cap.

you may also have to experiment with the position of the grid leak coil ---
sometimes it is necessary to have it 6-10" up the sec coil for best
results.

measure your grid drive current to be sure you are driving the 833 properly.

Dr. Resonance




On Fri, Aug 1, 2008 at 12:05 PM, Kris Grillo
wrote:

> I just finished my first VTTC. It's a single 833C closely following
Steve
> Ward's single 833A schematic. The specs:
> Secondary - 3.5" x 17" x #26
> Topload - 2" x 7.5" toroid, about .25" above the top turn.
> Primary - 29 turns #12 THHN on a 6.25" green pipe. Bottom turn level
with
> secondary bottom turn.
> Filament supply - rewound mot, 10V even (does get warm).
> Plate supply - very large (physically) old mot, shunts removed, voltage
> doubler 2.05uf, mo diode, 2nf10kv ceramic disk
> Tank cap - 2nf 40kV TDK UHV doorknob
> Plate filter - 12 turns #12 on 50ohm 20W
> Feedback coil - 20 turns #26
> Grid leak - 4k 125w variable, 2nf 40kV TDK UHV doorknob
> Everything is wired with #12 wire. All connections are tight.
> No staccato yet
>
> For some reason, I can't get past 6" - 8" arks and about 75
volts in from
> the variac without the plate starting to glow orange. I've tried 24 -
20
> turns on the feedback, 20 - 31 turns on the primary, varying the grid leak
> resistance between 1K - 4K, varying the feedback coil from .5" above
the
> primary all the way to 8" above. I tried going with 1.75uf and 2uf on
the
> votage doubler. I tried a 4" pvc secondary with 19.75" of #26. I
know the
> 833C can do better than 8" arcs. It's a brand new Taylor from RF
parts, by
> the way.
>
> I'm out of ideas. Anyone have any suggestions?
>
> Thanks
> Kris
>
>
>
>
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