Valve T-Coils (fwd)

---------- Forwarded message ----------
Date: Sun, 12 Jul 1998 19:50:03 +0200
From: Pete <casius-at-cyberserv.co.za>
To: tesla-at-pupman-dot-com
Subject: Valve T-Coils

Hi to All,

Been away on business for a while hence me not being heard on the list.
This weekend decided to try the 5762 triodes. Of the four valves I
obtained for a few $, one performed rather well. The other three were
poor to very poor. ( Over worked in the old AM transmitters I guess ).
All tests were done by removing the two 833's from the existing unit and
placing the 5762 on a bakalite base on top of the 833 unit. (A bit messy
and not very eligant). I changed the HT supply to 3000 volts via a 10uF
cap and diode. I used a 12 volt car battery for the filaments. Tied one
end of the filaments to ground. The grid feed back was left unchanged
with a 2.5k 50 watt resistor in parrallel with a 1000 pf 6kV capacitor.
Tank circuit was left as is.

At 70% variac position, the 5762 gave +/- 400mm discharge. At 80% variac
the discharge was +/- 500mm. At this time I noticed the old too familiar
smell of something burning. I glanced down to see "smoke" coming from
the variac. I shutdown the system and allowed the variac to cool down.
In a daring and maybe stupid urge, I quickly turned up the variac to
100% for a few seconds, the discharge was maybe 600 - 650mm. The variac
was hot even for this very brief burst. The old 4 amp variac definitely
can't handle this load. I all cases the ammeter in the grid feed back
showed 100mA to 120mA. I did not check for current drawn in the HT
supply. Will do so next time. Will fish out the 10 amp variac from some
where in the garage in one of many boxes. Which box is any ones guess. I
could connect directly to the mains but prefer not to at this time. If I
can nearly fry the variac, I'm a liitle concerend about the house wiring

Have now obtained a second (second hand) 12kV 30mA NST to put in
parrallel with the existing. Fired up the coil with the newly aquired
NST to check it. Once again only got 12" discharges. Before I double up
NST's , I want the coil to perform better. At the moment I am only
tuning by tapping one turn at a time.
I also need to put on a much bigger top loading. I'm  using a 9" x 1.5"
toroid at the moment which breaks out in all directions.
Question??? How critical is the tuning on an inverted saucer primary.
Does one have to be precise to 1/16 turn for maximum spark. How much
will 1/2  or a 1/4 turn out reduce performance??

Regards to all
Good Sparking
Pete Scully