Re: 833A Tube Coil (fwd)

---------- Forwarded message ----------
Date: Sun, 24 May 1998 13:05:19 EDT
From: FutureT <FutureT-at-aol-dot-com>
To: tesla-at-pupman-dot-com
Subject: Re: 833A Tube Coil (fwd)

In a message dated 98-05-23 17:00:24 EDT, you write:

<< Hi Guys & John Freau,
> Well here it is. My 833A tube coil. Fired it up today with two tubes.
 >Supply voltage 1600AC from a rewound microwave oven transformer. Spark
> length 180mm (7 inches).
> Not sure if this is what to expect under the conditions. At least it's
> working. Was supprised to that the anodes glow dull orange at this low
> supply voltage.

Hi Pete,

Congratulations on first light with this new coil.  If the MOT is still
current limited, it will limit the spark length to about 12" under the
best of conditions.  It takes some playing around with the adjustments
to optimize the output.  You may want to add a voltage doubler using
the cap and diode from the oven.  this can increase the output to at
least 20" sparks.

> Primary is 7" with two sets of windings, 9 turns each for the plate and
> grid coils, wound clockwise. Spacing between the coils is 1". Coupling
> could be too tight???

Tube coils seem to tolerate close pri to sec coupling pretty well.  I
usually make the grid coil to primary coupling adjustable.  

> The secondary is 4.3" x 19.5" wound with 23 gauge wire, wound clockwise.
> Blocking capacitor is 1250pF. The grid capcitors and resistors are
> 1000pF and 2.5K 50 watt resistors. Tuning appears to be very broad. I
> can vary a wide spaced variable a full 250pF without noticably affecting
> spark length. Tried various fixed caps in parrallel with the 250pF
> variable "2500pF, 2800pF 3250pF 3500pF and 3750pF. Found 3250
> combination to give best results.

Generally, as the tuning is varied through the range, the spark will
lengthen, and tubes will be cool, then at the center of the range, spark
will be the same length but brighter and tubes will get red, then at the
other end of range, the tubes will be cool again, but spark length will
remain the same.  I often set the tune point away from the center
of the range to keep the tubes cool.  Have you tried adding more
grid resistance and varying the distance between the grid and plate
coils to vary the grid feedback?

It is also possible that best tuning will be found outside the 
range of cap values you used?  (just a thought...this happened
to me one time).

Also, the tubes may run cooler using more primary turns and a
smaller tank cap, to raise the tank impedance.  I usually use about
18 to 23 primary turns with a .004uF cap.  I also use 23 primary turns with
a .0015uF cap, in a small coil with a 13mH secondary.

> There is RF getting back to the HT transformer despite a choke. The
> choke is a 2" toroid with 40 turns 1mm wire. I also noticed some
> "sparking" in my variac at full voltage.

I never use any chokes when running on AC and have not
seen any variac sparking in
my coils when run on AC.  Using pulsed or pulsed DC systems I
have had the variac problems, and had to use good chokes and
bypass capacitors.  Maybe your chokes are ringing?

> Not too sure if the tuning is critical too within a few pF to achieve
> best results. Does not appear so.

> Do not have a 4.5kV supply to really pump the beast up. Will have to
> scout around for a good HT transformer. I don't think my 5 amp variac
> will take the punch either.

The voltage doubler or *level shifted* approach may be an easy way
to get a longer spark.

John Freau
> I have the schematic with full details for anyone who is interested.
> Just Email me. The schematic works with 812A and 8005 triodes. Have
> tried both. The schematic is drawn with Microsoft Office Excel spread
> sheet.
> Happy Sparking
 >From a very happy Pete