833A Tube Coil (fwd)
---------- Forwarded message ----------
Date: Sat, 23 May 1998 19:50:22 +0200
From: Pete <casius-at-cyberserv.co.za>
Subject: 833A Tube Coil
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
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???
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.
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.
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.
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
>From a very happy Pete