Re: 833 coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Mddeming-at-aol-dot-com 

In a message dated 9/26/03 10:34:03 PM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:
I recently acquired an 833a tube, and would like to proceed with making a
coil.  The only requirement that I want to impose on this project is
longetivity, even at the expense of spark length.  The goal is to be able to
run for extended periods without worrying about tube damage, allowing for
various experimentation.

Hi John,

If component longevity is really critical to you rather than the 
"push-to-failure" that many people advocate, here are some tips:

If not using forced air cooling, Keep the plate voltage below 3300 volts 
(1500 watts). With forced-air cooling (RCA recommends 40 cfm 2" nozzle 
vertical between grid and plate seals) then V Max of 4000 volts (2000 
watts) is acceptable. Even at these ratings, the bulb temp. can hit 145C. 
and the plate will be red-orange.

Since the filament is the cathode, the insulation between primary and 
secondary of the filament transformer should be good for a minimum of 6000 
volts. Steve Flowers of Pineville, LA frequently has filament transformers 
salvaged from X-ray equipment with 10KV and up insulation for sale on eBay. 
I recently bought a 16 v one with 75KV insulation from him for about $10 
with a like amount for shipping. You can write directly to him at 
<mailto:SFLOW26426-at-aol-dot-com>SFLOW26426-at-aol-dot-com.

For best performance, a Variac is highly desirable and I would recommend a 
12 V x 10A transformer adjusted to give 10.0 V measured across the tube 
pins. Only 9.5 V will give poor performance, while 10.5 V will shorten tube 
life noticeably. It is a good idea to bring the filament up to voltage and 
stable temperature before cranking up the plate voltage and to shut down 
the plate supply before turning down the filament.

I would strongly recommend buying a reprint of the RCA Transmitting Tube 
Manual TT-5, available from Antique Electronic Supply of Tempe AZ. 
Carefully study the 833A data on pages 145-146 and circuit 5-22 on page 31, 
which is a single 833A class C oscillator designed for 450KHz operation.

It is a good idea to build in a couple of DC milliammeters into the circuit 
to limit plate current to <500 mA and grid current to <100 mA.

When buying used tubes or commercial pulse caps, as on eBay, you never know 
the age or life history of the item. However, these tips will stretch the 
available life out quite a ways, but will not set any streamer length records.

Hope this helps,
Matt D.