[TCML] Tube question

[David Rieben]

Ed,

I've also discussed my situation with Cameron Prince and he 
feels that my problem is likely with the grid resistance. I am
using a appr. 100 watt rated 0 - 25 K ceramic wire wound
resistor that I have set at 10 K. However, I epoxied the resistor
down to the base and the epoxy has also glued down the "ring"
of the slide tap so I would almost certainly destroy the resistor 
by trying to unglue it and vary the tap. Cameron stated that he
had to bring down the 10 K closer to 8 K to get his 833(C) to
work correctly with his coil after replacing the 833(A). So it
looks like I may have to opt for a high wattage variable rheostat
with a 0 -20 K resistance rating, or something like that.

David

----- Original Message ----- 
From: "Ed Phillips" <evp at pacbell.net>
To: "Tesla Coil Mailing List" <tesla at pupman.com>
Sent: Sunday, September 21, 2008 12:56 PM
Subject: Re: [TCML] Tube question


> 
> 
> David Rieben wrote:
> 
>>Ed,
>>
>>I am not currently metering the grid (or plate) current
>>in my setup, so I cannot answer the current question. I 
>>am really pretty dumb on vacuum tube coils and this one
>>was based loosely on Steve Ward's 833A VTTC that
>>is supposed to yield 22" sparks in stacatto mode. The
>>only difference is that I am using 2 seriesed MOTs
>>through the voltage doubler circuit, just like Cameron
>>Prince did on his. As a matter of fact it was my perso-
>>nal viewing of Cameron's coil's output that convinced 
>>me to take on this as my first VTTC project. Anyway,
>>I have used two or three different used 833As and always
>>got about the same output. However, I decided to or-
>>der a new (Taylor) 833C because Cameron had pur-
>>chased a used one on eBay and had reported good
>>results, mainly the plate stays cooler at a given power
>>level. However, when I tried my new 833C tube in   
>>my circuit, it had a very limited spark length output and 
>>the spark output would not increase beyond about 40%
>>on the variac knob but would instead start to take on
>>a "crackly" sound. BTW, the output at this point is only
>>about a pitiful 6 to 8", in continuous mode! Further ramp-
>>ing up of the variac beyond this point does not increase the
>>spark length any further but it is apparent that there is more
>>plate current as the graphite plate will begin to take on a
>>glow. BTW, the output sparks will continue to grow pretty
>>well linearly with increase in the input via twisting up the
>>variac knob when using the old 833A until I can't really 
>>turn it up much past 60% on the variac knob due to ex-
>>cessive plate glow (in continous mode), but the spark length 
>>at this point is around 16 to 20".  Any more suggestions, I'm 
>>all ears :^)
>>
>>--
>>David Rieben
>>
> David:
> 
>    Without current measurements I don't really know what to tell you - 
> there simply isn't enough information.  Do you have meters so that you 
> can do that?   I would suggest boosting the filament voltage and see if 
> that makes a difference.  The fact that the output doesn't go up after a 
> certain plate voltage is suggestive of limited emission and going from 
> 10 volts to 11 volts should make a significant difference and certainly 
> won't damage the tube in the short run [few minutes of operation].  You 
> didn't tell me what value of grid resistor you are using but it's 
> possible cutting it in half for a test might also increase the output.
> 
>    The increase in "plate glow" [hope it's a very dull red at most] 
> with plate voltage doesn't necessarily imply an increase in plate 
> current as the voltage increase alone will increase the power loss in 
> the tube, particularly as you don't get more output.  The plate input 
> power goes either into the load [streamers] or is lost in the tube.
> 
> Ed
> 
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