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@xxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx> 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) currentin my setup, so I cannot answer the current question. I am really pretty dumb on vacuum tube coils and this onewas 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 powerlevel. 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 thevariac 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 RiebenDavid: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 _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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