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Re: [TCML] Tube question



Hi All,

Does anyone (maybe in the over 50 crowd) remember about
tube characteristic curves and how to use them to determine
the proper operating points for VTs? It seems that 45 years
ago this was the right way to determine the proper bias values
for tube circuits, but I've never heard anyone on the list ever
discuss doing a proper analysis, so maybe it's a lost art that I
only vaguely remember?

Matt D


-----Original Message-----
From: David Rieben <drieben@xxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sun, 21 Sep 2008 6:04 pm
Subject: Re: [TCML] Tube question


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) 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 c
ontinuous 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 very20dull 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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