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Re: Grid-leak resistor (fwd)
---------- Forwarded message ----------
Date: Tue, 25 Feb 1997 07:37:35 -0500 (EST)
From: "Mark S. Rzeszotarski, Ph.D." <msr7-at-po.cwru.edu>
To: Tesla List <mod1-at-poodle.pupman-dot-com>
Subject: Re: Grid-leak resistor (fwd)
Hello Tube Coilers,
Tero Ranta <dixie-at-icenet.fi> said in part:
>I'm building a vacuum tube tesla coil using four 811A's in parallel.
>How can I calculate the value of grid-leak resistor?
>
For starters, TCBA News, V. 3, No. 3 shows a tube coil using two
811A's in parallel with a plate supply of 1450 volts. The author uses a
grid resistor of 4000 ohms. You might consider starting with two tubes to
get things going. This design works, although it is not optimal.
The specs on an 811A for operation at 1500 volts on the plate call
for a maximum grid current of 50 mA. For class C operation as an
oscillator, typical operating conditions are 40 mA grid current with a grid
voltage of about -70 volts. That means a grid resistor of about 1750 ohms
is appropriate for a single tube. The maximum safe grid voltage spec is
-200 volts. To be safe, start with a grid resistance which is near the 1750
ohm value, and adjust it slowly while monitoring the grid current with a
meter to keep it below 50 mA.
You may also need to balance the currents for each tube. This can
be accomplished by using a 47 ohm resistor from the grid of each tube to the
main grid bias resistor. Check the wattage of these components!
Finally, the grid voltage also depends on the amount of feedback
signal you are driving the tube with. You may need to adjust your operating
point a bit if your feedback voltage is too low/high.
Regards,
Mark S. Rzeszotarski, Ph.D.