Re: VTTC grid leak

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

Original poster: "James" <mustang3-at-cox-dot-net> 

Hi list,
     I have also wondered about the grid leak. I lean toward version "B". I
have never observed the grid to swing positive when the coil is in proper
tune. I feel this would lead to instant failure of the toob. On my VTTC's,
the grid runs from -100 to -3000 volts. The grid leak resistor needs to be
small enough to discharge the GL cap every pulse. As you change the GL cap,
the grid waveform and sound of the coil change. I have noticed spikes in the
grid waveform if the cap is the wrong value or the resistor too large in
value. These spikes will sometimes go to a positive value. This is a bad
thing. The tube heats excessively and draws more current. I run all these
tests using a variac to control the plate voltage. I also monitor the coil
with a Spectrum Analyzer. These odd pulses are not the coil's fundamental.
More evidence that the spikes are bad.

Later,

James
     P.S. I'll have the toob coil running at 15KW for the Teslathon. MORE
POWER

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, August 31, 2004 8:47 AM
Subject: Re: VTTC grid leak


 > Original poster: herwig.roscher-at-gmx.de
 >
 >
 > Bob (R.A.) Jones <a1accounting-at-bellsouth-dot-net>  wrote:
 >
 >  > Here is my version:.
 > - Thank you, Bob.
 >
 >  > The usually idea of the circuit in a class C  oscillator with a tuned
 >  > anode load is to automatically adjust the DC grid negative bias so that
 >  > a particular operating point or oscillation amplitude is archived.
 > - So the grid coil is responsible for driving the control grid with
 > correct phase and drive level and the grid leak combination sets the
 > desired negative bias. Are there interactions?
 >
 >  > Because the tube is biased on the loop gain exceeds one. So any noise
 >  > or switch on transient grows in to an oscillation.
 > - So the control grid voltage is in phase with plate current?
 >
 >  > Assuming the time constant of the grid leak C and R is long compared
 >  > to the oscillation frequency
 > - 500 pF and 680 Ohms equals 340 ns
 >    half cycle of 6.5 MHz equals 75 ns
 >    Measured swing of control grid voltage from -280 V to + 280 V. Bias
 >    therefore 0 V.
 >
 > Increasing the value of the grid leak resistor to 5 kOhm changes the
 > swing to -330 V to + 200 V. The voltage still is sinusodial
 > (unexpected and a little bit strange). Bias calculates to -65 V (tube
 > is 4-125A).
 >
 >  > So the tube only conducts at part of the positive peaks of the
 >  > drive signal ie class C operation.
 > - I assume that the bias should be around -200 V. Correct?
 >
 > Are my comments correct?
 >
 > Cheers,
 >
 > Herwig
 >
 >