Re: VTTC grid leak

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

Original poster: "Bob (R.A.) Jones" <a1accounting-at-bellsouth-dot-net> 


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, August 30, 2004 6:50 AM
Subject: VTTC grid leak


 > Original poster: herwig.roscher-at-gmx.de
 >
 >
 > Hi fellow VTTC coilers,
 >
 > Having built some well functioning VTTCs, I still am doubting that I
 > understand the grid leak circuit correctly  ;-)
 >
 > In the archives I found differing opinions. It is agreed on that "As
 > voltage is applied to the plate first, the tube conductes and the
 > tank circuit is pinged, transferring energy to the grid coil."
 >
 > But then:
 > Version A): The grid coil sends some *positive* voltage to the grid,
 > turning the tube further ON. The grid draws some current, which
 > develops a voltage drop across the grid leak resistor. It is stored
 > in the grid leak capacitor and builds up to form a negative bias
 > voltage, which cuts the tube OFF. Grid voltage swings from negative
 > to positive voltage.
 >
 > Version B):  The grid coil sends a strong *negative* pulse to the
 > grid, which turns the tube OFF. The negative charge is stored in the
 > grid leak capacitor. Grid voltage swings from negative to zero
 > voltage (GND).
 >
 > For both versions the story continues:
 > "Eventually the grid leak resistor bleeds off the charge of the grid
 > leak cap and the tube conducts again."
 >
 > Tending towards version A), I really would like to understand the
 > mode of operation.
 >
 > Bye Herwig
 >

Hi Herwig,

Here is my version:.

The usually idea of the circuit in a class C (valve only conducts for part
of half a cycle)  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.  i.e. automatic level/bias control. It
also possible to arrange the component values so that it oscillates in a
burst mode.

At turn on the grid leak capacitor is discharged so the tube is biased on.
Because the tube is biased on the loop gain exceeds one (if its designed
right) so any noise or switch on transient grows in to an oscillation.

Assuming the time constant of the grid leak C and R is long compared to the
oscillation frequency and R is not too small,  the positive peaks on the
grid draw current which charges the grid leak C  over one or more cycles
As the negative bias increases the conduction period decreases reducing the
oscillation amplitude.
Eventually the oscillation amplitude stabilizes when the grid current is
equal to the current flow thru the grid leak R and with a negative bias on
the grid. So the tube only conducts at part of the positive peaks of the
drive signal ie class C operation.

The majority of the AC signal at the grid, both positive and negative, is
due to the grid drive. Its the DC value that determines the operating point.
i.e. its not a relaxation oscillator using the RC/grid current  time
constant as part of the cycle.

Bob Jones