Re: VTTC grid leak

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

Original poster: herwig.roscher-at-gmx.de 


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

 >>  - 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?
 > Yes there must be some interaction.

Bob,

Increasing the grid leak resistor from 680 Ohm to 50 kOhm lead to an
increase of the grid bias from - 25 V to -145 V. However the height
of the brush became shorter with resistances larger than 15 kOhm. The
tube runs much cooler now.

 >>  The voltage still is sinusodial  (unexpected).
 > Why would you expect it not to be sinusoidal ?
- Because the impedance of the grid changes from high to low (grid
current). I expected the positive parts of the control signal to
become flattened.

 > Assuming no break out. I would have expected the total swing to
 > decrease because as you increased the negative bias the anode swing
 > should decrease.
- Without breakout the control signal is 370 Vpp and with breakout it
is 550 Vpp.

 > Put say a 1 ohm resistor in the cathode circuit to monitor  the
 > current.
- Unfortunately the cathode of the 4-125A is heated directly i.e. the
filament current of 6.5 A ac would flow through the resistor as well.
I will try to monitor the plate current instead.

As this circuit provides automatic level/bias control, I now
understand, why it is so difficult to amplitude modulate it. The
modulator tries to change the amplitude of the signal and the
oscillator tries to keep it constant.

Cheers,

Herwig


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Greed is the root of all evil!
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