Re: VTTC with 4-125 tube

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

Original poster: "Wim Bosma" <wbosma-at-stud.tue.nl> 

Hi Herwig,
A 4-125 (same as QB3/300)  will draw anode current with its screen at 0
Volts if the control grid is not negative enough. Zero bias mode use by
connecting control grid and screen grid is sometimes practized to simulate a
triode. From your description of a brush discharge without screen voltage I
concluded that your circuit is unneutralized, hence capacitive throughput of
HF energy through the tube and lessened max. modulation depth.
Greetings, Willem,  PA0TW


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, August 22, 2004 1:53 AM
Subject: VTTC with 4-125 tube


 > Original poster: herwig.roscher-at-gmx.de
 >
 >
 > Hi VTTC coilers,
 >
 > I am trying to audio modulate a 4-125 tube. As I was not satisfied
 > with the audio quality from control grid modulation, I am using
 > screen grid modulation now. Plate voltage is 2500 V and screen grid
 > voltage is 175 V +- 175 V (i.e. the screen grid swings between 0 V
 > and 350 V). To my surprise there is an oscillation with V p applied
 > but without V g2 (i.e. g2 tied to GND).
 >
 > Is the influence of the screen grid of transmitting tubes on I p so
 > small (tubes used in radios cut off any I p if there is no V g2) ?
 >
 > Cheers,
 >
 > Herwig
 >
 >
 >