[Amps] tetrode reverse screen grid current

[Ian White GM3SEK]

Hsu wrote:
>I have a thought about shunt bleeder resistance ,If we  use a HV MOSFET 

seial  a resistor to replace it, when the negative screen current 
>happened,
control  the total  resistance so  the screen voltage still 
>constant,  Is 
it a good idea?

It works for me... and about 800 others, so far (shameless plug :-)

<http://www.ifwtech.co.uk/g3sek/boards/tetrode-1.htm>

Negative screen current happens in many of the tetrodes that we use for 
amateur-sized power amplifiers. Whether this is a practical problem will 
depend on many factors: the tube (not only the type, but also each 
individual example); the tuning, loading and drive level; and the screen 
voltage regulation.

'Negative screen current' forces current backward into the screen 
supply. This will tend to drive the screen voltage upward, which 
increases the screen-grid dissipation and the temperature of the grid 
structure, and thus causes even more secondary electron emission. If the 
screen supply fails to control the situation, it can quickly lead to a 
dangerous runaway - often ending in an anode-screen flashover.

An active shunt regulator will hold the screen voltage far more constant 
than a passive bleeder resistor could, but it does need to be designed 
correctly. The voltage regulator also needs to be backed up by a current 
trip, to take the amplifier safely offline before any dangerous 
condition is reached.

For more details about tetrode screen supplies, please see my QEX 
article:

<http://www.ifwtech.co.uk/g3sek/boards/tetrode/tetrode-3.htm>



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek/boards