[Amps] tetrode reverse screen grid current

[Hsu]

Hi Ian,
   Maybe I can not  describ it clearly. Sorry for my bad English.I have read 
your article and the manuals carefully many  years ago, very good job!
But I do not think it can use my 4CX5000R project othewise modify the screen 
shunt power supply  greatly.
The shun regulater with low   efficiency although it  can hold negative 
screen current,So I do not think pure shunt regulator in big amp( l 
4CX5000/10000/15000) is good option
.but series  rugulator cann't hold  negative screen current, if we use 
series power supply for
secreen power supply , we have to put  a HV block diode and a  shunt 
bleeder resistor ,  when the negative current happened, the block diode is 
turned off and the powewr supply  output current is zero and negative 
current gos along
bleeder resistor  to ground, in this condition, the screen voltage   is not 
regulated. If  we combine a shunt regulator in a series rugunlator's outpuyt 
( use a smaller   bleeder resistor
series a HV MOSFET  as a shunt regunlator), when the negative  current 
happened, if the screen voltage gos high, and over series regulator's out 
put  a bit( I assume is 0.2V, as less as possible,just  only  keep the block 
diode  turn off) the shunt rugunlator will be  active. In this condition, 
screen voltage still regulated. in postitive current condition, only series 
regulator work, the shunt regulator will not active.
  Is It a good idea?
I have find  many commerical broadcasting transmitter's screen power 
supply's design, most of them use series regunlator+block diode+ shunt 
bleeder resistor. It is not perfect.
I 'm studyig Racal's design now, it is unique.

    73!Hsu



-----原始邮件----- 
From: Ian White GM3SEK
Sent: Sunday, January 29, 2012 4:36 PM
To: amps at contesting.com
Subject: Re: [Amps] tetrode reverse screen grid current

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
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