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@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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