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Re: [Amps] tetrode reverse screen grid current

Subject: Re: [Amps] tetrode reverse screen grid current
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Sun, 29 Jan 2012 19:56:58 +0000
List-post: <">>
Hsu wrote:
>  Maybe I can not  describ it clearly. Sorry for my bad English.

Don't worry, your English is fine, but sometimes we all need to use more 
words  :-)

>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

A shunt regulator is probably the best solution for tetrodes up to the 
4CX1000-1500W class, but for larger tube like the 4CX5000 or 4CX10,000 
the shunt circuit consumes a lot of DC power, so we have to look for 
something more efficient.

>.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 
> Is It a good idea?

I agree with that analysis - a series/shunt regulator will decrease the 
power dissipation in the screen supply... but there is a price to pay.

Instead of one regulator circuit, we now have two, so we have doubled 
the circuit complexity. It may not be easy to guarantee a smooth 
handover between the two regulator circuits under dynamically changing 
conditions such as SSB.

Also, any screen regulator circuit MUST be able to survive a direct HV 
arc from anode to screen. The screen supply MUST protect the tube and 
the socket (with the often-fragile screen bypass capacitor) and of 
course the power supply must protect itself. By "protect", I mean shut 
everything down without damage, and then come back online with a press 
of the Reset button.

That isn't easily done.

>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.

No, it isn't... at that power level, nothing is.

>I 'm studyig Racal's design now, it is unique.

Details, please?


73 from Ian GM3SEK
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