[AMPS] Screen protection WHAAAATTTTTT!!!!

[measures]

>
>Let me see if I have got lost here
>
>Rich said
>
>>Burning up a tetrode's screen from
>>>>mistuning is unlikely because screen supply power transformers usually
>>>>have fast-acting fuses in the primary.
>>>>
>
>Explain to me how this could be anyone only or primary protection against
>screen overcurrent and be considered effective?
>
The fuse opened when the screen current exceeded approx.  150mA.  Normal 
screen current was 120mA.  

>Ian G3SEK said
>>>Can anyone think of a *worse* way to protect the screen of a tetrode?
>>>
>
>In my opinion I have to say an emphatic no!
>
You should explain why.    

>Rich said
>
>>>Substitute a 20A slow blow fuse for the fast blow screen fuse?  Wrap
>>>aluminum foil around an open fuse and reinsert in the screen fuse holder?
>
>Sorry I stand corrected this is worse, well done Rich!
>
Thanks.  The aluminum foil shenanigan is clearly a slam-dunk. 
>
>>>-  During the entire time that the Plywood Box was in service, it
>>>consumed less than 1 box of 5 fuses.  Rated screen dissipation was 275w.
>>>The fast blow fuse opened at roughly 200w.
>
>That Rich is not a very scientific presentation of data is it?Are you saying
>that because 5 fuses 

I blew 4 fuses do to tuning errors.  Once I had a tuning chart filled in, 
I rarely blew a fuse.  

>actually blew that this constitutes proof of "adequate"
>protection?How roughly is roughly?Within 10 percent 74.35 percent
>what?Mmmm.............
>
The screen is rated at 275w.  
 
>Rich then wrote
>
>>>On AMPS, there are those who know what's what, and there are those who
>>>pretend they know.
>
>Could you please submit a list of those who know what's what and those who
>don't Rich?Just so I know who to ignore.
>
Watch for sea stories. 

>Tom said
>>Fuses mainly prevent drastic melt-downs.
>
>Rich said rather usefully
>
>>>[chortle]
>
>Why is that funny?Are you still suggesting that a fuse in the primary of a
>screen supply transformer is the "Best" protection?

The best protection is the simplest one that works.  

> Cmon Rich you can back
>down on this one and we won't hold it against you
>
1500v fuses are expensive. 250v fuses are not.  

>Tom wrote
>>Electronic circuits are
>>much more reliable
>
>Certainly more accurate and repeatable.
>
Shutting off a 600v-rated pass-FET in a regulated 1500v supply has 
consequences.  The first thing that happens is the pass-protect thyrister 
fires and the output goes to full throttle.  

>Rich wrote
>
>>>Transistors that C/E short offer zero protection.
>
>Ok so sense the voltage drop across a resistor across the screen supply,
>feed the transistor via an opto isolator,make the transistor trip a fast
>relay in the primary of the supply transformer.This is better and more
>reliable than a fuse surely.
>
Not a sound wager.  

>> and hundreds of times faster in most
>>applications.
>
>>>Screen potential was 1200 - 1500 V.  Duhhh.
>
>So what?
>
these are not easy potentials to interrupt.  
end
>
>I doubt it!
>
cheers, Conrad

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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