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previous: Enzo I've heard the fuse idea too, and I... -- 1141951037 View Thread

Re: Questions on screen resistors.

3/9/2006 7:42 PM
Carl@Zwengel amps
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Re: Questions on screen resistors.
Ok, first thing's first. you NEVER NEVER EVER use a resistor as a fuse! Whoever started this one needs a severe beating about the head and shoulders! If you want to protect an element of the circuit you use a fuse. THIS IS UNSAFE, and I wish people would stop even mentioning this crap. FUCK!!!!  
-End Rant  
 
Second thing and i kind of think this is the original thrust of the question and it was why select certain values. Like it's been said already, the value is somewhat fluid as long as you dont select a value that overdissipates the screen grid rating. Once you know you've met that goal it's pretty much up to you as to the value.  
 
If you look at the Ip/Ep transfer curves like you'd see in a tube manual, normally a pentode will exhibit a steep rise in value then level off to a near flatline slope...sort of logarithmic if you will. By changing the resistor value you can shape the curve to look much more linear and exhibit a more triode-like behavior or create a sharper rise to the shelf.  
 
In the first case generally you'll get a smoother onset of distortion whereas in the latter the tube will stay linear then hard clip. Of course all this depends on about 5 million different variables including but not limited to input signal magnitude, bias points, plate loads, plate and screen voltages, etc.  
 
HTH  
 
-Carl

 
Replies:
Matt Earlier I said:[QUOTE]There is the ... -- 3/10/2006 7:43 AM
Le Basseur [QUOTE]Ok, first thing's first. yo... -- 3/10/2006 10:05 AM