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Bypassing Screen Resistors

5/2/1998 10:51 AM
Bypassing Screen Resistors
Why do you bypass screen resistors  
in certain applications? Is this  
an issue in guitar amps, or different  
output tubes, or ultralinear con-  
nections. Why, why, why? Values?
6/5/1998 9:48 PM
Paul Markwalter

What's your meaning of bypass?  
A cap across the resistor?  
Or a cap from grid to ground?  
On power tubes or preamp tubes?
6/5/1998 11:42 PM

I'm not sure what I'm asking here as  
I don't know much about the different  
configurations of screen bypassing.  
That having been said, I have read  
about caps bypassing the screen resis-  
tors on power tubes. I never though  
about an input pentode to tell you  
the truth. Does the practice of by-  
passing screen resistors in this way  
have any application in guitar amps?
6/6/1998 3:11 PM
What's your meaning of bypass?  
    I think the beginning of this thread has scrolled off the board, but doesn't "bypass" mean running a jumper or another component across the component in question (the meaning of which can usually be determined by the context).  
    I know that YOU know that, Paul, having read some of your other excellent posts here- but I just wanted to explain the term for any newbies who might stumble on to this thread.  
    I understand your point- someone might say "bypass the screen resistors" without a clue as to what you are to bypass them with- a jumper, a cap, an artery (as in heart bypass). I personally think that veins sound better than arteries for heart bypasses because they have the blues... [man, did THAT one stink! I better post this one anonymously...]
6/8/1998 2:56 PM

In electronics, there are generally two places where capacitors are used (I'm excluding timing or tuning circuits here). To pass an AC signal (and block DC) from one stage to another a "coupling" capacitor is used. To shunt a particular AC signal (usually unwanted noise) to ground a "bypass" capacitor is employed. (Power supply caps are considered bypass caps.)  
Screen grids, when in a standard grounded cathode pentode hookup, need a clean DC source for proper (or expected) tube performance. To make sure the DC to this anode has no noise or AC ripple riding on it, which would modulate the signal being amplified, the screen grid will have a bypass cap to ground. This is especially important in a pentode preamp stage. Power amps don't always have a dedicated screen bypass cap, but one is effectively close by as part of the HV power supply.  
The jargon becomes kind of loose, so when someone says "bypass the screens", they mean bypass capacitors installed at the screen grids.  
Bypass does have connotations that mean to skip past or go around, but they're not usually implied in electronics.  
I'm not sure what the accepted term is for a capacitor placed across a signal attenuating resistor (for tone shaping) like the 470k//500pf in a Marshall circuit. You might just say "shunted"?

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