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grid stopper

5/30/2000 10:29 AM
grid stopper
I want to build a Marhall 50 clone and was wondering everyones opinion about using grid resistors on the OP tubes.I know the old Marshalls didn't have em at all, if the amp runs OK leave em off?  
thanks David  
5/30/2000 12:36 PM

yes, use them. anything over 1k will do. the higher the value, the better in terms of protection from oscillations, but your HF may suffer. find a balance you can live with.  
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5/30/2000 8:11 PM

thanks for the reply,I'll probably go with 1k5 and see how she sounds!  
5/30/2000 8:11 PM
Peter S

In my opinion.....they need to be on there. I like to use 5.6k or higher on most of my Marshall clones, and real Marshalls too.  
5/30/2000 8:15 PM

Peter,are your clones 50 or 100's ?just wondering what effect increasing the value will have on the tone?  
regards David  
5/30/2000 9:43 PM
You can calculate what a grid stopper does to you in terms of frequency response.  
There is a phantom capacitor from the grid of a tube to its cathode, and another from the plate to grid.  
The plate to grid capacitor value acts like a cap that's the voltage gain times bigger in parallel with the grid to cathode capacitance. Both values are usually specified on the tube's data sheet.  
The series grid resistor and the two paralleled parasistic capacitances form a simple RC lowpass filter at the grid.  
For pentodes, the plate-grid capacitance is pretty low - that's one reason the screen grid and suppressor grids are there.  
For 12AX7's, the actual results in circuit are pretty close to what you'd compute with the simple RC filter approximation.  
5/31/2000 2:24 AM
Peter S

They are 50 watters......the effect on tone is so slight that it is hard to hear, but it does shave off some of the high frequency buzz when the amp is cranked with a Strat or Tele. At low volumes it's not noticeable and you cant hear any difference with a Les Paul.  
Peter S  
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