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previous: Joe Yes. If you use a higher value pla... -- 1110474385 View Thread

Re: Plate Resistor Value vs Tone?

3/10/2005 11:24 AM
AlNelson
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Re: Plate Resistor Value vs Tone?
Generally, a larger value plate resistor will result in more gain for that stage. One popular rule of thumb is to use a plate resistor that roughly halves the supply voltage, so the stage can swing plus or minus the maximum amount from that midpoint.  
 
However, I can not recommed depending on a rule of thumb here. The relationship between supply voltage, current and plate resistor are important and can result in a whole range of sounds. You need to measure the supply for that stage to select the "best" value.  
 
Also note that "best" can mean different things. For instance, if you want that stage to run out of supply on the negative-going peaks first (smoother), positive-going peaks first (more aggressive) or both at once. Of course, bias is important too.  
 
For instance, taking standard Fender values: 100k plate, 1.5k cathode and say, 270VDC supply at 1ma, The plate voltage will be about 170, voltage gain will be about 57 (35db).  
 
Moving to a 220k Rp will drop the plate voltage to about 50VDC. Gain will be increased to about 70, but it won't do you much good. However, if your supply current was only .7ma, the plate voltage only drops to 115 (w/ a 220kRp) and the stage can now swing from -50 to 105.  
 
This is also why I would be careful about sticking a 12AT7 (or whatever) into a socket meant for a 12AX7. While a triode in an AX7 usually runs at .5 to 1.5ma, an AT7 normally runs in the 3-10ma range. So, dropping one in at one stage may change the supply for multiple stages.

 
Replies:
Joe I have also read that a good rule o... -- 3/11/2005 6:41 AM