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previous: Bob DeRosa If I were to make a single channel ... -- 1132602129 View Thread

Re: PI Plate Resistor Question

11/22/2005 10:36 AM
MattRe: PI Plate Resistor Question
Bob,  
This is from the Aiken article (not complete, I skipped some parts) NOTE: R1/R2 are plate resistors, R6 is the tail resistor, C1 is the input cap, C2 is the cap for the second grid (but it isn't directly grounded, it's grounded 'through' the Presence pot):  
[QUOTE]It should be noted that there are actually three inputs used in this type of phase splitter. The first input is the obvious one, the left side of C1. The second input (the lower end of C2) is useful as a feedback input, a reverb or effects return input, or as a second channel input. In the circuit shown above, the second input is used as a feedback return input, taking the signal off the junction of the feedback divider.  
 
The third input is not so obvious; it is the lower end of R6. If a signal is input at this point, the phase splitter will produce an output signal on each output that is in phase with the other, rather than 180 degrees out of phase, and also in phase with the signal input at the lower end of R6. This means that if a signal of equal phase is applied to the first input (C1) and the third input (R6), it will subtract from the out of phase output (R2) and add to the in phase output (R1). Likewise, if an equal phase signal is applied to the second input (C2), and the third input (R6), it will subtract from the in phase output and add to the out of phase output (this is because the out of phase output is actually in phase for the signal applied to the second input, C2, and the in phase output is out of phase). This third input is useful for balancing the feedback signal by subtracting from the in phase output and adding to the out of phase output in order to compensate the unequal gains to each output from the feedback input. The gain is much less than the gain into the first and second inputs.  
 
The two outputs provide (nearly) identical signals, except for a 180 degree phase difference between them. This is exactly the type of signal needed to drive a push-pull amplifier, so this circuit is commonly seen in higher-power guitar amplifiers.  
 
The output voltage is developed across the plate resistors (R1 and R2), and is proportional to the current changes from the tubes in response to the input signals.  
jump  
When only one signal input is used (ignoring feedback inputs) R1 is usually made 10% - 20% lower than R2 to compensate the unbalanced gains of the two tube sections and make the two output amplitudes equal.[/QUOTE]  
 
So it appears that you should use the 82K/100K combo when you're not using that second grid as a second input. I 'think' the reason you use 100k/100k when you use the second input is because while an 82k/100k combo tends to balance the PI when the first input is used, it also would increase the imbalance when the second input is used.

 
Replies:
Bob DeRosa Good info Matt (and of course Randa... -- 11/22/2005 12:08 PM
Shea Something I read in O'Connor's firs... -- 11/23/2005 9:51 AM