PI resistor question

[EFK]

Maybe this is overly simplistic, but in the attached drawing, how do the PI resistors valued 1.2K and 47K affect the behavior of the PI tube as compared to the same tube in a more JTM45 circuit w/ the "typical" 470 ohm/10K values? Just wondering about this, not sure if it is a meaningful question or not... There is no negative feedback involved here. Thanks!

[Chuck H]

Yes NFB is involved. Along with some other things. There is an excellent artical on the Aiken amps website about PI construction. You should check it out.

The 470/1.2k resistor is the bias resistor and sets the central operating point for both halves of the PI. The 47k/10k resistor does effect the gain and voltage via localized NFB and sheer resistance. Much of the aiken artical goes over my head so you should read it for yourself. But in my own experiments these resistors effect the bias point and the gain (respectively) of the PI. You will commonly see the higher values with EL84 amps because EL84's don't require much drive voltage with their lower Mu. Big bottle amps often use the lower values due to their higher Mu. And then there's the EL34 amps, specifically Marshall's. EL34's have a lower Mu than 6L6's but commonly use lower values than their 6L6 "Fender" counterparts. Some of this just comes down to individual amp design. The Aiken artical will give you the basics on how to do it "correctly" but experimentation is not discouraged here. In fact...

I recently designed an amp that uses EL84's that are intentionally pushed VERY hard. The bias resistor I landed on was an 820 and the tail is a 47k (+1k shunt for the feedback loop). Point is, this is giving me what I want. And that should be your first criteria. So read the artical and glean what you can. But don't be afraid to go out of bounds if it suits the design. Remember that guitar amps are built to INTENTIONALLY color the tone. Not just accurately reproduce what they hear at their input, like a PA or Hi Fi amp.

Chuck

[Robert M. Martinelli]

To complement Chuck's good explanation, if you look at a "typical" LTPI, you can see that the "tail" resistor forms a voltage divider together with the bias resistor and the tube itself. The tail resistor affects the PI symmetry and gain, a higher tail resistor makes for a better PI symmetry and a lower gain, while a lower tail resistor obviously goes in the opposite direction. As Chuck wisely says, guitar tube amps are made to "color" the sound in many ways, so a (LT)PI can be used to "reshape" the waveforms just like any gain stage.

Hope this helps

Best regards

Bob

[EFK]

Most of Aiken's material is way over my head! But it is interesting and I'm getting there. Slowly. This has indeed been helpful so thank you for the explanations!