Questions and observations on PI grid load resistors

[thejaf]

First post here I beleive, long time lurker. I have a few questions and observations about the value of the grid load resistors in a guitar amp long tailed Phase Inverter. I apologize in advance if this has been covered before, but I searched and didn't find. Anyway, here goes.

In my recent "modified AB-763" build, I started off with a pair of 1meg grid load resistors in the Phase Inverter, but then changed to a pair of 330K when I was tweeking the amp, which was the value of my go-by refrences. I noticed that the 1meg pair gave a stiff feel and great punch to the clean sound, but the 330K softened up the attack and smoothed out the overdrive tone (post PI LARMAR type master volume).

Has anyone experimented with the values of these resistors in an amp, and noted similar results?

Also, what would the effect be of installing a dual-ganged 1meg pot in place of these resistors?

Thanks!

[David Root]

Fender used lower values, including 330K, in some of their bass amps. Lowering the value of these resistors improves the DC balance of the LTP PI, and bass amps need good DC balance more than guitar amps do. I have seen hi fi schematics that do it too.

However, as you noted, it changes the tone a bit. If you want better DC balance in a guitar amp, keep the 1M grid resistors and get a well balanced PI tube ie each triode performs very much the same on DC voltage, then either play with the plate resistor(s) or, easier, put in a 10K trimpot a la Dumble.

A dual ganged 1M pot would work, but you are then putting DC on the pots and you'll get scritch-scratch as you adjust them. If that doesn't bother you, fine. Just don't use an el cheapo pot for that as the DC will eventually wear it down.

[Gaz]

I can't really hear too much of a difference between 470k and 1M, but you better believe I'm gonna test it tomorrow!