value of resistors in voltage divider?

[HTH]

ok, so you've got a voltage divider like the two 470k resistors coming off V1b's plate in a Marshall 2203.

By keeping the two resistors the same, the voltage at the end is the same whether they're 220k, 470k, or 1M.

I'm assuming the difference would be the current across the 220k will be different to the current across the 1Megs.

What difference in tone does this give?

I've found that keeping a 470k 'in line' but increasing the resistor to ground gives a thicker tone and similarly lowering that same resistor to say 220k gives a thinner tone.

Anyone got experieces with this?

[cbarrow7625]

When you start changing either resistor you are doing three things:

1.) You are changing the total value of the resistor that is seen by the coupling cap. Smaller resistor with same cap = less bass. That is part of the reson for the thin tone. 470k + 1M to ground = 1.47M vs. 470k + 220k = 690k, almost half the value of resistance as seen by the coupling cap.

2.) You are changing the load that the preceeding tube stage is working into. A tube stage that is less loaded down tends to have a more "open" kind on sound because the tube is not fighting as hard to create the voltage swing that it want to.

3.) You are changing the grid leak resistor for the following stage. Larger values of grid leak resistors make it more difficult fo the tube to control the fluctuating voltages within itself. That is one of the reasons why you can make a grid leak resistor too large, the tube completely loses control. A value of 100K (or less) for the grid leak resistor can make the gain stage sound overly tight & constricted as the tube is able to control the fluctuating voltages too well.

One other effect that has to be taken into account is miller capacitance. That is, the internal capacitance of the tube. Larger resistors in series with the grid will cause the high frequencies to roll off at a lower frequency than when a smaller value series resistor is used. That is one of the resons Marshalls have 500pF caps around the 470k resistor & Fenders have bright caps on the volumes (marshalls too!).

Hopefully that helps. I prefer a straight 470k + 470k voltage divider between stages when I am going for a HTH tone.

[HTH]

thanks for the explanation - that's getting saved for future reference.

I've been tinkering this afternoon and used the typical 'Marshall' value of 470k between stages and it sounds much better now. I think a 470pF cap will give it just the right amount of 'cut' too (I'm using 1000pF on the next stage and it gives a really agro midrange snarl, heh).