coupling cap & voltage divider question...

[HTH]

If I have a 22nF cap coming off the plate of V1a into a pair of 470k resistors setup as a voltage divider, would a 47nF cap and 220k resistors give the same tone?

I seem to remember something about doubling the cap size if you half the resistor size to keep the same frequency response.

As per a thread I saved recently, the lower the resistor values, the lower the load presented to the preceeding stage giving a more open tone. Also, lower grid leak resistors keep the tone tight (you can probably see where I'm going with this).

btw, thanks to everyone who helped with my recent thread where the extra gain stage was unstable - that's all sorted now. I think the main culprit was the lack of an extra HT node for the extra gain stage and also insufficient attenuation between the extra stage and the following stage.

[Earl]

I don't know anything about "tone", but essentially what you have is a cap off the plate going to a series resistance to ground.

That RC combination will have some value of AC resistance, or Impedance. That value will act as part of the 1st stage plate load, and have an effect on gain, over the frequency band it operates at.

Here's another case where you could make a program to graph the response and gain over frequency...plug in different part values, and see what happens.

This probably isn't much help with your question .. :-)

[bbartilson]

If I have a 22nF cap coming off the plate of V1a into a pair of 470k resistors setup as a voltage divider, would a 47nF cap and 220k resistors give the same tone?

The formula you want to use is:

1/ (2 * Pi * R * C) = Cutoff frequency (-3db)

At least I think that's what you're asking. Same 'tone'. Change anything and it's not likely to have the same 'tone'. More or less same or similar frequency cutoff?

so load of 940k and .022 = 7.7 hz cutoff
440k and .047 = 7.7hz cutoff

Sounds like your math is good to me.

This is, of course, assuming low source impedance and high load impedance, which can affect the cutoff and slope to a degree. But for your purposes, same circuit otherwise, your substitutions work.

Lots of online calculators too, like:

http://www.sengpielaudio.com/calculator-RCpad.htm?

[Earl]

I did a few calc's with a 12AX7, and it looks like the lower R's and bigger C will net you about 20% lower midband gain as well.

Not a big surprise though... :-)

[HTH]

This is getting interesting - so I can get the same frequency cutoff, but slightly different gain in the lower mids.

Time to heat up the soldering iron again.

[Earl]

HTH - Since the two different combinations have the same Fcutoff, the response of the two filters is relatively the same across frequency.

Impedance-wise, the big cap/low R looks like a lower R to the Plate. This R forms part of the gain equation for the tube....It's basically in parallel with the Plate Load resistor.

The two curves will look very similar with the exception that one will cause the tube to have slightly less in circuit gain.

[HTH]

HTH - Since the two different combinations have the same Fcutoff, the response of the two filters is relatively the same across frequency.

Impedance-wise, the big cap/low R looks like a lower R to the Plate. This R forms part of the gain equation for the tube....It's basically in parallel with the Plate Load resistor.

The two curves will look very similar with the exception that one will cause the tube to have slightly less in circuit gain.

Thanks for that Earl, this is all going in the archives for future reference.