Marshall PI Bias Feed Resistors

[FourT6and2]

Not sure if that's the technical term for these. Grid leak/bias feed/bias splitters... the resistors on the output of the phase inverter right before the (usually) 5.6K grids on the power tubes.

Let's take a typical Marshall 50-watt 2204/JCM800 circuit. The PI has 0.022uF output coupling caps with 220K bias feed resistors, 5.6K grid stoppers on the power tubes.

What does changing the value of these resistors do to the sound of the amp—while sticking with EL34s? I know lowering them was commonly done on Marshalls shipped to the US with 6550s instead of EL34s. And that a lower value can help with tube longevity in the case a runaway with modern tubes. But does the value affect the actual sound of the amp?

Let's say going from 220K down to 180K, 150K, 120K, 110K. EL34s in each case.

[FourT6and2]

Some folks on another forum a few years ago said something like:

Lowering them might help a Marshall that's too bassy, a little too clean, and too loud. And that the lower you go, the browner/mushier the amp gets.

But I'd love some more detail on the mechanics.

[nsubulysses]

Lower value resistors have less bass content in general and I *think* more stable -DC bias value with changes in signal amplitude. I can't remember exactly on that last part.

There is a calculator to look at this phenomenon.

https://www.ampbooks.com/mobile/ampl...ias-excursion/

[nickb]

Some folks on another forum a few years ago said something like:

Lowering them might help a Marshall that's too bassy, a little too clean, and too loud. And that the lower you go, the browner/mushier the amp gets.

But I'd love some more detail on the mechanics.

Lowering the resistors will
1) Reduce the likelihood of runaway
2) Cut bass: -3dB at 72Hz instead of 33Hz. The perceived effect may be less in practice due to the existing preamp and speaker response.
3) Lower the gain a little, thus making it cleaner all other things being equal
4) Reduce the time taken to recover (blocking distortion) from heavy overdrive

Whether you like it or not or how you might describe it is up to you. Try it and see.

[FourT6and2]

Lowering the resistors will
1) Reduce the likelihood of runaway
2) Cut bass: -3dB at 72Hz instead of 33Hz. The perceived effect may be less in practice due to the existing preamp and speaker response.
3) Lower the gain a little, thus making it cleaner all other things being equal
4) Reduce the time taken to recover (blocking distortion) form heavy overdrive

Whether you like it or not or who you might describe it is up to you. Try it and see.

Thanks. Yeah, I have a few amps here with various values. One has 110K, another has 150K. Stock Marshall is 220K.

[Justin Thomas]

Hmmmmm. Does this work on cathode-biased 6V6s, too? (Within reason... I'm not even currently sure what's in my amp, but some "browning" might be nice...)

Justin

[FourT6and2]

Lowering the resistors will
1) Reduce the likelihood of runaway
2) Cut bass: -3dB at 72Hz instead of 33Hz. The perceived effect may be less in practice due to the existing preamp and speaker response.
3) Lower the gain a little, thus making it cleaner all other things being equal
4) Reduce the time taken to recover (blocking distortion) from heavy overdrive

Whether you like it or not or how you might describe it is up to you. Try it and see.

Oh one more thing, how are you getting 72Hz cut and 33Hz cut?

Stock Marshall values are 0.022/220K with an impedance of 40-50K (I've been told). According to that calculator, that equals a cutoff @ 27Hz. Lowering the resistors to 110K raises the cutoff to 46Hz. You'd have to lower the resistors to 55K in order to raise the cutoff all the way to 72Hz. I've never seen them that low in any amp of this type from any manufacturer. Conversely, keeping the resistors at 220K, you'd have to lower the caps to 0.0083uF to get 72Hz cutoff.

[nickb]

Oh one more thing, how are you getting 72Hz cut and 33Hz cut?

Stock Marshall values are 0.022/220K with an impedance of 40-50K (I've been told). According to that calculator, that equals a cutoff @ 27Hz. Lowering the resistors to 110K raises the cutoff to 46Hz. You'd have to lower the resistors to 55K in order to raise the cutoff all the way to 72Hz. I've never seen them that low in any amp of this type from any manufacturer. Conversely, keeping the resistors at 220K, you'd have to lower the caps to 0.0083uF to get 72Hz cutoff.

f= 1/2/3.14/R/C

The R is the total resistance in the grid circuit and C is the coupling capacitor. When I did the calculation I used 100K and 220K as the total resistance. Strictly speaking I should have included the source impedance of the driver stage, typically around 40K, but I chose to ignore it for simplicity and that accounts for the difference.