Cathod follower bias resistor

[Rod]

Hello!

The standard cathode bias follower value for marshall is 100k, but in the Marshall JVM they put a 47K resistor in R118( as you can see in the attached part of the schematic), if i put the R118 value to 100K, what results can i expect in the amp tonewise?


https://drive.google.com/file/d/1t3r...ew?usp=sharing

[Helmholtz]

Hello!

The standard cathode bias follower value for marshall is 100k, but in the Marshall JVM they put a 47K resistor in R118( as you can see in the attached part of the schematic), if i put the R118 value to 100K, what results can i expect in the amp tonewise?

Less grid current distortion.

[Rod]

Hello, Helmholtz!

Less grid current distortion is bad or good? What does it make to the tone controls, bass mid and treble?

Thanks a lot for your answer.

[Helmholtz]

Less grid current distortion is bad or good? What does it make to the tone controls, bass mid and treble?

I actually don't know as I never tried 47k in a Marshall. IIRC, the VOX AC30 uses 56k there. Grid current distortion varies with individual tube samples.

Should be easy to find out in your amp.

[vintagekiki]

... if i put the R118 value to 100K, what results can i expect in the amp tonewise?

It's a matter of taste, it's best to try it and share your impressions with us

[J M Fahey]

Itīs not *really* a"bias" resistor.

Bias is fixed at 100 something volts at grid, cathode voltge will be a couple volts above that, cathode to ground resistor is actually the load resistor, go figure, and output signal is pulled from it.

Its influence on sound is subtle.

cathode followers re "low impedance", thatīs why they are used as followers, buffers, etc. but that happens at low signal levels.

When driven balls to the wall the harsh reality takes command.
In the upper swing, circuit can supply quite few mA, easily 5 mA and with some tubes even more.
On the downwards swing only the pull down resistor is working.
Yes, the cathode to ground one.
So with a 100k resistor you have, say, 150V/100k=1.5mA (actually even less but just simplifying things)
With 47K you have 3 mA , in any case less that whatīs available for the upper half.

No big deal if you drive a high impedance load (say, >220k) but a tone control is a complex circuit, acomplexload, and at some frequencies becomes a lowish impedance load.

End result being that even a cathode follower becomes current starved one way and clips unsymmetrically, big time.

This is one important component of the "Marshall sound", and even "Tweed sound".

Now: will going from 100k to 47k change sound much?

As I said before, any change would be subtle.

[Guitarist]

I will sometimes parallel three 100K 1/2 Watt resistors on that CF resistor then you can try the other tube types u7 - y7 - t7 - . It then is simple to remove one for some better impedance matching downstream. Especially if an effects loop follows. You also dissipate the extra current flow via the now 1.5 Watt set of resistors. Or that's how I kinda understand it
I suspect changing the tone slope resistor will self-affect more.