JCM 800 gain mod

[acceptance]

Can anyone give me a more technical explanation for a layperson of what this mod does? Second, is this part common to other preamps and if so what values to other amps use for comparison?

Locate the 10K cathode resistor (R6 found on V1B) and replace it with a 4.7K resistor (or just solder another 10k in parallel on PCB amps). This will beef up the gain. This seems to be the "sweet-spot" on these amps. You can also try other values like 6.8k, 3.3k, 2.7k, 1.5k If you want insane gain use a 1.5K (but I think it's too much and doesn't sound as good). The 4.7k sounds just right and the gain doesn't get mushy. Since there is no cathode bypass cap here, all the frequencies in this stage are amplified normally (flat response).

 

[Code001]

https://robrobinette.com/How_the_Marshall_JCM800_Works.htm#Cold_Clipper

 

[acceptance]

https://robrobinette.com/How_the_Marshall_JCM800_Works.htm#Cold_Clipper

Is that mod above the cold clipper though? It's V1B.

 

[Code001]

Is that mod above the cold clipper though? It's V1B.

When in doubt, look at the schematic. Look for the 10k cathode resistor and compare to your amp:

Official (you want the bottom one for the 800) : https://www.drtube.com/schematics/marshall/jcm800pr.gif
Redrawn by Rob: https://robrobinette.com/images/Guitar/Marshall/JCM800_Schematic_DIYLC.pdf
Redrawn by Mark Huss: https://mhuss.com/MyJCM/JCM800_2204.gif