What is the JCM-1 Anniversary Boost button doing?

[jchrisf]

I really like the sound of the JCM-1's boost button and was curious what it is doing to the circuit when engaged? Here is what the manual says:

5. Boost Button Pressing this button changes the pre-amp from the classic ‘Master Volume’ voicing to the later ‘Split Channel’ boost sound. When active, the sound is smoother and more distorted. With the button out, the sound is more dynamic and responsive to the guitar’s volume control. The effect of this control is more apparent when the Master Volume is at lower levels (less than 2 o’clock on the dial)

I'm not sure if this is an accurate schematic or not?

 

[mcstinger]

Very quick view, I hope not to mislead you:

Disengaged:
- clipping diodes are lifted (inactive)
- R107/R106 attenuates voltage before it hits V2A
- operating point of V1B is shifted in a direction of cold clipper behavior with less gain, my guess

Engaged:
- clipping diodes engaged
- no attenuation in front of V2A
- V1B configured as a classic gain stage

 

[jchrisf]

Very quick view, I hope not to mislead you:

Disengaged:
- clipping diodes are lifted (inactive)
- R107/R106 attenuates voltage before it hits V2A
- operating point of V1B is shifted in a direction of cold clipper behavior with less gain, my guess

Engaged:
- clipping diodes engaged
- no attenuation in front of V2A
- V1B configured as a classic gain stage

Thanks.. I'm hoping to someday add this mojo to my Ceriatone 2204. I'll study that schematic more with what you have described and see what I can figure out. That is very helpful.