Technical questions about the JCM800 2210 (regarding master volume, channel switching and clipping)

[d3vCr0w]

Hi people, hope y'all are doing great!

I have some questions about the Marshall JCM800 2210:

1. Is the Master Volume located before or after the phase inverter tube/valve?
Is it VR11? I followed the signal coming from Valve 4A (point Y in the schematic) which leads right to Valve 5 which is the phase inverter and the only variable resistor I see nearby is VR11 but I got kinda lost there and don't know how to properly read that.

2. What does the CA3046 integrated circuit do, is it only used for channel switching?

3. What/where are the clipping diodes? I've read people talk about solid state clipping in this model.

Here's the schematic by the way: https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_splitch_100w_2210.pdf

Thanks beforehand and have a good one.

 

[LyseFar]

VR10 is MV.

 

[LyseFar]

Vr11 is presence.

 

[d3vCr0w]

VR10 is MV.

Thanks for replying. I wasn't expecting the master volume to be so early in the circuit, it's then not only before the phase inverter (V5) but also before V3B / V4B (which, correct me if I'm wrong, drive the reverb tank) and also before V4A.

Is V4A a gain stage or do you know the purpose of it?

 

[Blake F]

This circuit (also 2205 head) is quite a departure for Marshall from their earlier models. Schenker uses the 2205's and front-ends it with a bunch of pedals. I've only heard one live recording where his 2205 tone sounded really good like his old tone but I'm not saying this amp is bad just haven't heard one live.

The front-end is a total re-design, master volume as noted and tone stack. I don't have time to look up what that IC does but there are 2 diodes going into it that could be clipping diodes, and there's one on the cathode of V1B that's probably doing half-wave clipping. The 2203's 'cold clipper' circuit actually does half-wave clipping and leaves the other half of the wave unclipped- that's probably what they're replicating with the diode D1.

V4a appears to be driving the phase inverter V5.

 

[Pete Farrington]

2. What does the CA3046 integrated circuit do, is it only used for channel switching?

It’s only for channel switching.

3. What/where are the clipping diodes? I've read people talk about solid state clipping in this model.

They're a matrix formed by FWB package W005 and D6.

Is V4A a gain stage or do you know the purpose of it?

It’s a gain stage, recovering the signal level from the line level of the fx loop, so that its big enough to fully drive the power amp.

 

[anitoli]

The CA3046 is a 5 transistor chip that takes the place of discrete parts. It's job as Pete said is for channel switching.

https://rocelec.widen.net/view/pdf/hqlk54qn0l/RE_DSHEET_CA3046_REI.pdf?t.download=true&u=5oefqw

 

[Pete Farrington]

The CA3046 is a 5 transistor chip that takes the place of discrete parts. It's job as Pete said is for channel switching.

https://rocelec.widen.net/view/pdf/hqlk54qn0l/RE_DSHEET_CA3046_REI.pdf?t.download=true&u=5oefqw

Thanks, I thought it was a MOSFET device; maybe the part number reminded me of something in that range