Marshall lead 20 (has anyone owned one before?)

[Reg18]

Just picked this up for not much due to it needing a little TLC. The pots are needing some contact cleaner and possibly even replaced but trying to figure out what the original sounded like.
The gain pot on both low and high sensitivity has a steady/linear increase until you get to around 3/4 and then it’s like it engages another gain stage entirely, pushing into high gain JCM800 territory. Is this a faulty pot or how they were built?

[Wairarapajack]

I've not owned a lead 20, but have a lead 12 which is in the same series. They definitely do a good impression of a bigger 800 when cranked. Apparently it is a feature and not a bug, not sure where it normally kicks in on the gain pot though.

[FuzzMonkey]

Having a quick look at the schematic of both the Lead 12 and Lead 20, the Gain control sits between the two op amp stages of the pre-amp section in both amps.

Someone will correct me if I am wrong, but to me it looks Gain control (22k linear) is pulling double duty and attenuates the output from the first op amp in on one side of the pot as a traditional voltage divider and decreases the Gain of the second op amp stage on the other of the pot much like the arrangement you see in a MXR Distortion + or DOD 250 with the middle lug is tied to ground.

Maybe a reverse audio taper pot might work better for this application?

[Reg18]

Having a quick look at the schematic of both the Lead 12 and Lead 20, the Gain control sits between the two op amp stages of the pre-amp section in both amps.

Someone will correct me if I am wrong, but to me it looks Gain control (22k linear) is pulling double duty and attenuates the output from the first op amp in on one side of the pot as a traditional voltage divider and decreases the Gain of the second op amp stage on the other of the pot much like the arrangement you see in a MXR Distortion + or DOD 250 with the middle lug is tied to ground.

Maybe a reverse audio taper pot might work better for this application?

Interesting, although I’d rather keep it original if in fact it is supposed to act like that. After a good clean of the pots it might shed some light on what’s what.

[Delayman]

I had a Lead12 and preferred it clean, so never noticed the taper of the gain knob. Great amp, I imagine the Lead20
would be cool too.

[FuzzMonkey]

Interesting, although I’d rather keep it original if in fact it is supposed to act like that. After a good clean of the pots it might shed some light on what’s what.

I imagine that the linear taper was just the best compromise between the two funcations. But a good clean of the pots will do it the world of good no doubt.

[Reg18]

Giving the pots a good clean this morning did the trick, everything works well now.
The gain pot is acting a bit more appropriately however it does still have a big gain increase in the last 15% of its rotation. I added a speaker jack on the bottom (not seen) so I could hook it up to a cab and it certainly sounds significantly better than through its 10” Celestion.
Here’s a couple of internal pics, made in 1984 by the looks.
Could do with a bit more bass but not sure it’s worth modding it?

Edit: in doing some research I found out Tom Morello used a Lead 20 to record “Tire me” which won a Grammy in 1997 for Rage Against the Machine. Makes you reconsider every justification for gear after hearing that.