Marshall JCM 800

JMP220478

I was going to suggest there must be a simple mod to correct the poor volume taper on this - I have a late 70's MV JMP2204 and its reasonably balanced - and supposedly the basis of the earlier JCM's ?

But having googled the differences between vertical and horizontal JCM's it would seem its not quite so clear - there may have been some changes to pcb mounted pots / reduced caps / lower quality transformers etc ..

Perhaps ICBM / MJW etc can comment n advise

Ps - I use mine with a powerbrake n avoid super efficient speakers because hi input cranked its total Spinal Tap !

SidNewton

JMP220478 said:

I was going to suggest there must be a simple mod to correct the poor volume taper on this - I have a late 70's MV JMP2204 and its reasonably balanced - and supposedly the basis of the earlier JCM's ?

But having googled the differences between vertical and horizontal JCM's it would seem its not quite so clear - there may have been some changes to pcb mounted pots / reduced caps / lower quality transformers etc ..

Perhaps ICBM / MJW etc can comment n advise

Ps - I use mine with a powerbrake n avoid super efficient speakers because hi input cranked its total Spinal Tap !

From what I read the opinion was it only affected the 100 watt versions. The change to horizontal inputs had no affect on the 50 watt amps. What I read was related to the overall sound rather than poor volume taper and again ICBM will know this better than me.

Either way the amp sounds great the way I'm using it so I can't see me changing it anytime soon.

ICBM

JMP220478 said:

I was going to suggest there must be a simple mod to correct the poor volume taper on this - I have a late 70's MV JMP2204 and its reasonably balanced - and supposedly the basis of the earlier JCM's ?

But having googled the differences between vertical and horizontal JCM's it would seem its not quite so clear - there may have been some changes to pcb mounted pots / reduced caps / lower quality transformers etc ..

Perhaps ICBM / MJW etc can comment n advise

Ps - I use mine with a powerbrake n avoid super efficient speakers because hi input cranked its total Spinal Tap !

The quality of the pots is just poorer - and yes, they're PCB-mounted. You can't really replace them all since they also support the PCB, but you can replace just the volume with a better one connected to the PCB with flying leads - that doesn't reduce the suport much.

The filter caps thing is interesting - despite having fewer caps - three instead of six - these amps have *more* filtering than the vertical-input models! That sounds impossible but it's because in the verticals the caps are in stacked series pairs, which halves the effective capacitance; in the horizontals they're paralleled which doubles it... so actually the horizontals have twice the capacitance, 100uF in each of the main filter stages rather than 50uF. This is why they sound slightly tighter and brighter.

I don't see any evidence that the transformers are lower quality at all - as far as I can remember I've never had to replace a single one, whereas I have a (very) few in the vertical models.

The Powerbrake is excellent with these amps, yes. I tend to find it best to let the Powerbrake do most of the work and keep the amp volume not fully cranked, but you can use any combination of the two and find the sweet spot.