JCM800 Series Combo to head conversion: wouldn't the knobs turn backwards?

[Axderson1987]

Hey folks,

I recently bought a 2558 and I started seeing other cool combos, also people whom mentioned converting from combos to heads, and I noticed that even though construction wise the combos are the same as heads schematic wise except for one value I believe in the mid pot of the 4101/2204, would not making a conversion from combo to head make the knobs function backwards?

 

[Matthews Guitars]

No. Knobs still turn the same. Still clockwise to turn it up either way. However the panel legends should be printed "upside down" when making the conversion. An example is to look at a Lead 12 head and a Lead 12 combo. All the control positions are reversed left to right, because all they do is use the same chassis upside down.

Now, to be clear about it, the "upside down" version Lead 12 is rare, but I know it exists because I have one.





I have made custom panels for converting a 2204 to a 2104 combo. Marshall didn't even do that. But I make it. It allows you to read the control legends "right side up" while standing in front of the amp rather than behind it.

Making replica Marshall front and rear panels for most JMP era models has been a sideline of mine for a few years now.

 

[fitz]

would not making a conversion from combo to head make the knobs function backwards?

Just upside down - here's a pic of my 4210 in a head shell.
From 3 feet away, you'd never even notice.

 

[Axderson1987]

Just upside down - here's a pic of my 4210 in a head shell.
From 3 feet away, you'd never even notice.

View attachment 140268

But you still have to go clockwise but ¨0¨ start at a diagonal angle right?

Green being the normal head and orange a combo conversion

 

[fitz]

View attachment 140271

But you still have to go clockwise but ¨0¨ start at a diagonal angle right?

Green being the normal head and orange a combo conversion

"0" starts at the upper right, instead of the lower left.
Same diagonal angle, just off by 180 degrees.
"Noon" knob settings are now "6 o-clock".
Not the first amp I've inverted a combo to a head.
TBH, I've got the 2 channels pretty much set the way I want, and I've memorized where the MV is...

If it bugs you, there's always the option of a tubes down head shell with the chassis on top and all the knob labels "normal".

 

[Axderson1987]

"0" starts at the upper right, instead of the lower left.
Same diagonal angle, just off by 180 degrees.
"Noon" knob settings are now "6 o-clock".
Not the first amp I've inverted a combo to a head.
TBH, I've got the 2 channels pretty much set the way I want, and I've memorized where the MV is...

If it bugs you, there's always the option of a tubes down head shell with the chassis on top and all the knob labels "normal".

Well I am actually more interested in learning if the pots are reverse taper pots and if they could just be changed to normal taper as I internally do not see the board being backwards lol

 

[Axderson1987]

Just upside down - here's a pic of my 4210 in a head shell.
From 3 feet away, you'd never even notice.

View attachment 140268

Very cool!

 

[Whizzercone]

Just upside down - here's a pic of my 4210 in a head shell.
From 3 feet away, you'd never even notice.

View attachment 140268

I think it's cool AF up close!

 

[fitz]

Well I am actually more interested in learning if the pots are reverse taper pots and if they could just be changed to normal taper as I internally do not see the board being backwards lol

Upside down is not inside out.
It's still clockwise for more.
A custom faceplate or even some peel & stick labels and re-orienting the knobs would make the controls look normal.
Reversing the pots would make them got counterclockwise for more.

 

[Matthews Guitars]

The pots are normal. All that changes is how the control legends are "clocked". You'd most often use knobs that can be placed in any orientation on the pot shafts, so you just rotate them to match up to the legends.

Oh, I was wrong about having a Lead 12 with an inverted face. Not sure how I got that idea but both of them are the same in that respect.

 

[anitoli]

Just upside down - here's a pic of my 4210 in a head shell.
From 3 feet away, you'd never even notice.

View attachment 140268

That isn't anything a six pack wouldn't sort out.

 

[Axderson1987]

Thanks guys! I think I'll just rock it backwards then, maybe might some time try and twist 180° the pots on a vertical input JCM to correct them and then order a custom faceplate.

But with the 2558 I dig the odd look