My Layout is Backwards (Front Facing JTM Combo)

[chinrest]

Here's a "DUH," a laugh, and some questions:

I decided to drill my own chassis (I've done 2 or 3 before) for a new front-facing JTM50 combo amp. My intention was to invert the usual chassis, and have custom control plates made. As i was cleaning up the holes on the top of the chassis, I noticed that I had suffered from carpenter's dyslexia—I forgot to invert my drawing before drilling. "DUH"

Since I hadn't drilled the front and back panel yet, I figured I'll just invert the circuit board (preamp on the right, power on the left), and then I can use standard JTM control plates!

My questions now are:

1. Has anyone else suffered from amp designer's dyslexia, and ended up with happy accidents or boat anchors?
2. Does anyone know of any production amps that use a "backward" layout?
3. What do I stand to screw up on an inverted layout, like wiring the pots backward? (I use the Hoffman Plexi 50 layout for reference)

[bob p]

2. Aren't *ALL* Marhsall layouts backwards?

Or at least over under sideways down?

[chinrest]

Or at least over under sideways down?

Wait. When referring to amp layouts, do we use "stage left/stage right?" or "driver side/passenger side?"

[Enzo]

Fork side or spoon side.


Marshall models that come in both head and combo styles use the same chassis. The head will have the chassis at bottom, and the combo at the top. The combo version will have an upside down printed panel escutcheon so it reads right.

[chinrest]

Fork side or spoon side.

Ah, a civilized chap with table manners. Now we're getting somewhere. I'm talkin' the input jacks are on the right, but it's a top-mounted combo. Bass-ackwards, and the fork side is my ass.

[Enzo]

The problem is the classier amps, they have the input jacks where the soup spoon should be and the presence control over by the butter knife. The finger bowl won't get a good sustain. The napkin ring modulator sounds nice though. The setting should start with a charger, just in case the battery is flat.


The Chinese amps only play wok 'n roll, and I am not sure how chopsticks works into it, though I do play chopsticks on a piano.



OK, OK, I'll stop...

[chinrest]

Alright, enough messing around. I'm seriously frightened:

[Jazz P Bass]

I would think that as long as you keep the power transformer away from the input circuits you will be fine & dandy.

[bob p]

1. No.
2. All Marshalls, if you're a Fender guy like me.
3. I don't think there's going to be any problem with an inverted layout if you pay attention to what you're doing and you reverse everything. With eyelets I can't imagine there being a problem as they're the same on both sides. With turrets you just need to make sure that you mount them on the opposite side of the board. Basically, you'll end up building the project as a mirror image of what you had originally intended. The only difference is that I wouldn't reverse the pot wiring from the normal layout. I'd want to keep the pots rotating in the same direction.

[chinrest]

I've encountered the first two issues with a "mirrored" layout:

The switch terminals face the chassis bottom (not terrible).

Since I can't mirror the pinout of tube sockets, and I already drilled (and flipped over) a standard turret board, I will have a lot of wires criss-crossing.

[Steve Conner]

You can't mirror the pinout of a tube socket, but you can turn it through 180 degrees which is almost as good. (a "first-order approximation" )

With dual triode tubes you may not even need to bother as the pinout is symmetrical. Just swap triode 1 for triode 2.

[chinrest]

You can't mirror the pinout of a tube socket, but you can turn it through 180 degrees which is almost as good. (a "first-order approximation" )

With dual triode tubes you may not even need to bother as the pinout is symmetrical. Just swap triode 1 for triode 2.

Ahhh, you're getting the dyslexia now. Rotating the tube socket gives me the same problem--it just moves the heater pins. What I really need is a tube socket and tube with a pin configuration of 3-2-1-4-5-8-7-6-9 - or - I need to flop the position of components in each section of the board.

(I understand that I can use the "second half" of a dual triode as the "first half." I'm trying to avoid a cross-crossing lead dress.)

[Steve Conner]

So you're saying the first-order approximation isn't good enough?

The circuit should work fine with wires crossing, as long as they are the plate, grid and cathode wires of a single tube section. After all, they're going to meet inside the tube anyway.

It's when wires belonging to different stages cross each other that things start to get scary.

[chinrest]

First order approximation might be fine for some guys, but not me, man! The rest of your post was very encouraging indeed Thanks. I'm getting ready to wire in the board. I have this paranoid, sinking feeling that I'm going to get near completion, and there will be a big horrifying "duh!" "The damn blahhblah is backwards!!!!!!" Doom! Doom!

[chinrest]

The amp sounds great, but everything I play sounds backwards!!!! Aieeeeee!!!!