How to wire bass pot (JTM45/100 type)?

[BlueX]

Can someone help me understand how to wire the bass pot in the JTM45/100 clone I'm currently building?

In photos provided by @neikeel and @william vogel , it looks like the lug closest to inputs is not connected (and wiper connected to treble pot, and the other fixed lug connected to mid pot). This seems to correspond to Ceriatone layout (1 Aug 2015).

The #7026 schematics I follow (2024-02) shows something different, where all lugs on the bass pot are connected. The Tube Depot layout shows a third variant, where the bass wiper is connected to the mid pot.

A general bonus question: How to read pot orientation from schematics (0-100% or 100-0% sweep, if I made myself understood)?

 

[neikeel]

How I do it is the correct way

 

[BlueX]

How I do it is the correct way

I thought so, and that's how I'm preparing.

Wonder if there is a mistake in the BKW schematics for the #7026, but it's been around for a long time so it should've been spotted if so. Maybe another way to wire and get the same result. I can't tell, yet.

 

[Blake F]

It's wired as a variable resistor with the middle and right terminal used (looking at the chassis upside down in a stand) and the left terminal unconnected (although it can be shorted to the wiper). The wiper (middle terminal) goes to the junction of the .02uf and 250k treble pot (which is on the treble pot). The bass pot right terminal goes to the mid pot left terminal.



The orientation works like this: when you turn the pot fully clockwise, the wiper will be a full short to the left terminal and full resistance (whatever the pot's resistance is) on the right terminal; and the exact opposite when you turn the pot fully counterclockwise.

 

[TassieViking]

I always connect the unused lug to the wiper, if the wiper is a bit dirty you don't loose the whole resistance on the wiper.
It is hard to remember which way the bass pot is connected since it can be drawn in different ways, you can not tell if lug 3 is on the top or bottom on the schematic unless they show orientation on the drawing.
You just have to look at different schematics that have the lugs numbered or an arrow for clockwise rotation and copy that.

 

[BlueX]

Thanks for your input! I think I know enough to wire now.

Seems like there is no real standard how to show pot orientation in schematics.

 

[playloud]

Yes, as you've discovered, you can't discern the physical orientation from schematics (unless otherwise indicated).

If you think about it, what the BKW schematic shows is equivalent to a rheostat (variable resistor) anyway. Given two parallel paths - one with some resistance and the other with none - you can guess where the current will go...

There is a variation on the JTM45/5F6A tonestack in which the mid pot is wired as a variable resistor, instead of a voltage divider. You can see this in AB763 amps, for example.

 

[BlueX]

Thanks, again!

The tone stack is the part of this type of circuit where I still can't follow what happens. Need to read the Tone Controls chapter in Merlin's preamp book, while waiting for parts.

 

[Blake F]

Thanks, again!

The tone stack is the part of this type of circuit where I still can't follow what happens. Need to read the Tone Controls chapter in Merlin's preamp book, while waiting for parts.

Rob Robinette has a great section on the TMB tone stack. https://robrobinette.com/How_The_TMB_Tone_Stack_Works.htm . It's basically four passive filters hooked together although that's an over-simplification. Don't be confused when you first read it and he talks about the Fender Bassman- the Marshall tone stack was taken directly from the Bassman tone stack without modification (until Jimi Hendrix's amp tech developed a modification that Marshall adopted). The Hendrix mod changes the slope and treble cap from 56k/250pf to 33k/500pf- this doesn't alter the tone much and gives more throughput, increasing gain a little.

IMO nobody has really improved on this tone stack, although there are other tone stacks. But it's far from perfect IMO. To *really* dial in a tone, it's touchier than most people realize. This is because changing one pot doesn't just change that part of the filter- all the other filters are affected to some degree. I can't count the times I've almost gotten it sounding killer, then adjusted one knob and couldn't get it back. I use little stickers now to exactly mark settings when I get close, then I can revert if needed.

 

[Sigs]

How I do it is the correc