Bass knob question

[isaac_teller]

Hello. I built a Plexi head, stock with only the addition of a 1MA pre-phase inverter master. The treble and mid knobs work fine. For some reason the bass knob has zero effect on the bass level. I replaced the bass pot and checked all the wiring at least 25 times. Any ideas what could cause this?

[Latole]

We need to see your schematic as build.
By the way, it is not a knob issue, it is a bass pot issue

I test different MV and a pre-phase inverter MV is  one I don't like.

[dwinstonwood]

If everything is wired correctly, and you've double checked all of the resistor and capacitor values, you could try changing parts values in this calculator until you get the bass control you want (select Marshall in the drop-down menu):

https://tonestack.yuriturov.com/

[isaac_teller]

Yes there is a single-gang 1MA pot where the red circle is. This is the schematic that I used.
https://el34world.com/charts/Schematics/files/Marshall/Marshall_1987_jmp_lead_50watt_el34.gif

[Latole]

You must have made a mistake in the assembly. Otherwise what else?

[ac427v]

The schematic shows a jumper wire connecting the center terminal of the bass pot (wiper) with the outside terminal. Check that the jumper has good solder joints.
I prefer the pre pi master volume on Marshalls...or both a pre and a post pi master if I am in a knob spinning mood.

[Latole]

Yes there is a single-gang 1MA pot where the red circle is. This is the schematic that I used.
https://el34world.com/charts/Schematics/files/Marshall/Marshall_1987_jmp_lead_50watt_el34.gif

[isaac_teller]

Found the error. I mistakenly used 270K mixing resistors after the volume controls instead of 470K. Now the bass works properly.

[Latole]

Found the error. I mistakenly used 270K mixing resistors after the volume controls instead of 470K. Now the bass works properly.

I don't see how a 270k would cause a problem.  Almost any high value may work.
Even less that it would prevent the bass potentiometer from working.

You must have “corrected” the bass potentiometer problem without noticing it, and it may only be momentary, but it will come back.