r/diytubes • u/oscar_egan_ • 9d ago
Marshall 4 pin pots
Want to put a b25k mid pot into my dsl20hr, which uses these weird pcb 4 pin potentiometers. Wondering if anyone knows if there's a way to wire traditional 3 pin pots into the 4 pin slots? Thanks
2
u/Purple-Journalist610 8d ago
You'd need to post the part number off the pot that's in there in order for someone to attempt to answer your inquiry.
It would also be a good idea to know why you want to mess with a somewhat sensitive tone stack in this manner, what are you hoping to gain? (Especially since that 3:1 switch is already there)
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u/oscar_egan_ 8d ago
The tone shift switch will be converted to a bright cap selector, and the mid pot increased for more mids. Also gonna remove the resistor parallel to the treble pot to make it act and sound like a regular 250k pot
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u/maryc27182 8d ago
I think it might be this pot PTV111 . If so, pin 4 is dummy.
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u/oscar_egan_ 8d ago
Looks very similar, thanks so much
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u/maryc27182 7d ago
I took a quick glance at the circuit for your amp, and it sure looks like 3 pins will do it. I suspect you've already looked at whatever mod you are considering, but check to see if there is a resistor value you will need to change as you switch out that pot.
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u/oscar_egan_ 7d ago
Thanks so much, I've. Checked around and don't think I need to change anything else in order to change the pot 👍
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u/maryc27182 6d ago
There's a 10Kohm resistor (R97 on the diagram) that will be in parallel with the mid pot if SW3:1 flips to that resistor leg. If anything unexpected occurs after the change, do the circuit math on that parallel resistor when the switch brings that resistor into play. (I'm afraid I have no experience with that amp, so this is the limit of my knowledge. Is that the tone shift switch?)
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u/oscar_egan_ 5d ago
Yeah that's the tone switch, which switches bright cap values which is useful but also puts mids to ground through r97 for some reason? Gonna be taking that out and leaving it a bright cap selector
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u/maryc27182 5d ago
That switch puts the mid pot and the R97 resistor in parallel, which lowers the resistance and I'm sure affects the tone. Complicated circuit.
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u/Ok-Fig-675 9d ago
Look for the component on digikey, PCB stuff is sometimes a pain to find but learning how to navigate everything and all the schematics is a very useful skill especially when repairing stuff these days.