Volume and tone pot not working

[Mark]

So, I’ve got the amp together and it sounds pretty damn good! The only problem is the volume and tone pot aren’t working right. Even with the volume all the way down, it’s still pretty loud, and turning up the volume doesn’t increase the loudness much. The tone pot doesn’t do anything at all. I swear things are wired correctly, I’ve checked it at least a dozen times. It must be something simple I’m not seeing. Any suggestions?

I suppose the pots could be bad. They’re new, but just some cheap ones I bought from Micro Center. No name brand.

[Andy]

I doubt the pots are bad, it’s usually a grounding issue. This is not an uncommon 1st build issue 😉

Double-check that all the grounds are connected properly. I usually connect all the tone and volume control grounds onto a bus wire then connect that to the three-pin ground terminal. Also, double-check the coax wire from pin 7 on the 12AX7. That should have the shield grounded only on the one side, the tube side. The pot side should be trimmed back and shrink wrapped if possible.

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   Tone-and-volume-Wiring-scaled.jpg
   

[Mark]

Thanks Andy. I guess I’ll check for the 10th time. I’ll post a pic if I don’t see anything.

[Mark]

I don’t know Andy. I’ve looked this thing over and over and over again. I’ve looked at the wiring of other people’s builds too. I don’t see anything wrong this (see pic). The preamp start ground is highlighted by a green circle. The buss wire is an unnecessary paranoid addition because I was afraid that maybe the pots weren’t contacting the chassis very well. Anyway, the problem remains. Tone pot does nothing. Still get considerable sound with volume turned all the way down. Thoughts? Anyone?

By the way, I was “trained” to ground shielded leads at the jacks/pots, not the tube socket. Also, I learned that the 68k grid stopper(?) should be close to the tube. So, that’s why the shielded leads look the way they do.

[Mark]

Image attached.

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   Control-wiring.jpg
   

[Mark]

SOLVED!! The pots were, indeed, bad. They weren’t passing any signal at all! Replaced them with new Bourns pots. Everything works perfectly. I knew the problem wasn’t me wiring, confirmed it by tracing the layout and schematic ala Hoffman’s simple first step for troubleshooting. Highly recommended!

http://el34world.com/Forum/index.php?topic=17701.0

[Author]

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