Metropoulos Forum

The bass control on my JTM is not working, ie. it sounds exactly the same at 0 and at 10?? I've checked and double-checked everyting, any ideas? treble & mid controls are fine.

Any advice would be much appreciated

Measure the 1Meg bass pot with a DMM.

Make sure that pot is grounded properly.

That pot is not grounded at the soldering lug at all.

The pot is fine, and as axeman says, the bass pot isn't grounded. I'm having trouble figuring out just how it's supposed to work, it's wired to both the treble & mid pots, which are both working fine???

John_A wrote:The pot is fine, and as axeman says, the bass pot isn't grounded. I'm having trouble figuring out just how it's supposed to work, it's wired to both the treble & mid pots, which are both working fine???

Treble pot 'bottom' lug to bass pot wiper lug. Bass tone cap to treble pot bottom lug. The bass pot functions as a RHEOSTAT (variable resistor), not as a divider like the treble pot. Look at this layout, which makes the JTM45-style tone stack hookup VERY clear:


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Thanks for the assistance guys!!

Yes that's exactly as I have it wired up?? The only thing is I'm using an 18W transformer set, could that make a difference?

I've also noticed that when jumping the channels rolling on the normal channel really overpowers the bright channel even when it's only turned up a little bit.

Looking at the way it's wired up if I disconnect the bass pot it should be 'off' with a 500k fixed resistor the bass should be at '5' is that logic correct?

John_A wrote:Thanks for the assistance guys!!

Yes that's exactly as I have it wired up?? The only thing is I'm using an 18W transformer set, could that make a difference?

I've also noticed that when jumping the channels rolling on the normal channel really overpowers the bright channel even when it's only turned up a little bit.

Transformers should not be a concern..they have nothing to do with the tone stack. Check to make sure that the pot is the right resistance between the two outer terminals, and that the wiper is not open and is functioning properly.

The first thing I did was replace the pot, as I couldn't see anything else that could be wrong, especially as the treble & mid were working fine.

A bit more info if it helps

I noticed that both the treble and middle pots seemed to affect the same frequency so checked the value of all the pots. I had a 250K pot for the middle pot instead of 25K, changed it and it works fine!!