Missing Treble Control - Fixed!

[Chris-in-LA]

I have two distinctly different 2204 builds with the same problem in that the treble control does not seem to do much. This is not usually a problem with 2204 style amps. It almost sounds as if the treble is too high to be heard and the mids control is the new treble control.

My newest amp is a Valvestorm 2204 kit but I also have the same problem on a Bassman that I built to 2204 spec’s. Any suggestions?. With the Bassman and I tried a number of different things, different value silver mica caps, different slope resistors things like that. Made no change. It’s not the pots, they were taken out of the circuit and tested.

I have 2 Valvestorm builds with the same basic components but I only have the lack of treble control with the most recent one.

Any suggestions?

 

[junk notes]

Is your Bass pot a 1m analog and your Treble 250K that is attached to it?

not up on the technical, but try lifting the lead off on and off the board to hear changes in and out of the circuit.

It’s not the pots, they were taken out of the circuit and tested.

Also could be a cold solder joint. .. yes twice.

 

[Chris-in-LA]

Bass pot is 1megA and the treble pot connected to it is 250KL. I was just testing the amp and it seems like neither the bass or treble pot has much effect. The mids pot and presence have the most effect. The sweep on these pots does seem strange. The mids pot has the most effect between 1 and 5.

I did just check my other Valvestorm 2204. The bass on that one works and the more bass, the less the treble works on that one. Maybe my hearing is going. I need to check my Marshall 2204 to see if it’s my ears.

 

[anitoli]

Have you seen this? Try it and look at the frequency curves for a given value(s).

https://www.duncanamps.com/tsc/download.html

 

[Pete Farrington]

The mid control value acts towards setting the range of control of the T and B. Hence check the mid pot.

 

[junk notes]

The mid control value acts towards setting the range of control of the T and B. Hence check the mid pot.

Thank you for that @Pete Farrington. This also helps explain a lot to me with my Plexis. Guessing the right area, and in short I thought verify those components. Probably then guessing to check path for continued troubleshooting..

 

[Chris-in-LA]

It’s not my ears, the real Marshall works. It must be something in the clones’ circuitry.

 

[Chris-in-LA]

The mid control value acts towards setting the range of control of the T and B. Hence check the mid pot.

Check what on the mids pot?

 

[Pete Farrington]

The track resistance value, eg needs to be 25k not 250k. Hgher than 25k will reduce the range of control of the treble and bass control, less difference between min and max.

 

[Chris-in-LA]

Is your Bass pot a 1m analog and your Treble 250K that is attached to it?

not up on the technical, but try lifting the lead off on and off the board to hear changes in and out of the circuit.

Also could be a cold solder joint. .. yes twice.

Which lead should I lift?

 

[Chris-in-LA]

The track resistance value, eg needs to be 25k not 250k. Hgher than 25k will reduce the range of control of the treble and bass control, less difference between min and max.

You called it. The treble pot reads 25k and the mids pot reads 250k. I have them in backwards. That’s food for thought on 2 of my other builds.

 

[Chris-in-LA]

That fixed it. @Pete Farrington is a genius.

 

[Gunner64]

Nice. Pete's the man.

 

[junk notes]

Is your Bass pot a 1m analog and your Treble 250K that is attached to it?

You called it. The treble pot reads 25k and the mids pot reads 250k. I have them in backwards.

Ohh good!

 

[Pete Farrington]

The treble pot reads 25k and the mids pot reads 250k

The mids pot has the most effect between 1 and 5.

The 2nd point makes me wonder whether that 250k pot may be log / audio taper, rather than linear?

 

[Chris-in-LA]

@Pete Farrington , the 2204 works great. But I also have a Bassman wired like a 2204, same values for the components. The treble control on that one is nonexistent. The treble pot on the bassman is stamped 250KA and it read 250K on my meter. Could that be the problem? The mids pot reads 25KL, I have not checked it yet.

 

[Pete Farrington]

Bear in mind that most of the electrical change with an audio taper treble pot will happen between 70-100% rotation.

 

[Chris-in-LA]

Bear in mind that most of the electrical change with an audio taper treble pot will happen between 70-100% rotation.

The way that the treble pot in the Bassman acts now is funny. Basically nothing happens until I hit about 8 or 9. But even then any change is almost negligible. At 12 and 1 I get some deadening. I did email Mojotone and told them that they sent me a 250KA pot instead of the 250KL pot. My plan is to put the right pot in and see if it works.

 

[AndyD]

The mid control value acts towards setting the range of control of the T and B. Hence check the mid pot.

Good call Pete!

 

[Pete Farrington]

At 12 and 1

So treble is being reduced at full max or minimum?
If so that’s weird, have you got a scope, checking for oscillation may be a good idea?