What makes a tone circuit work/ fail?

[louderock]

Assuming the Treble pot is functioning properly, what else would cause the treble circuit not to work? I have a Selmer Treble and Bass amp and the treble pot on the bass channel doesn't really do anything. I measured across the pot and it does go from 0 to 100k or resistance with the center measuring around 50. So, I guess the pot is good. What else would cause it not to work?

[Clutch21286]

You could try reflowing the solder connections. You could have a bad cap or ground connection. Good luck!

[drgonzoguitar]

Is it the MKI? Try replacing c15 (.0047uF) cap that goes to ground from the 3rd leg of the treble pot.

A tone circuit requires a capacitor to create a corner frequency to roll off from. Check out:

http://www.muzique.com/schem/filter.htm

[louderock]

Thanks guys. It's a MkII and I swapped the cap with the cap from the other channel and get the same result. I'll measure all the components again.

[louderock]

Still can't figure this one out. It's almost like the amp gets louder and fuller sounding when I turn the Treble pot all the way down. As I turn the Treble pot up, the amp gets quieter and smaller sounding. ?????

[donnyjaguar]

It sounds like the top (CW) side of the control is disconnected. Either internally to the control or via a bad solder joint. Replace the control but also look for what caused the problem to begin with. Guessing shorted capacitor here.

[louderock]

Any idea which cap could be bad? I don't really know how to check a cap.

http://www.schematicheaven.com/bargainbin/selmer_treblenbass_mkII_50w.pdf

[donnyjaguar]

A capacitor should register an open circuit on an ohmmeter when tested out of circuit. Check for 100kΩ across the Treble control first to confirm my theory above. Test also the wiper on the pot with the ohmmeter. This can be tested in-circuit on this amplifier. Just make sure everything is discharged first as this ampliifer doesn't have a bleeder resistor.

[louderock]

Here's the culprit:

http://i238.photobucket.com/albums/ff227/louderock/P1010410.jpg

This was wired across the volume pot on the bass channel. Remove it and the Treble pot works fine. Next question.... What the heck is it? I know it's a cap but what is the 4700J???? Is it a 4700pf cap?

[louderock]

Which cap do I need to replace this with?

http://store.triodestore.com/sprag71ordro.html

What's the difference in all of these caps? Silver mica, orange drop, polypropylene....

[Trout]

Here's the culprit:



This was wired across the volume pot on the bass channel. Remove it and the Treble pot works fine. Next question.... What the heck is it? I know it's a cap but what is the 4700J???? Is it a 4700pf cap?

Yep, Polystyrene Film . Those are especially heat sensitive, plus the very fine leads tend to break easily.
They do however sound REALLY nice in some circuits, but IMO not worth the potential failure problems.

Just Radio's (http://www.justradios.com/polystyrene.html)carries them really cheap.

Best to just use a Silver Mica as a replacement.

[louderock]

Thanks for the info and link. What makes one 4700pf cap different from another? Silver mica, orange drop...... Are they tonally different? Do some work better in some applications than others?

[Keyser Soze]

What makes them different you answered yourself - it is the material used to create the dielectric (silver mica v.s. polypropylene film in the case of orange drops) and yes people will tell you they sound different. In amplifier applications I tend to agree and lean towards silver mica for the type of tone controls you are dealing with.

But for the minimal expense buy a few of each and experiment to find what suits you best.

[louderock]

Does anyone know what value 4700J is? I know the J is +/- 5% but what is the 4700? Is is 4700 pf? mf?

[epluribus]

That calculator website...Fun link! 'Preciate it!

[SatelliteAmps]

Does anyone know what value 4700J is? I know the J is +/- 5% but what is the 4700? Is is 4700 pf? mf?

.0047uf.

[TweeDLX]

.0047uf.
Also 4700pf :)