Need alittle 5150 help!

[LosingReality]

So i got this 5150 and the presence knob would only work at certain points.

Actually, at 0 it would sound like it was wide open, and as you turn the knob to 12 oclock it would have less presence and then going past 12 oclock would add presence again. Make sence?

Now, the presence AND resonance, make no change to the amp tone what so ever.

Before it was just the pres....but now the reso too!

I have cleaned all the jacks as it did have the volume drop issue with the FX loop. However that is taken care of and is not an issue anymore.

Any ideas on what it could be? Bad power tubes? (they are old, but all glowing nicely) Bad pots?

 

[LosingReality]

Any suggestions?

 

[Trevoire520]

Best bet would be to take it to a tech, seems a pretty strange problem.

 

[lolzgreg]

I believe when the knob is off it removes it from the circuit. It definitely makes sense; when at a reasonable volume, the controls do the most at the end of the sweep. As you increase the volume, the controls sound better dialed back a bit.

 

[onqel]

Based on your information it sounds like a bad pot to me
edit: or a bad cap (if you have a schematic, it's the 22uF that decouple's the feedback signal going into the PI I'm thinking about)

 

[LosingReality]

Based on your information it sounds like a bad pot to me
edit: or a bad cap (if you have a schematic, it's the 22uF that decouple's the feedback signal going into the PI I'm thinking about)

K So alittle update. I did some maintence to the amp...cleaned the pots/jacks and reseated the preamp tubes....and i have it back to where the reso works fine and the pres is back to acting werid like i said at 0 its wide open and so on.

So Im confused, i guess i can try to replace the pot and if that fixes the pres ....then thats the issue and if not then its somthing else.

How expensive is it to replace a cap? And is it that hard to replace a pot? Just desolder and solder the new one in right?

 

[Wolfeman]

If it's the bad cap problem, it usually screws with your master volume and tone controls as well. It's like a $2 part but you have to remove the board to change it.

The pots are a little rough since they are pcb mounted, so the board need to come out to change just one.

Pretty much a pain in the ass more than anything. Otherwise most problems on amps are cheap to fix.

 

[onqel]

Since you got the resonance working there's no issue with the capacitor I was talking about .. presence pot is the first thing I would check/replace at this point..