Amp experts, question

[Chad]

I am getting ready to do a cap job on my Musicmaster bass amp. While I have it out, I am going to replace the pots (they are pretty scratchy and have little control, IE, you get very little sound until 3, then it is full on). The schematic has 250k linear for the volume and 1M audio for the tone. Is there something I can swap out to get a better, more even sweep with the volume?

 

[wagdog]

@Chad Can you post a link to the schematic for the amp?

 

[GilmourD]

Is this the amp?

If so, this is the schematic.

 

[Chad]

Is this the amp?

If so, this is the schematic.

@wagdog

Thanks, that is it. On a side note, the amp in the picture is the version of the Musicmaster with 6aq5 power tubes not the 6v6 ones. Fender made 2 different versions and the ones with the silver skirted knobs have the 6aq5 tubes in them. The ones with the traditional black knobs have the 6v6 tubes like mine and the schematic you posted. I'm not sure why they did that.

 

[Help!I'maRock!]

@wagdog

Thanks, that is it. On a side note, the amp in the picture is the version of the Musicmaster with 6aq5 power tubes not the 6v6 ones. Fender made 2 different versions and the ones with the silver skirted knobs have the 6aq5 tubes in them. The ones with the traditional black knobs have the 6v6 tubes like mine and the schematic you posted. I'm not sure why they did that.

Cheaper parts later into the CBS years?

 

[GilmourD]

It's interesting that the PI is a stransformer.

 

[wagdog]

I suppose you could try a 250k audio taper for the volume. I'm not certain, but there may be interplay between the vol and tone pot and it might be affected. I'm sure you'll be able to dial the same tone you did before, but it might at a different place on the dial.

I would order both a linear and audio taper and try them both.

 

[wagdog]

It's interesting that the PI is a stransformer.

Yeah, I don't understand how that circuit works. I've seen that before... it's interesting, that's for sure.

 

[realtree71]

I suppose you could try a 250k audio taper for the volume. I'm not certain, but there may be interplay between the vol and tone pot and it might be affected. I'm sure you'll be able to dial the same tone you did before, but it might at a different place on the dial.

I would order both a linear and audio taper and try them both.

Agree with this. Good news is pots are cheap.....

 

[GilmourD]

Yeah, I don't understand how that circuit works. I've seen that before... it's interesting, that's for sure.

I think the center tap is the reference so each side of the transformer gets the top or bottom of the waveform.

 

[wagdog]

I think the center tap is the reference so each side of the transformer gets the top or bottom of the waveform.

Makes sense. I'm stuck on how the power tubes are biased.

 

[GilmourD]

Makes sense. I'm stuck on how the power tubes are biased.

Looks like a really weird way of notating that they're cathode biased by a 220 ohm resistor and a 25uF in parallel with it for both 6V6 cathodes.

 

[wagdog]

Looks like a really weird way of notating that they're cathode biased by a 220 ohm resistor and a 25uF in parallel with it for both 6V6 cathodes.

Ah - good catch, didn't notice it.

 

[GilmourD]

Ah - good catch, didn't notice it.

Yeah, the way it's diagrammed looks really weird but when I convert it to an actual layout in my head it works.

 

[Flying Guitarist]

Using a center tapped transformer is not an uncommon way of constructing a phase inverter. Just look at a typical DI box and you'll see that's how the balanced XLR signal gets generated.

But you don't see it often in amps. I believe even today transformers are most costly than a tube and definitely more than a transistor or op amp. $$$$

 

[Modern Saint]

Love the way the AT is being used as an OT for V1B hence used to drive the output stage much like how old ham radios were done.