Fender reverse taper volume pots

[Xris]

Hi all.
It's been known that fender installed reverse taper pots on their volume controls on certain amps.
The code is 50k-30C.
Everyone complains that the volume goes from
0-"jesus, that's loud!!!" with little movement, and not much after.
On the amp there is a tone control pot labeled 50K-15A,
Which is a forward taper pot.
So, the question is,
Can I swap these pots over so I can have a usable volume control?

Cheers.

[lowell]

I'm not familiar with Fender's pot tapers. However, taper can be adjusted to some extent by adding parallel resistors to a pot. If a pot say, turns up too fast, then most of it's resistance is in the sweep where the most volume change occurs. So, say you turn a pot UP, clockwise, and it's smooth until 12 o'clock, then it jumps much higher from 12-6 o'clock. You would want to make the resistance from the center lug to the clockwise lug LESS. Therefore placing a resistor between said lugs. Good place to start is using a resistor about 1/4 of the pot value.

[Xris]

So as it is it sweeps up too quick from 0-2.
So would I pop a 12.5k resistor on lug 1 and 2?

[lowell]

With the back of the pot facing you, and lugs on top, the resistor would go between the left and middle lugs. 12k is more common value. See if that helps.

[Chuck H]

Lowell's suggestion can work. You'll need to ear test it. It's important to note that adding resistance across a pot changes it's value. You need to start with a higher value pot to end up with the stock, intended resistance. Even in these cases it also changes the stock divider relationships to something that isn't stock. That is, the net pot value actually changes relative to setting. This is fine if it works and is usually not a problem, but it should be known for the purposes of circuit analysis.

I'm not aware of any guitar amp that uses a reverse log volume pot. I could be wrong. Reverse log pots are typically used elsewhere. I'd be less surprised to find one indicated incorrectly on a schematic or incorrectly installed at the factory than I would be to find out it's the correct spec. YMMV. In any case, just use whatever the equivalent 50k standard (not reverse) log pot is. All the same adjustments will be there, just at different places on the pot. That is a bracket mount or snap in pot (I found the same part number applied to both) so I expect the board it's on isn't really conducive to kludging in extra resistors. And anyway, why buy an over value pot just to adjust it with resistors when I'm certain a normal 50k log pot is available with some searching?

So, try Lowell's suggestion. But if you need to order a pot, just get the taper you want. In fact, they're not expensive. Get a linear taper AND a log taper. Use whatever seems to "feel" best. Electronically they're the same.

[Xris]

Yes. It's the space and the mountings I'm concerned about. That's why I thought it would be an easy yet cheap way to sort out the volume jump by switching the 30c with the 15a.
They are the same dimensions and mountings.
Im not to fussed how quick the treble may turn up between one and two, but the volume seems more important.
Theres lots of posts on this issue, and think this could be a quick and cheap solution.
Correct me if I'm wrong lowell, but the taper introduced by your example would still rise quickly, slowing down in the middle??
Cheers.

http://forums.fender.com/viewtopic.php?f=13&t=89288
http://www.strat-talk.com/forum/amp-...dio-taper.html
http://music-electronics-forum.com/t34045/
to mention just a few sites where the volume between 0 and 2 is an issue.

cheers

[lowell]

Good call Chuck. Xris, theres really no way to predict precisely how it's going to act. It's imperfect. However for educational discussion here's whats happening. The pot is 50k.

So, resistances in parallel are the reciprocal of the sum of reciprocals. Sorry that's a goofy mouthful. More simply put. Rtotal = 1/((1/Ra)+(1/Rb))

So when your pot is cranked up you have
1/((1/50k)+(1/12k)) = 9.7k

As Chuck said, this is a big difference from 50k. Ideally you'd use a 250k pot with a 68k paralleled. Issue being that now the load or input impedance to the following stage is much lower. Effect being less gain and possibly treble. You may or may not notice it too much though.

Anyway back to the pot. The paralleled resistance value and sweep is dependent on the fixed internal taper, so to some degree you are stuck with what you got. The added resistor simply gets you in the ballpark.

[g1]

Which amp is this for? Leo mentioned the op amp feedback loop in the other thread, I seem to recall something similar discussed recently. There was a very simple fix in that circuit, I think just one single resistor change.

[Xris]

It's the fender deluxe 112. Im fixing it up for my step son as a gig / practice amp.
They do practice in the house allot, so a controllable knob is preferred.

[Enzo]

Always good to mention what you are working on in the first post.

There are two kinds of volume control. The common kind we see in tube amps all the time are simple voltage dividers, but in SS amps, many are feedback elements in op amp circuits, and since the feedback resistance sets the gain, the value matters, and the tapers may not work the same as your basic tube amp divider. Just something to consider.

And as a practical matter, in the time it has taken to discuss swapping the two pots to see if they satisfy you, it could have been done already to find out.

[lowell]

Good points. I ASSumed it was your typical plate load voltage divider. Different story if in an op amp feedback loop. Might still work, but different nonetheless because the overall value matters more in the case of the latter.

[g1]

Which amp is this for? Leo mentioned the op amp feedback loop in the other thread, I seem to recall something similar discussed recently. There was a very simple fix in that circuit, I think just one single resistor change.

Not sure if the circuits are at all the same, but this is the thread I was thinking of, same issue with taper of volume control in Frontman 65R:
http://music-electronics-forum.com/t39179/

[Xris]

I have indeed swapped them. And indeed, happy with the results.
On the deluxe 112, the clean channel treble and bass pots are 50k log. These are now both volume knobs, and vice versa.
Admittedly, now the treble and the bass rise quickly and peter out. But now I've got my volumes sweeping nicely into a usable amount in the home.
When I take it out, I expect not much more gain from the mid sweep, but will get a nice rise at the top in gig situations.
Why they didn't do that in the first place, ill never know.
I can't see there being any danger of this set up, is there?

[Xris]

I must say though, a big thanks on the insight of pot path manipulation.
Gonna have a look at my guitar pots.
The volume on that creeps up too fast thinking about it.

[lowell]

Down the rabbit hole then.