Help w/2000 Fender Pro Jr Tweed Combo

StratUp

Most Honored Senior Member
Sep 5, 2020
8,155
Altered States
A LOT of Fender amps have this "most of the volume is between 1 & 3" feature.

Internet rumor is that it's a marketing feature so that when you play the amp to test it, you're amazed at the volume low on the dial and believe it must be incredibly loud turn up high. But you've already got most of the volume.

The other internet rumor is that they actually use reverse taper pot to get that intentional volume boost down low. Which makes sense in terms of what you hear, but I've never pulled one out and tested it... and the schematic never indicate a "C" type control.

Are you handy enough to pull the pot and test it before ordering? Have a backup amp you can use while it's out to avoid having to solder the board too many times?
 

zozoe

Strat-Talker
Dec 19, 2018
473
Florida
The bright cap, as the name implies, brightens the tone. Clipping the bright cap will actually drop the tone down, which sounds like the opposite of what you are wanting to do.
It is C2, and you can see that it dumps the high frequencies past that 470k resistor ahead of the volume pot.
Clipping it will run all of the signal through that resistor, with will kill the top end.

Again, has it always been like this, or has something changed? If something has changed, it's time to take it to a good amp tech.
If things changed after changing tubes, then it definitely needs to be looked at, as Fender biases these things extremely hot.

Increasing the value of the volume pot will perhaps shift the peak volume level up into the higher numbers, but it's not going to change the taper.
The volume pot is attached to the incoming signal, and the grid of the next tube.
Down low, the grid is shorted to ground. As the control is moved up, the pot sweeps through the voltage divider, until reaching max where it is connected directly to the grid.
This won't change with a higher value pot... and going from 250k to1m might give you an amp that does little to nothing below "5"

The Fromel mods include changes to the tone stack.
Were the new capacitors installed in the correct location? Reversing them will cause issues.
OK CB,,, Check out the pics, and I was able to get readings on the Tone pot, & it becomes evide that it is a Linear, though I don't recall it behaving as such.... Probing points 1 & 2, then 1 & 3, revealed a 227k pot,, but after fiddling about & trying to get any reading on the VOLUME proved fruitless. On the back-lit photo, you can see what I've done & where I'm stuck,,, Perhaps at a glance, you can point out which are the live terminals.
But I distinctively recall the Volume jumping right out of the gates, by 3 pointing towards it being an Audio taper,,, I'll get it from AES & see what happens.... I'll let you know how it goes.... iirc, that damn Tone pot didn't behave like a linear, but your test said it all, & I'm OK w/it....
Thnx again
Kenny
 

Attachments

  • Screenshot_20220702-201716_Gallery.jpg
    Screenshot_20220702-201716_Gallery.jpg
    121.2 KB · Views: 2
  • Screenshot_20220702-201700_Gallery.jpg
    Screenshot_20220702-201700_Gallery.jpg
    112.7 KB · Views: 2
  • Screenshot_20220702-201645_Gallery.jpg
    Screenshot_20220702-201645_Gallery.jpg
    121.5 KB · Views: 2
  • Screenshot_20220702-203950_Gallery.jpg
    Screenshot_20220702-203950_Gallery.jpg
    175.4 KB · Views: 2
  • 20220702_213349.jpg
    20220702_213349.jpg
    120.9 KB · Views: 2

CB91710

No GAS shortage here
Double Platinum Supporting Member
Feb 24, 2019
10,904
SoCal
The contacts for the pots will be the 3 smaller pads.
The two outer, larger pads, are connected to the pot case.

Projrpots.jpg
 

Wound_Up

You can call me Duane 😁
Jan 23, 2020
4,733
NW LA
From this diagram, Vol seems to be 250k Audio, but Tone has a 'B' designation which I always thought was Linear....? View attachment 572603

Don't just ignore values on a schematic. There's a reason that volume potentiometer is listed as 250k 10a and not just 250k or 250k(A)

To put it simply, there are multiple 'A' tapers. So they have to identify each of them. As a result, you get 05A(aka A1), 10A(aka A2), 15A(aka A3), etc...

They didn't just add the 10 in there for no reason. That's a 10A(aka A2) taper pot. Google "fender 250k10a potentiometer" and you'll see that 10a is an, important part of that number and not something to remove and ignore. They didn't put it in the part number because they felt like it. It's actually a critical part of identifying the correct part for your amp.


Fender 250K 10A Taper Snap-in-Style 1/4" D Shaft Potentiometer for Pro Junior and Blues Junior Amplifiers https://a.co/d/arbMxwf

And the other post IS a "B taper" pot.

Look at this graph of different tapers. You can see 10a listed on the graph. Because it's an INTEGRAL piece of the part number. So don't just randomly take numbers out an identifying part # because you don't know what they mean. That's how you end up buying the wrong things. The 10A is just as important to that number as the 250k value.

Screenshot_20220703-012733_Drive.jpg
 
Last edited:

CB91710

No GAS shortage here
Double Platinum Supporting Member
Feb 24, 2019
10,904
SoCal
So measuring the tone pot from the large pad, that;s running from ground, which is actually the opposite side of C4 and would not show a direct DC resistance, instead it is running through the 22k resistor R6 to the pot wiper, so it'll throw off your reading on the tone pot.

Tone pot needs to be measured from "5" to "4" and "6".
The volume pot can be measured in-circuit from "2" to "1" or "3", but the presence of R6 will alter the apparent taper. Measuring between "1" and "2" at zero will read 22k. Rolling the volume up, it will steadily increase to about 100k, then it will again decrease back to zero ohms when the volume it at full.

Getting an accurate reading on the volume pot would require lifting one end of R6, which may be that pad below and to the right of "1", but I can't be positive, as we can't see the connections that would also be needed to R5 and C2, unless the connection between those two pads to the right of the volume mounting lug is a straight wire jumper.

Since the tone pot is connected only to capacitors on "4" and "5", it can be measured accurately in the circuit.
 

Stevem

Senior Stratmaster
Sep 3, 2014
1,498
NY
I don’t find your tone pot to be a issue.

What I have found on amps that have only a tone pot is they play feel wise and sound better if 1) the tone pot is wide open.
2) the amp is set up for a little more top end then I need and then just roll that back to my liking with the tone on my Axe.

At that point you may also want the treble bleed mod on your guitars volume pot if that works for you.
 

zozoe

Strat-Talker
Dec 19, 2018
473
Florida
I don’t find your tone pot to be a issue.

What I have found on amps that have only a tone pot is they play feel wise and sound better if 1) the tone pot is wide open.
2) the amp is set up for a little more top end then I need and then just roll that back to my liking with the tone on my Axe.

At that point you may also want the treble bleed mod on your guitars volume pot if that works for you.
Thnx,,,, I build pickguards for my own entertainment & skill savvy, but any extended knowledge base, ends there, & amps are a entirely foreign world... Oddly enough, I am & have ALWAYS been an 'amp guy', but I never pursued any furthur understanding except for when I'm forced to, like now!!

I don't want to retain any potentially rolled off high end coming from the guitar, & I'll probably need to settle on just finding the sweet spot on the amp & leave it.... BUT the Volume pot's jump between 0 & 3 is something I'm more anxious to rectify.
Because of these amps' delicate nature, I myself choose not to even go there, & allow my tech, who really just pulls off any PCB tight soldering and/or troubleshoot issues on an old PA amp find, to do whatever it is I ask. Handwired stuff is way more in my comfort zone...

The above graphs are remarkable, & does give me a better understanding of what it is I'm dealing with, & thank you..... I still remain firm (in my head) that the Vol pot needs to be changed to a Linear, but now w/all the variations in taper, I may just be stuck with simply accepting whatever taper AES has to offer...
Options are few & far between.
Kenny 🕉🎵🎵
 

Attachments

  • Screenshot_20220703-121318_Chrome.jpg
    Screenshot_20220703-121318_Chrome.jpg
    69.6 KB · Views: 1

zozoe

Strat-Talker
Dec 19, 2018
473
Florida
Don't just ignore values on a schematic. There's a reason that volume potentiometer is listed as 250k 10a and not just 250k or 250k(A)

To put it simply, there are multiple 'A' tapers. So they have to identify each of them. As a result, you get 05A(aka A1), 10A(aka A2), 15A(aka A3), etc...

They didn't just add the 10 in there for no reason. That's a 10A(aka A2) taper pot. Google "fender 250k10a potentiometer" and you'll see that 10a is an, important part of that number and not something to remove and ignore. They didn't put it in the part number because they felt like it. It's actually a critical part of identifying the correct part for your amp.


Fender 250K 10A Taper Snap-in-Style 1/4" D Shaft Potentiometer for Pro Junior and Blues Junior Amplifiers https://a.co/d/arbMxwf

And the other post IS a "B taper" pot.

Look at this graph of different tapers. You can see 10a listed on the graph. Because it's an INTEGRAL piece of the part number. So don't just randomly take numbers out an identifying part # because you don't know what they mean. That's how you end up buying the wrong things. The 10A is just as important to that number as the 250k value.

View attachment 573985
WOUND_UP,, to your knowledge, is there a Linear version of what you linked me to? I know how to navigate AMZ, but I just get too many darn hits to weed thru....
 


Latest posts

Top