Taming My Fender Pro Jr Amp....

Z

zozoe

New member
Hi all~ It's a fine little tweed-like amp, but it could use a few tweaks in the potentiometer department, like better, smoother & better tapers...
I was hoping that I could throw in a capacitor or some resistor swap to get more of a gradual tone feed, as well as a more even tempered Volume...
OR can this be done only w/a Pot Value change? & if so, to what values & types?
Thanks folks ~
 
This is a great question. I have a Pro Jr. (In Tweed!) that to be honest, kind of roars. I use a TS9 to tame the volume, but yes...a slightly tapered pot would be great!

I too will wait on the experts...

FYI I have a Weber Blue Back speaker in mine.
 
I tried one a few times but never bought it. Nice amps.

My money is on the pot being linear to impress folks at Guitar Center (wow, it’s just in 2 and it’s super loud) and you would be good if you changed it to audio
 
Blille said:
I tried one a few times but never bought it. Nice amps.

My money is on the pot being linear to impress folks at Guitar Center (wow, it’s just in 2 and it’s super loud) and you would be good if you changed it to audio
Click to expand...

Fender needed a place to put all the left over Hot Rod Deluxe volume pots, so they used them in the Pro Jr.
 
Can you just get a volume pedal and throw it in front? That's why my friend used to do with his old Hot Rod Deluxe that had the same problem (although he put his in the loop - if a pro jr. has a loop, this would work even better than in front).
 
GuitarStv said:
Can you just get a volume pedal and throw it in front? That's why my friend used to do with his old Hot Rod Deluxe that had the same problem (although he put his in the loop - if a pro jr. has a loop, this would work even better than in front).
Click to expand...

You won't hit the pre-amp as hard that way. You're basically just turning your guitar down if the volume pedal is out front.
 
The volume pot Fender chooses to go with is a weird taper for its use and location; it is, in fact, a linear taper pot. Replacing it with an Audio Taper pot will fix the off-on nature that it has. I don't think it came down to surplus so much as cost and likely use. If you can use the amp at anything above 2 on the volume dial, the level of volume control after that point is great. For bedroom playing, not so much. The most prevalent pot taper in amps is 250k linear, usually used for Treble and or mids. Now imagine catching a HUGE bulk deal if you ordered yet another linear pot to use on another amp line? Most of the decisions for OEM amplifiers are made out of economy, not because of what the original designer spec'd. If they can save 5 cents per amplifier, they will! The lowest common denominator will always win.
 
Benjy_26 said:
You won't hit the pre-amp as hard that way. You're basically just turning your guitar down if the volume pedal is out front.
Click to expand...

Yeah, I was thinking of putting it in the loop . . . it works great to tame stupid volume pot taper there. In front it would probably not work as well.
 
Theoretically speaking -

If this is about the main volume, a lot of times when people complain about the taper, it's due to the bright cap on the volume pot (no different really from a guitar bright cap on its volume pot).

People will desolder the amp volume bright cap and remove it (or just one leg) - or replace it entirely... and then it's manageable.

Mind you if anyone should want to go poking around, the requisite safety tip: drain the caps first!

And if you don't know what that (drain the caps) means, take it to a professional.
 
Thx EWIZARD,, So I should stick w/the same value, but just go audio for the Volume? And what about the tone control, where the high end lives between 8 & 10??
thnx again
 
LLL said:
Theoretically speaking -

If this is about the main volume,
Click to expand...

Pro Jr. is only 1 vol 1 tone. That's all.


But yeah the setting is Off, I hear it, Loud clean, and then progressive shades of breakup from 'hit hard' for a little crackle to 'Cheap Trick at Budokan'
 
I would just change the pot for an Audio taper equivalent. A 250K pot for a master volume is already pretty low in value. Most tend towards 500k to 1Meg. The higher the value the more overall gain you get, the lower in value, the less gain you get; relatively speaking anyway. You can use a 100k pot and it will darken the sound a little while also reducing the potential level sent to the amplifier section of the amp. If you don't use the amp for its natural breakup, this may be ideal? It will give you a more usable range and you will have pretty much the same clean headroom as before. As for tonal shaping, the tone circuit is surrounding the master volume, which means that the tone circuit becomes less and less effective the higher you set the master volume. The Tone circuit in this amp is set up sort of like a variable master volume bleed. The values in the tone circuit are pretty standard and not much improvement will be seen by messing with the values. Turn the tone knob down and it acts more as a Hi-Cut circuit, turn it up and it acts as more of a Treble bypass circuit letting more upper mids and highs go around the master volume pot. As you can imagine, when the master volume is all the way up, the tone control will only allow the Hi-Cut portion of the tone circuit to operate. The amp will be as bright as it will ever be when the master volume and tone pots are all the way up. At lower volumes, you can sort of make it brighter and darker as needed.

As for other tonal shaping things to do, there are only a couple that are really viable without doing a full preamp reconstruction. You can remove C2, which will reduce the very highest of highs ( call it air if you will ) of the amp. You can place a bypass cap on V1A, which will add lots of gain to drive the PI and help get some more bass depending on how large of a cap you go with? You can also make some changes in the feedback loop to add some rawness back or stiffen it up a bit more. The only thing I would look to change would be lifting C2 out if you think the amp is too bright.
 
Hey Ewizard,, after I locate the C2 you mentioned, w/o have to take the chassis out (no biggee if I do), can I simply clip one end of the cap? But it's not really the amount of high end it gets, but more so the taper, where most of the tonal changes occur from about 7 or 8, to 10. & similarly, most of the volume comes in before even hut 3 or 4.... I think there already Audio pots in there(?), but will changing them both to 500k's yield me a more even taper on both controls, & I wouldn't even need to mess with C2...?
Thnx~
 
C2 can simply be clipped on one side of it. It is likely a ceramic ( brown colored disc ) cap. Cutting it out will not change the taper of how the highs come in or out. It will simply reduce the overall amount of very high frequencies that you hear no matter what the tone and volume are set at. According to the schematic I saw, the volume pot is a linear taper pot. Linear taper pots are typically used for tone circuits, they don't work as well for volume because of the reasons that you are already aware of ( it turns the volume into an off-on switch ).

Going to a higher value volume pot will actually make your problem worse. Not only will it increase potential gain, you now have that much more resistance to sweep through in the same amount of sweep. I.E. If 250k is already too loud at 3 - 4 on the dial, a 500k pot will have the same output at 1 - 2 on the dial. A lower value pot does the opposite by reducing overall gain and having less resistance over the same sweep range, making the sweet spot larger.

The tone circuit is interactive with the volume knob. So as you adjust the volume, it changes the tone, and you have to readjust the tone knob to suit the volume you have set. The tone circuit acts more or less as a Hi-Cut type tone circuit. Where it is clever is that it acts as more of a treble bypass at higher tone settings ( it allows the mids and highs to sail over the volume pot ) reducing the amount of highs it cuts the higher you have it.

The tone circuit has rather standard values. The Hi-Cut portion of it is what I feel is the best option, so I wouldn't mess with that. The Treble bypass portion of it can be messed with to allow for more or less of the midrange to sail over the volume pot. It currently has a .01uF labeled as C3. This will allow a pretty large portion of the mids and all of the highs to sail over the volume pot. Going with smaller values will allow fewer and fewer mids to sail over. The highs will always be allowed by because this part of the circuit is a Hi-Pass circuit. So going with a 500pF cap would allow only the very upper midrange to slip by. Conversely going with a .02uF would allow pretty much all of the mids and most of the lows to bypass the volume pot. .01uF is a pretty standard value that I feel works pretty well.

Using different taper pots will only change where the two parts of the tone circuit balance out at, within the tone knobs sweep. It may turn the current setting of 7 into a setting of 5, but it will not change how much potential each section has. So 10 on the knob will always be what 10 was, and zero on the tone knob will always be what zero was. The only change will be where in that sweep the changes take place. In your case, the sweet spot on your tone control sets around 7 to 10 on the dial. I don't see a problem with that. Changing the amount of high or lows the amp has beyond that will require more in-depth component changes. C3 is one of those changes. Cutting C3 out will reduce the highs, but only for very high frequencies and it may make the amp too dark for you?

I say start with the simplest and most apparent problem first and simply swap the volume control for a 250K audio taper pot. See how you like it after that, and then make a new assessment of your needs.
 
Yeah, Ewiz,,, You nailed it as I don't want, or plan, to change either control's potential,,, just more even tapers on both Vol & Tone... As I mentioned, on 3 or so, the Volume is really up there, leaving me with fewer potential sweet spots,,,, Likewise on the tone, the bulk of it's function is between 8 & 10....So, it's 250k Audios for both, & I'll let the caps remain??
Thnx again
 
I wouldn't use an audio taper for the Tone portion of the circuit. I think it will make your problem worse. An audio taper pot uses a logarithmic sweep. It won't have the same relative output that the current pot does until the dial is around 8-9. A reverse audio taper may do what you want, but it is probably not possible to find one that will work for your amp ( PCB mounted like the current ones ). You can buy replacement pots in audio taper for your amp currently ( they will solder right in ).

I think you will find after replacing only the volume pot with an audio taper one, that you may need the dial back the tone knob a little anyway. Once you do the volume pot swap, your volume's sweet spot will be much wider and more in control, which means you should not need as much tone dialed in to get what you want? Try just the volume first and then go from there.
 
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