Aftermarket Knobs: Bad Idea … and a fix
The Blues Junior and Pro Junior have plastic-shaft potentiometers that are soldered to the board, but do not attach to the face plate with a nut. They use a special chickenhead knob to stabilize the shaft and prevent breakage. This is not a great design, but it is what it is. Fender doesn’t make any new amps, even inexpensive ones, this way anymore.
Aftermarket chickenhead knobs come in pretty colors, but don’t have the stepped flange at the bottom that prevents damage to the shaft. They also feel very wobbly and imprecise. Here’s an illustration of how easy it is to bend the shaft.
Also, since there’s nothing to prevent the shaft from being pushed in, any pressure on the knob or dropping something on it can easily break the fiber wafer that forms the back of the pot.
Here’s a detailed look at the stepped skirt on the stock knob and an aftermarket knob, which attaches with a setscrew. Above them is a pot that had an aftermarket knob on it. The owner leaned over to adjust it, lost his balance, and put too much pressure on it. It doesn’t take much.
The stock knobs sometimes don’t fit as well as they should. Here’s an example of an amp that came in for mods. One knob was too tight and simply would not go all the way down:
This is just as bad as an aftermarket knob–lots of wiggle, no protection for the pot or shaft. Here’s the solution:
It may seem crude, but trimming 1/4 inch or so off the end of the shaft is the quickest way to get the knobs to engage the hole in the faceplate and rest fully on it.
Don’t risk pot breakage with aftermarket knobs and make sure that your factory knobs fit properly. The feel of the controls is much better and your amp will stand up to rough treatment on the road.
If you’ve absolutely just gotta have some other color than black, maybe you can find some plastic tubing that’s 1/4 inch diameter on the inside and 5/16ths inch on the outside to fill the gap between the shaft and the faceplate. Then you can find a thick washer to go between the bottom of the aftermarket knob and the faceplate.
Oh, and there are no aftermarket pots that both attach to the faceplate with a nut and fit the solder pads in the circuit board. Sorry. And besides, would it be worth all that just to have different-color knobs?
Update:
When Fender did a run of the Texas Red Blues Juniors, they just stuck cheap aftermarket white chickenhead knobs on it, with no regard for stabilizing the shaft. Here’s what happened to a Texas Red as a result:
The reverb and volume knobs both got smashed down and both blew out the pots, as you can see here:
After I replace the pots, I’m going insert spacers and tubes to provide at least some protection in the future.
Above is the cure for wobbly knobs on the Texas Red Blues Junior. This will also work for other cheap crap aftermarket knobs, so if you’ve just gotta have some color other than black, here’s what you have to do:
- Get some 3/8 beveled faucet washers. The ones above are white nylon, and are no longer made. Current production are black, reddish, or orange. It doesn’t matter, since they’re going under the knobs and won’t be seen unless someone gets down on their hands and knees and peeks. There is some variance in the outside diameter of the washers, so take a knob with you to the store and make sure they fit.
- Get a 15/64″, letter A, or 6mm drill bit. They’re all .234 inch in diameter, the same as the black plastic shaft. Do not listen to anyone who suggests that you use 1/4 inch instead “because it’s so close” or because that’s what you have in your tool kit.
- Drill the washers out to .234 inch. Do not attempt to freehand this. You will hurt yourself or destroy the washers. Use a drill press and a vise with a notch to hold the round washer firmly. The quality of this hole determines how well the fix will work.
- Press the washer all the way down onto the shaft. It will be a tight fit. You must support the pot from the back as you press down, especially on the little black plastic dot where the shaft comes through the back wafer (see above). Get firm contact with the faceplate.
- Attach the knob so it rests evenly on the top of the washer. The washer is doing most of the work, so don’t overdo it.
You’ll find that the knobs are stable and that your pots are protected.
Here’s an even better solution to the wobbly shaft/unprotected pot problem, and it’s from Fender’s own parts bin. It took me a while to find them, so it’s no surprise that some fresh-faced engineer at Fender didn’t know they existed:
The bushing fits the shaft and the panel hole perfectly. It stabilizes aftermarket knobs and protects the pot.
You can order from several sites online; I got some from Darren Riley:
http://www.darrenriley.com/shop/cart.php?m=product_detail&p=1093
Here’s another approach to custom knobs that work well, but won’t allow the pots to be damaged:
Custom wood knobs have thick felt spacer. Plastic bushing goes around shaft.
I made custom maple knobs for this Blues Junior, to better match the pine cabinet that the customer had selected. You can see the thin plastic tubing around the pot shafts to eliminate side-to-side wobble. I turned the knobs on a lathe and drilled the .234 inch hole in the bottom for a firm press-fit onto the shafts. I made the pointer slots on the milling machine with a slitting saw. I punched a hole in some self-stick heavy felt furniture pads and applied them to the bottoms of the knobs. When pressing knobs onto Blues Junior shafts, you have to support the pot from the back so you don’t overstress or crack the fiber wafer.
I just want to compliment you again.
The modification to the pot shafts (shorter) now makes the feel on the knobs just right. Thank you for the FREE advice…Rare these days.
I find your answers to questions to be very informative and without the attitude I’ve seen from other “experts”.
A wise man once said “there are no stupid questions.”
Therefore, You’ve gained a customer. Not a big purchase, but I’m going to try the Basic kit, Presence control and the Switchcraft jack.
Thank you.
GG
Too bad I didn`t see this sooner. The volume pot on my amp just broke in exactly the way described here. Turns out the factory knob did not sit low enough on the shaft. Some of the other knobs are the same way, so I cut them shorter and now they fit properly.
Everyone with a Blues JR should do this, unless you want to have a broken amp right before a gig!
I second this. When I got the replacement pots from the BillM kits, the shafts were shorter. I noticed that with the knob sitting on the panel, not only was it far less wobbly, but the knobs felt firmer, which is more to my liking.
So I clipped the rest of them with those flush cutters he’s got in some of the pictures on this site, and I’m much happier with the knobs. Plus, my acne cleared up and girls like me better, too.
Hi! Many thanks for your very informative site. It helped me decide to buy this amp.
My new BJ:r (built 2006, it has the small anniversary button on the back) has this fault. It seems to be an easy thing to solve but the knobs sit very tight. I am afraid of doing damage to the board by just trying to pull the knobs off the shafts. What can I do?
Jan Mortenson
You can use a pair of spoons as levers to lift the knob from both sides. Protect the chrome faceplate.
How do you trim the shafts? What do you use to cut them?
Thanks
Dan
I use a Dremel tool with a cutoff wheel. It cuts/melts through the plastic quickly. If I didn’t have one of those, I’d use an X-Acto or similar small hobby saw.
Hi guys and gals,
When I did my shaft mod, I unplugged of course, opened up the back, discharged the power…(see Bills explaination how to do this elsewhere on the site)…and slid my finger under each pot as I was pushing the knobs back on, you may need to push a little and wiggle the knob a little to get the larger ring down inside the faceplate, So support the pot from underneath before you break your board!
Rock On,
Gary Galleher
Is it possible to replace the pots by some with metal axles ans a thread to screw them onto the chrome-plate?
Would the chicken-head-knobs still be under the upper wood-panel?
Or is the installation not practicable or is it too much work?
I covered this point in the article above. There are no pots that solder to the circuit board and also thread through the faceplate. If you use pots that thread through the faceplate, you have to run individual wires from the pot terminals to the circuit board. Since the connections would be under the pots, this would be a difficult thing to do and it would be very easy to break the wires if you needed to service the amp.
There is nothing wrong with the stock pots if the knobs are correctly installed.
Thank you, it is just that I wanted higher quality and easier turning, it is sometimes difficult if you only touch the tip of the chickenhead.
This includes the white chicken head knobs that are on the Red special edition! My amp tech in Milwaukee (John Blick) noticed the issue and resolved it by placing rubber washers under the knobs help them settle.
Yes, putting unsupported knobs on the Texas Red was a stupid idea on Fender’s part. You should put some tubing around the shaft, however, to provide more side-to-side stability as well. I’m glad you have a sharp-eyed tech!
Bill:
As always, you’ve saved the day and helped us to get beyond some short-sightedness on Fender’s part.
I blew out my tone pot, something must have been set on top of it and blew out the bottom during travel, is it dangerous to still play it? how easy of a fix would this be?
It won’t hurt to play the amp, but you will either have limited control or no control over the tone. A broken treble pot could silence the amp. You have to get to the back of the circuit board to replace the pot. See instructions here. The pots are inexpensive and are available from Antique Electronics, http://www.tubesandmore.com.
Bill, as others have said already, THANKS – Great info. I have this exact problem with my Volume pot/knob on my Pro Junior and I intend to fix it today. Additionally, I’ve done most of your Pro Junior Mods on my own – thanks for your guidance on these mods too! I am experiencing one issue and I’m thinking it might be a blown pot.. My Pro Junior works fine; however, when the volume is set at Zero, I still hear my guitar as if it was on 1 or two. Any ideas appreciated!
Some pots just don’t go all the way off. The only cure is to replace the pot and hope that the new one goes full off.
Bill, those maple knobs are just beautiful! Kudos to you! Can I get some? Happy holidays!
Sorry, no time to make more maple knobs. But I’ve seen some nice wooden knobs at Allparts that might do the job.
Many thanks for the info on this page Bill !
I have a Tweed NOS Blues Junior with C12N, love the amp but the wobbling knobs (most of them) feel so cheap and badly engineered that I actually queried it with the shop at the time thinking the securing nuts had been ‘left off’. It’s always made me feel the amp is very fragile.
Anyhow I levered off all the knobs except the Volume (yes that many were badly fitted!) with the help of two spoons and a cloth as suggested, clipped them slightly with very sharp hobby snips (which cut through the plastic with butter), refitted the knobs…job done
What a silly design but it feels much better now and I know the pots are protected by the flange.
Been experimenting with tubes too, found that a 12AY7 in V1 allows me to crank up the Master higher and the amp gets nicely bouncy and responsive, also keeps rather cleaner
Thanks again
will a 250 k linear pot designed for a blues jr. work as a tone pot for a pro jr.?
i can’t find tone pots specifically for the pro jr. at the link you gave, tubesandmore
thanks.
Yes, the BJr 250K linear pot is identical to the 250K linear in the PJ.
i just finished replacing some parts on a Fender 65fmr amp. under the control knobs of this amp is a sleeve with two diameters. smaller dia fits into chassis. and larger goes against the knob. control shaft fits easily through it and it prevents the shaft from rockin sideways…they are a buck each at Darren Rileys. link-> http://www.darrenriley.com/shop/cart.php?m=product_detail&p=1093
I recently discovered these bushings too. Darren Riley is the only source I’ve found. They’re a great solution–highly recommended. I’ll add them to my page when I get a chance.
i ordered and received the pot, it says snapin, can i snap it into position or do i have to solder it?, any info would be very welcome, as i dont want to force the pot into position,but would want to do it myself if possible.
thanks again.
Snap-in is the name for the kind of pot. The other kind is called panel mount. You must solder it in place.
Bill,
Music Parts Guru also has the bushings, FEN-003-3206-049, $6.00 for a bag of 12.
I had to replace the mid pot on a Blues Junior. I had no idea that they would use such junk in an amp. I replaced the pot with a dime sized pot from Small Bear Electronics using three short wires to attach it to the solder pads. It works well. I should replace all of the rest of the pots. My BLues Deluxe uses real pots but still isn’t fun to repair. The amp sounds very good. I am gooing to try the tone stack mods that you suggest. Also I am going to do the bias mod and set it to 9 watts idle current like you do. Thanks again Bill! I’ll try one or more of the mod kits when my students bring me Blues Juniors to repair.
I see a lot of Blues Juniors that are 10 and more years old where those pots are working fine. But I agree that they’re pretty cheesy. As you discovered, you have to use a very small pot in order to get clearance with the board, and the small wires are fragile–that’s six solder joints per pot, more chances for breakage.
what other knobs can fit with out modification?
No other knobs fit the Blues Junior properly unless you use the bushing shown above.
I clipped them and the original knobs are now snug. Thanks for the advice. If I went to a brown chicken head fender replacement with set screws, they should work fine now. Correct?
thanks for all your help !
No, if the knobs don’t have the step on the bottom, the pot shafts are in danger. Now that you shortened the shafts, you may not have enough shaft for the setscrews to grab. I think you’ll learn to love the black knobs. :^)
Bill,
Been doing my research before modding or changing anything on my Hot Rod BJr. Your site and the info are awesome, but still pale in comparison to the craftsmanship you display in your work. The maple knobs you made are incredible and demonstrate the professionalism and attention to detail that make you THE reference for BJr mods!
Thanks!