Ok campers, I get quite a few questions in emails asking me about how to repair channel switching problems on newer Fender and Peaveys and such. Most of the time the actual repair is simple to an experienced bench tech but to someone sitting at home wondering if they should fix it themselves to save money or take it to the shop, the answer most of the time from me is take it to a shop.
But with that said, maybe some of my visitors would enjoy seeing what the problem most likely is. And some may decide to repair it themselves. BE WARNED, THIS INFORMATION IS FOR ENTERTAINMENT ONLY, you go into your amp and screw it up, don’t come back here complaining.
Here is a definition of a FET: (FET) A voltage controlled transistor in which the source to drain conduction is controlled by gate to source voltage. If you want to compare it to a tube, think of gate=grid, drain=anode, source=cathode. Of course they are different animals. And if you’re really want to dig, find out how a field effect transistor (FET) is made and works.
Now let’s move on. On a lot of Fender’s and Peaveys and other amps that have channel switching, there are little FET’s like the J111 that go out. This isn’t the problem all the time but a lot of the time.
The first thing to check is something simple like the foot switch or channel switch on the control panel. Remember to always check the simple things first.
Look for clues, like does it switch with the panel but not the foot switch? Does the LED lights change but not the sound? This can help you narrow it down.
I’ve included a partial schematic of a hot rod deluxe for an example. In the first pic notice the red flares, these will be places of interest. Study the whole circuit but for this post I will focus on the main areas of interest.
The circuit in the first pic is responsible for generating signal voltages to do switching is a good way to look at it. Notice to the left you have the foot switch input and the channel select switch.
These are basically shorting and lifting a ground on the input of the IC’s to vary the input voltage.
One chip delivers current to drive the LED lights and relay and the other is driving the input to our focus point of the post the J111 FETS. Look at the MORE_DRV
Notice on the second pic the MORE_DRV, this means a continuation of the same circuit, which is an actual copper trace on the circuit board, sometimes wires.
Ok, this is where it gets interesting. Notice the MORE_DRV is tied to both J111 FETS.
Which in turn, the J111’s are tied to the cathode’s of V2A and V2B, thus shutting down or turning on the tube according to the voltage that the J111’s receive from the circuit of the first pic.
Now that we have the ability to turn tubes on and off, how do we control them individually? By the relays K18 and K28 that again are controlled by the circuit in the first Pic.
If you want to study them closely you can figure out which one is doing what at which time, personally from a repair tech point of view, my aim is to get in, repair the circuit gracefully and get out and move on.
Of course from an engineering point of view, every fine point would be studied and understood.
Now back to point of this article. I know from years of experience that most of the time in a switching problem is going to be the FETs. Why? Because usually it’s the weakest point in the chain, it’s the nature of the component to fail for various reasons: heat, voltage spikes, excessive current for a short amount of time, alignment of the planets, pick one.
The beauty of a FET and the reason it is used, FETS have the ability to interface seperate circuits with reduced supporting circuitry. The accountants with the manufacturing companies love them. And they do their job well, the downside is they are vulnerable to the items I mentioned above.
So it is a tradeoff and if you happen to be one of the unlucky ones, you have a reapir job on your hand. Believe me FETs have come a long way in their dependability. In the early days of solid state electronics used in music gear, after a major lighting storm that causes large volume of static electricity, the gear would march in the door the next day. Especially in the early effects units.
How do you test if it’s bad? The easiest way is to take voltage readings while your using the switching components, like the foot switch. You should have a swing of so many volts between the different feet of the FET. It can vary between amps
The simplest thing to do is A/B with good ones in other part of the circuit. Watch how they behave as you switch between circuits. A bad one just sits there and maybe switches .5 volts, there is little variance. Now make sure you have continuity from pic one to pic two for example. If there is a physical break between the two circuits, then the FETS are not the problem, the problem is they are not getting a signal.
What I usually do is watch it real quick on my scope and see how it behaves. Make sure it’s getting it’s switching voltage from the PIC one, and because I’ve seen so many fail, I replace it, I don’t do a lot of troubleshooting.
But anyway back to the article. Here’s the dilemma on the newer style amps like the reissues. This little inexpensive part takes awhile to replace correctly.
To get to the part, everything has come out, like the main board, jumpers, pots have to be freed and you have to carefully contort various items to get the board out and be at an angle you can get to it.
Then once there you have to remove the transistor, which I have a desoldering gun that makes it a snap but can be done with a spring loaded solder sucker or solder wick.
Your repair soldering chops are going to have to be up to par or you’re going to gimp up the board and have more work on your hands.
Now some techs might just clip the transistor from the front-side and tack on another one. I’m not going to comment on that one way or another, each tech will decide that one. Each situation is different.
I’m including a video that explains FET transistors with examples pretty well. It’s talking about MOSFETS but it is basically the same thing.
This is kind of a “the basics” of transistors but it has a section on FET’s
In this tutorial I’m going to go over the basics of a popular transistor and show you how you can use it to control different gadgets. What is a transistor? A transistor is a device that allows you to use small changes in voltage to switch things on and off. They are kind of like a valve in your plumbing system but instead of controlling the flow of water, you’re controlling the flow of electric current. To make things as simple as possible I’m only going to talk about the easiest type of transistor to work with: The N-channel MOSFET. Basically they work like this: When the transistor is off, no current can flow. So it’s as if one of the power wires on your gadget has been disconnected so obviously the gadget will stay off. When the transistor is on current can flow and it’s like both of the power wires on your gadget are connected now so the gadget will activate. So where can you get an N-channel MOSFET? Well there are many different types of N-channel MOSFETs but they all work in pretty much the same way. You can get an N-channel FET from Radio Shack or you can scavenge them from from old computer hardware. They usually look like this. Google the part number on the transistor to double check exactly what you’re working with. Here I have an IRFZ44. Anything else you need? Well in addition to the transistor, you’re going to need a couple of other things. You’re going to need the gadget that you want to switch on and off. And I’m going to use a car’s headlight as an example here. You’re going to need an external voltage supply that your gadget would normally require. And in this case it would be the car’s twelve volt battery. And finally you will need some sort of signal that is either 0 volts or 5 volts. Basically a digital logic signal… and I’ll give you a few examples later. Okay so you’ve got all that? Let’s talk about how you connect the transistor. N-channel MOSFETs always have 3 pins called Gate, Drain, and Source. I know the names are kind of funny sounding but you will have to memorize them. Gate Drain, and Source. Drain is the pin that current will drain into. Source is the pin that current will flow out of. And Gate is the pin that will turn the transistor on and off, kind of like how a water gate valve will control the flow of water. Connect up the transistor like this: The source is connected to your circuit ground. Connect the negative side of your load to the drain of your transistor. Connect the positive side of your load to the positive terminal of your external power supply. Now whether the transistor is off or on will depend on whether the gate is at 0 volts or 5 volts. Here is the equivalent circuit when the gate is at 0V. The transistor stays off, so no current can flow, so the headlights stays off. Here is the equivalent circuit when the gate is at 5V. The transistor turns on and starts acting like a very low resistance current path so current can flow. Current will flow from the power supply through to your load, into the drain of the transistor, and then out from the source of your transistor into ground. So when the transistor is on, your gadget will turn on too. Now let’s talk a little more about the signaling voltages that are going to the gate. There are a lot of different ways to do it and that’s why transistors are so much fun. Here is an example with a little mercury vibration switch to turn on the transistor. When you hit the switch, the gates receives five volts so the transistor turns on. Here’s an example with a computer’s parallel port pins. When the parallel port outputs a 1 (which would be five volts) The transistor turns on. And here’s another example with a 6 volt solar cell. When the light shines on solar cell, the gate receives at least five volts, so the transistor turns on. And there are hundreds of other ways you could switch the transistor on so basically you can control anything with anything. Now I would like to clarify something for safety’s sake. Over here on the gate side, you want to keep the signaling voltages less than fifteen volts. 0-5V is fine, 0-12V is fine but if you try to signal things with a 0 to 30V signal you will blow something up. However on the Drain side of things you have a lot more freedom in the voltages you can use. The only limitation is what the transistor can handle. This IRFZ44 is rated for up to 60V so it can switch 12V loads, 50V loads, whatever I want all the way up to 60V DC. So I could switch LEDs on and off. I could switch a string of low voltage christmas lights on or off. If you add a diode over here you can switch a motor on and off, or switch a solenoid on and off, or switch a relay on and off. And once you have a relay being switched you can switch light bulbs on and off, you can switch toaster ovens on and off and you can switch your refrigerator on and off. Basically if you can get a system that puts out a 0 to 5V signal, you can attach a transistor to it and you’ll be able to switch any gadget on and off. Now remember, I just showed you the basics of one type of transistor. There are many kinds of transistors out there with many different operating modes. If you are interested in learning about other kinds of transistors, google “NPN transistor tutorial” “PNP transistor tutorial” “P channel MOSFET tutorial” and “JFET tutorial.” That should be enough to give you a headache. But for now, you know how to use an N-channel MOSFET and that is all you need to turn any DC powered device on or off.
So hopefully this article will give you a better understanding of your guitar amp. Sometimes this very same arrangement is used in rack gear and synths, etc. Either for switching or what is known as clamping circuits.
For example to clamp down the output audio signal chain while all the digital circuits stabilize so you don’t get annoying pops and clicks. Maybe this is another article.
46 thoughts on “Guitar Amp Channel Switching Problem and How to Repair”
Thanks for suggesting the “The Guitar Amp Handbook” (Understanding Tube Amplifiers and Getting Great Sounds)!!! Fantastic resource. I love your site because it is good for the layman like me or for people who need to expert help. Your tutorials are excellent and to the point. Great site!
Keith Guitar Stanton
Thanks for the kind words. If anyone is interested here is a link to the book, a guy at Groove Tubes turned me on to it:
Hi, my problem is Fender Champion 300 amp, Channel one does’nt work, channel 2 works, any ideas what may be wrong with it?? Thanks.
I would read the above article if you havent, check the easy stuff, if it’s not the easy stuff, probably a bad FET.
Where can you get the J111 FET – Looking at all the info on it (and there is not a lot) it seems to be a bit of an odd fish – Peavey only list it as 35v Special Application – other specs are
J111 (TEM) N-FET-J 35V IDss> 20mA Rds30R TO-226AA (TO-92)
Is there a modern easy to get FET that one can pop in or is this something that needs to be ordered from Peavey
J111 is popular, just punch it into google. It looks like you are in UK, I just saw a place called Amp Doctor that has them. You can always do a cross reference check in google to…there’s something that all the electronics companies put out called a semi conductor cross reference book so you can get it at your local electronics parts house.
If all else fails just walk in and tell them you need a FET J111, TO92 case style…just make sure the legs of the FET are the same, by that I mean the Gate, Drain, Collector have the same orientation as the old one.
With that said the FET may or may not be your problem, I would check voltages first with one in the circuit that you know is working. And there is nothing to pop in, it has to be soldered. Some people cut the legs and tack the new one in and some pull the board and desolder correctly and replace…
Thanks Gary! Your advice was spot on.
I have a Fender Champion 110 which is my first amp. I bought it about seventeen years ago and then two years later my friend plugged a Radio Shack amplified microphone into the clean channel and it went dead. Well, not exactly dead because you could control the volume through the dirty channel but the switch had no effect and both channels sounded like the dirty channel.
I always liked the clean channel and have missed it all this time. I tried a Frontman 25R and it seemed flimsier and not quite the same. So I followed your advice.
There are two J111 transistors on the board. The board has to be flipped to get at the soldering side but I took my time and finding the screws was not too bad. The only things I had to unplug were the speaker and the reverb. The knobs on the front pull off and the 1/4 plugs unscrew with a socket (I just used my fingers and no wrench so as not to apply two much pressure).
I didn’t test the transistors because I don’t have the instrument and it was easier just to replace both. I also figured if one went bad then they were probably low quality to begin with.
I used my RadioShack solder sucker gun with the little pump on it to pull off the old solder and the transistors came out easily. Soldering the new ones in was a little harder because I’m terrible at it and have shaky hands, but there’s enough space it’s not impossible. My one mess up I was able to fix with the solder sucker.
Anyway I reassembled everything and fired it up and it works.
A plug for my transistor source: Newark Electionics.
This company is awesome. They charged 17 cents a transistor and shipping was like $6.50. They put them in a nice box with foam protection in it and a static free pouch for the transistors. I’m sure they lost money on this transaction (I ordered 10 transistors just to have some spares), but now they have a customer for life.
Safety-wise I don’t recommend anyone else copy me but all I did was leave the amp unplugged for a week beforehand and I wore shoes and tried not to touch too much stuff. The back of the amp says that there are no user serviceable parts inside.
Thanks again. I love this amp and am so glad to have it back fully. And you saved me at least $90.
Wow, you’re one of the very few that actually uses this info, congrats! Sounds like you did it perfectly. If you want to know more about discharging a supply look around the net or drop me an email and I can give you a few pointers.
Great site. I’m trying to help a buddy who has a switching problem with his Champion 110 – just like Jack(above). Your advice looks promising!
just curious, do older Boss pedals use this same type of system?
Thanks for your site!
From what I remember yes. Mainly for the reason for reduced parts in the supporting circuitry on a circuit that is switching the signal path. With that said, in a pedal it might also just be a mechanical switch with circuitry to clamp the annoying pop but keep it quick so you will probably need to look up the exact model schematic.
Here’s a problem my fender blues deluxe(1997) just developed.i was playing thru the clean channel when the drive channel light went on and i lost output.then the light would go out and the output would come back.this happened on and off consistently
That has a pedal right? If so switch the pedal between the ch. a lot, also if you can unplug it and plug it back in a couple of times. Hopefully it’s something simple at this point.
this is Joe with the drive channel problem on my vintage blues deluxe.these are the things I did…..
pushed the drive switch numerous times; o.k.
used a foot switch numerous time;o.k.
unplugged the foot switch numerous times;o.k.
left the amp on for 4 hours(about the length of rehearsal)
noticed that when I turn off the power switch,the drive channel goes on( i don’t think that matters,heck,the amp is off).
plugged in my guitar and……..,I could not duplicate the problem
oh ,how I hate intermittent failures.quite time consuming
joe in PA
I have a Fender Dual Showman Redknob which has developed a loud buzz on both channels. When I select the clean channel, the gain pot on the other channel increases the volume, (and the noise). I cleaned the pots with switch cleaner, blew off the dust with compressed air and sprayed the pcb’s with laquer. The amp sounded sweet with no noise for about ten minutes then the buzz came back.
Could this be something simple or should I take it to a shop? (expensive here in the UK)
I don’t know, could be the power supply if it’s common to both channels.
Probably shop time.
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Hey man I have a Fender Hot Rod Deluxe. Today it randomly started to have an issue after a drive, it was facedown in the back. The Master Volume knob has no control over the clean channel anymore, the clean channel has a low volume, probably a standared of 3 on the master volume if it would control it but it doesn’t. However the drive channel does work with the Master Control, so I’m really confused. I don’t know what to do and its so frusterating!!!
With the problems you’re describing, not sure how laying it in the back of the car would cause anything, something was probably ready to go anyway. Make sure all your tubes are in good. Really not much you can do except take it to a shop.
wooow this is very interesting and very helpful thank you….
What a great site!
Hello Gary. Played a gig Sat’ using my dear old Yamaha G100 (a very old transistor amp). It’s been a real workhorse with a lovely clean/bright tone. Half way thru’ the set it kept jumping Chanel’s. During the week prior to the gig I left it in the boot of my car and it’s been really damp and cold here in England. I don’t mind taking it apart, I just need to know where to look. Really appreciate any help/ advice, I wouldn’t want to part with it just yet.
Just saw this. I can’t remember on those amps if the relay between channels is a transistor switcher or an actual relay. If it’s a transistor (FET) you would need to find that particular one which involves troubleshooting and reading a control voltage, etc. If it’s a relay, maybe you could pop the top and clean it with a good silicone spray…
there is some solid info for us all here! awesome site!
i have question regarding my bugera 6262 amp, i have the same problem as david
where the amp jumps from lead channel to clean channel at random . it seems to start when
the amp gets hot ,as it is a 120w valve head .ive tried putting a heat sinks on the 7815 voltage
regulators(there are 2 of them) as some people say they switch when they get hot , that didnt work .
it does it with and without the amp control pedal plugged in
my question is basicly ,do you thonk the 7815 voltage regulators could be the cause of this problem , i want to attempt to fix the amp myself as we do not have a reliable repairmen in my small town and the amp is out of warranty
any help is greatly appreciated
Don’t know why I missed this. Maybe it’s fixed by now. If you’ve found online that people say this corrected the problem but not for you, double check the caps or whatever is smoothing the dc of the regulators but really if you have to get this precise, I would suspect the switching circuits themselves. For example (I don’t know this amp),,if they’re using FET’s to switch, I would look at those first. Also the associated circuitry since it’s a heat thing. If there’s a switching jack input from the floor make sure the amp side of the connection, is clean and making good ground. (if that is what it’s meant to do)
thanks for these useful tips. I datenifly will be using this information. I see they work wonders. its good to have a man with such experience on this site. I can see your influence and knowledge bettering the quality of music for this site. Keep them comming. Your music if exiteing.
fender blues deluxe only one channel working switch it go to full volume for 10 sec then click and goes back to clean channel
Sorry for the late delay, you may have it fixed already. If it’s doing it without the foot switch being plugged in, I would suspect the switching transistor. Maybe a cap or diode in the same circuit but probably the transistor.
I recently developed a problem with my Fender Evil Twin (red knob) that sounds like it could be the FET as mentioned above. I was plugged in parallel mode (both channels, using a piezo in one input and a bridge pickup in the other). The mistake was made of unplugging one of the channels while turned on in parallel mode. Instantly, my ability to switch from one channel to the other was lost and now I don’t get reverb. In looking at the Evil Twin schematic, I don’t see anything that resembles the FET above. How would I locate the FETs on either the schematic or on the amp? I’ll just replace them all (or whatever makes sense). Thanks so much in advance.
Hi, I happen to see your message. Without an exact schematic it’s hard for me to say what it is. If I remember correctly the red knob series may have switching coils (relays) or some light diode arrangement. Also you shouldn’t lose your reverb on both channels I would imagine. You may have something else going on. Any way long story short, you probably won’t see the FET arrangement in that amp.
Hi I also have a champion 110 with a bad j111 I replaced it and it worked fine for a day then went out again now my clean channel doesn’t work again do you think I should replace both or could it be something else?
Hi, might be a good idea to replace it, it’s usually the weakest point in a chain..good thing you know how to replace them, they are pretty inexpensive. If it goes out again you may want to check components around it but in thinking about it, they are times when I needed to replace more than one.
I’ve got a Mesa Boogie. Early model. I think a Mk II b . No reverb or equaliser. Bought it new nearly 30 years ago but the foot switch has always been a problem . It has been dead for most of the time I’ve had the amp. There is no voltage at all going to the relay. Should be 5 – 8 V according to the schematic I have. This amp has no transistors at all. I have suspected the relationship might be at fault but I really don’t know how to tell for sure . Do you think I could be right ?
My auto correct turned relay into relationship . Sorry about that.
Hi…when you say no reverb or equalizer, are they both controlled by the foot switch? I can’t remember. If it is, I would try all the easy stuff first, if the pedal or connecting circuitry has a mechanical issue (or ground issue) see if you can figure that out (best thing to do is switch out with known good one) If you’ve done all that and you’re sure the voltage is missing where it should be approach that the same way, check the easy stuff first…continuity in the circuitry…ground issue someplace…seems like those units had a optic issue but I can’t remember if it was an issue or they were just a bear to get to. Sorry it’s been awhile and I’m probably not that much help.
Hi Gary ,
Thanks … they certainly are a bear to get to alright. The preamp board is held on by the 3 12AX7 s. Soldered in to place . So I’m not going to undo that. The foot switch is fine. Don’t quite know what you mean by optic issue. I believe the foot switch and equaliser are controlled by the foot switch but , as I mentioned my amp does not have these . So really I only have the first and second 12AX7 working . The overdrive , controlled by the 3rd 12AX7 is not working at all. It will not switch in. Mesa Boogie are pretty reluctant to help out. Otherwise the amp sounds pretty good. Thanks for your comments. I guess I’ll sort it out one day.
What I meant by optic, if I remember correctly on a lot of those amps had opto isolaters but upon looking at a schematic I see this one doesn’t.
Check the relay’s, what’s feeding them from the supply, it’s kind of hard to tell from what I’m looking at but one of the neg voltages might be feeding the relays. The other is bias.
V3, make sure the heater is working, plate voltage and 2v at the base…if you have that I would go back to the relays and see why or if they are switching.
I wrote the voltages down on the three 12AX7 s. V1 & V2 were within the specified limits …. but V3 has 112Volts
on the grid . That is definitely wrong. The heater voltages are about 3V . That is wrong too . Should be 6.3V.
I am still trying to work out where the relay gets it’s supply from. There are definitely some differences from the schematic to how the amplifier is built. I am assuming the pre amp board is double sided . This complicates things .
Writing down all the voltages was a good idea. I didn’t check the phase splitter yet.
I am going to keep searching. I wonder if replacing the valves would help. There could be a short ? Though I did swap the valve around and that did not help.
My Dad has a Fender Band Master re-issue, the clean channel works perfectly but when you switch to the drive channel there’s almost nothing there. If you turn up the volume you can here it but it’s still very quiet. What could cause this?
Hi…Sounds like a switching transistor, which is what the above article is about. Make sure it’s not the foot switch, if it’s doing it without the foot switch from the front panel. If your repair chops are not up to speed I would strongly recommend taking it to a music repair shop with knowledgeable repair people and they can fix it. With that said, I don’t have a band master reissue schematic to look at, if there’s a preamp tube in that section of overdrive it may be bad and possibly be the issue but since it’s a reissue fender, from my experience it’s the switching transistor.
Hello Gary, thanks for the great resource! My Tube amp, a Yamaha T100 does some of these above things. At first power up it was working, then, the clean channel went out (faint fizzy signal heard only). However the green channel indicator still goes on and the dirty channel is apparently unaffected (red light goes on, comes through loud and clear). Same scenario whether or not it is switched via footswitch or push-pull pot in the dirty channel for switching channels on the front. About to open it up to try to locate the faulty component, any advice appreciated!
Hi..make sure the female part of the footswitch in the back is clean…since it’s a gradual decline may not be but always do the simple stuff first. I don’t know if these units use transistors or relays for switching but monitor the supply voltage to whatever is switching, maybe you’re losing it but if a FET is doing the switching, that’s usually the weak component in the chain…quicker to just replace it than try to troubleshoot it. Just eyeball everything, you may be able to see the issue. Also make sure you’re up on your desoldering licks before messing with a nice PC board…Practice on blown boards and make sure you have clean desoldering equipment. You never want to screw up a nice amp. Learn on the dogs. You may know all this stuff but I don’t know where your chops are at present. Good Luck!
Hi, awesome post, i kind like electronic, anyway…
I have a problem with a 5150 channel switching, it has a delay between the change like a old pod line 6’s , just a little gap between the change, do you have seen it?? Can you give me a feedback about it????
Hi. I haven’t seen one in years so can’t help you there. Try giving PV a call, customer service. They used to be helpful with issues like this. Just a quick search turned up this thread https://www.harmonycentral.com/forums/topic/1378805-5150-iii-poping-noise-when-switching-channel/ Post on there, seems to be several 5150 owners discussing technical issues. Good Luck!
I have a old Crate GX-60. Although I can plug the guitar in and get a clean sound, none of the knobs or dirty channel responds. Nothing.
Any help would be greatly appreciated.
Hi! I have a Fender FM 212r and it is stuck in the More Drive Channel, but when I plug the footswitch it work OK, and I can change to the other channels… what could it be?
Hi If you can change channels with the footswitch, I would say something is wrong with the panel switch or associated circuit..It can be cleaned for starters.
Hi I don’t know those amps, if it has a footswitch see if that will work. If not you may have a switching transistor out which would be a repair job by an experienced tech. Sometimes just plugging in the footswitch a couple of times or working the panel switch quickly will temporarily clean it.