Troubleshooting your Amp

Not every modification goes according to plan. Sometimes you break something, sometimes your soldering skills are not what you thought they were.

While you can always send your amp to me for repair or modification, you may be able to fix it yourself. Circuit board repairs are fairly easy if you have a steady hand, and we have a page that describes how to do them in detail:

The larger problem may be finding the break. It’s often hidden under a solder joint that looks perfect. If you put a signal into the input jack — any kind of audio tone — you can trace it through the amp and see where it stops. The usual tool for this job is an oscilloscope, but that’s an expensive proposition for, perhaps, a one-time use.

The video below will show you how to make this audio probe:

Click for larger image.

Update: I slipped the probe into a pen barrel, per a user’s suggestion:

Click for larger image.

It’s a simple device that you plug into another amplifier, so you can hear the test tone. In many cases it can be faster than an oscilloscope in finding the problem.

It’s simply a capacitor soldered to a piece of coaxial audio cable, with the shield lead brought out to a ground clip. Shrink tubing makes it rigid enough to use as a probe and a finishing nail on the end helps it stand up to wear and tear. The .01uF 400 volt capacitor I used is a Mallory 150, which was about the smallest diameter I could find. It fits nicely under the shrink tubing. If you get a male-to-male mono 1/8 inch cable, you can cut it in half and make a spare probe.

Note: There’s some indication (feedback from users) that even a small cap like the .01uF cap may send too large a signal to the Radio Shack mini-amp and blow it out. You should probably use a smaller cap, such as a .002uF.

The little Radio Shack utility amplifier has been in stores for as long as I can remember. Here’s the catalog link:

I like using something that’s both inexpensive and battery-powered so I don’t have to worry about ground loops, more wires, or potential hum problems from an AC-powered amp. But you can use another guitar amp for this if you want. If you get loud hum from the guitar amp, it’s probably because of a ground loop. Since you have a common ground through the electrical grounds on both amps, you may find that it will be quiet when you remove the ground clip.

You could also use the audio input of a notebook computer, but I hesitate to recommend it because, with the very wide range of loud and soft signals coming from the amp under test, there’s the outside chance that you could blow up something expensive.

This next video shows the audio probe in action:

When you set up your test tone, use your multimeter’s lowest AC voltage range and set the voltage from your signal source (probably a headphone jack) to around 10 millivolts (.010 volts). Even good multimeters are very sluggish when measuring small AC voltages, so be patient and give the meter time to detect and respond. If you listen to the tone with headphones, it should be “there” but not very loud.

After you’ve verified the proper working of the output section with the reverb wire test shown in the video, turn down the master volume. Keep a speaker plugged into the amp the entire time that it’s on, however.

Now is the point at which you say, “But I can’t read schematics!” Sure you can. It’s just like reading a roadmap. If you don’t have one, here’s the address to the Blues Junior schematic at Fender’s site:

The important part is knowing which parts are important and which to ignore in your trace. I’ll use cream board part numbers to describe the flow, with the green board part numbers in green:

R2 (R1): Grid resistor from input jack. Signal should be present at the top of R2; it comes directly from the input jack.

R1 (R2): Sets the input impedance. Not important.

R3 (R3): Plate resistor for first preamp stage. Will have around 200 volts DC on both sides, so be careful. The test tone should be loud on the lower (tube) side of the resistor.

C2 (C1): First-stage coupling cap. The tone and DC voltage should be on the lower side, just the tone on the upper side. From here the tone goes to the volume control. We’ll assume that the control is OK.

R7 is the grid resistor for the second stage. It gets the signal from the volume control. There is no equivalent to R7 on the green board Blues Junior. The signal on both sides of R7 should get louder or softer with the volume control. Test the green board at R5.

R8 (R10): Plate resistor for the second preamp stage. High DC voltage, as above, and strong signal that can be varied with the volume control. From here the signal goes into the tone stack. All tone from the stack exits from the center terminal of the treble control and flows to

R15 (R30), the grid resistor for V2. V2 re-amplifies the signal, which gets diminished in the tone stack. You should hear the tone on both sides of R15 and you should be able to vary it somewhat with the tone controls.

R16 (R39): Plate resistor for V2. High voltages again, but the signal should be present on the tube (lower) side of R16.

C8 (C16) is the coupling cap from the second stage. It strips off the high DC voltage and passes along the signal to R18.

R18 (R40) separates the signal that goes to and comes from the reverb and feeds the combined wet (with reverb) and dry (no reverb) signal to the master volume. If you don’t hear the signal at R18, you have a bad connection at C8.

There are other places where things can go wrong, such as the broken ribbon cable in the video, but following the signal through the first three tube stages, as we just did, should solve the great majority of problems that people encounter.


  1. Ash00 says:

    Hey Bill,
    Thanks again, Love your work!
    I’m very happy with my Blues junior Tolex .( basic mods , tone stack , presence control, recap, cathode follower , T020 OT, with Jansen C12N speaker).
    I followed the steps with the probe and found the problem atR18. After investigating further I found what Id done wrong DUH! .
    Mustve been in a hurry when I did the basic mod and left the iron on too long( no thermistat ) , ruined the soldering pad and then made a jumper wire repair, took it to R 18 and also down to the trace below.
    So I checked the continuity 100 times, assuming everything was in the right place, Did my head in good and proper.

    All the best

  2. Jasonblanchard says:

    My jr. is in need of some smelling salt right now, but I do have a backup amp until full recovery takes place. I’ve constructed an audio probe to diagnose the amp, and thanks I’m like a kid in a candy store! I love this stuff. R3 is questionable, it has 174v on one side and 264.8v on the other. Is this ok? C2-R18 all checked out perfectly though. I still can’t get audio through the speaker. A while back my fat boost went out, but I could live without that, but it’s now the silence that I can’t deal with. I’m sure you can shed some light on my subject.


    • bill says:

      That R3 voltage drop is normal. Work through the schematic. There has to be a spot where the signal goes away. You can also work backwards through the schematic, from the phase inverter resistors, to see where it first appears.

      • Jasonblanchard says:

        I finally went through the amp again retracing all of my steps, and repaired a lot of lifted and broken traces. I’m very glad to announce that my amp is once again up and running, but as before my fat switch has no affect on the amp. Can you isolate this circuit for me so that I may narrow down the chase? Your assist is greatly appreciated.

        • bill says:

          You probably need to replace the J111 transistor. It acts as a switch and cuts the 100K resistor out of the circuit, which allows the 22uF cap to act on the cathode of the 12AX7 stage, boosting the gain and widening the frequency response.

  3. Denny says:

    Hey Bill –
    Great stuff Thank You.
    Sorry if I missed it, but I still have a few questions:

    1. Do you need any kind of Speaker/Dummy load while using the audio probe.? Or can you just unplug the speaker.?
    2. I guess you just need to turn the Volume/Tone knobs enough to let some signal through for the test.?
    Thanks Again

    • bill says:

      If the amp is powered up, it absolutely needs a speaker or an 8 ohm dummy load.

      It’s better to work with a speaker because you can hear stuff that’s going on, which may help you troubleshoot. In the video, you can hear how pressing the broken ribbon cable connection back together restores the amp signal.

  4. highfigh says:

    In the event that the signal is too hot for the amplified speaker’s input, why not include a resistor on the ground of the cable to the amplified speaker or run the cable through a small project box with a voltage divider and maybe a small isolation transformer? A stereo isolator can be bought from Rat Shack for less than $20 (they don’t usually know what they’re for, so they don’t bother to keep them in stock) or from a car stereo installation shop. This can be opened and cannibalized, either for making two of these, keep one transformer as a spare or as an in/out box for other uses. I don’t think there’s any need to use a Jensen isolator, but….

    Jim (’58Bassman)

  5. pwittbold says:

    Hey Bill,

    I have a blues junior (made in 2003) Which worked fine about until 2 weeks ago. I had bought new tubes JJ EL84 and JJ preamp tubes. I cleaned the sockets with contact cleaner, and put the tubes in correctly. I also swapped out the speaker to an eminence swamp thing. The amp sounded great for a few days, and then all of a sudden the fuse blew. I replaced the fuse, and 2 minutes later the fuse blew again. I put in a new fuse, but I noticed there was a disconnected red wire from the power light, so I reattached it with solder and the light worked, but the amp still blew its fuse 2 minutes after turning it on. I have been reading around and have found that it could either be , a bad tube, bad socket connections, or possibly bad filter caps?? I read the 2003 models had bad caps. I was wondering if you could help guide my troubleshooting.



  6. pwittbold says:


    It was definitely a tube problem. It ran fine without the EL 84s in place. Now is there anything I should do aside from just swapping the tubes out? I.e. checking the tube sockets or soldering connections?

    • bill says:

      Make sure that the new tubes are tight in their sockets. See the socket retensioning page on my site. If new tubes blow the fuse, you’ll have to dig deeper. It could be a shorted output transformer.

  7. pwittbold says:

    I will be sure to re tension the sockets prior to putting in tubes.

    How can I determine which of the two tubes is the faulty tube, causing the short circuit? Also, how can I determine if it might be the socket causing the problem, and not the tube itself?



    • bill says:

      Without a tube tester, finding out which one is the bad tube will cost you another fuse. Why bother? Your output tubes should be matched so toss ’em both. If the socket is bad, it probably would have caused a short with or without a tube inserted.

      And if the amp fails with new tubes, take it to a tech.

  8. averygtars says:

    Bill : thanks so much for this tutorial . Just what I needed. My B.J. is back running great .The probe did the trick.
    Can’t say thanks enough !! Bruce

    • averygtars says:

      Just used the probe again to fix one of my buddies B.J.After doing many of your mods . I’m hooked. I just sent for the presence and sparkle controls. Your site is the best , Thanks again – Bruce

  9. Paul.w says:

    Hey Bill, I know this is a Very broad question but my blues jr has developed a distorted like fart at the end of a note that seams to be more active with bass .. Ive change the valves, used external speaker and in both cases its still the same. Any pointers would be welcomed, Thanks Paul.

  10. tobie.munroe says:

    Great site bill! I recently made some mods to my blues jr. I purchased your twin stack mod and used all the containing pieces then made a custom 2×12 combo cab and wired it to two 12″ v30s.
    It’s sounds amazing at low room volume but when I turn it up it crackles and cuts volume intermittently. Any thoughts? I opened it up and light wiggled caps and drop pins but nothing stuck out as an issue. Also recently retubed it and still the same issue. Any thoughts would be greatly appreciated! Thanks in advance!

    Ps could when I remove one speaker it still has this issue.


    • bill says:

      Crackling is some kind of bad connection. It could be a bad tube, socket springs, or socket solder joints. Poke around the board and all the connections with a wooden chopstick to see if you can locate the problem.

  11. sscottmca says:

    Bill, I just bought a 91 green board and ordered the basic mod and a new jack today. Was playing the amp for the first time and really loving the premod tone and thinking its only going to get better. Then IT STOPPED WORKING! No sound at all. I swapped out all 5 tubes from an amp that works and not a sound. Tubes all glowing. Speaker works with a different amp head. Wonder what to do. You’ll see my order from earlier today. Don’t want mod a dead amp. What can I do to troubleshoot this?

    • bill says:

      You’re on the troubleshooting page; it’s all here.

      Start with the reverb buzz test. Then you’ll know whether it’s a preamp problem or power problem. Then you can do some signal tracing. It’s likely to be a simple problem.

    • sscottmca says:

      Bill, fixed all the problems that came up with much help from your site. Finished the basic green board mod yesterday and it sounds great. I had an intermittent problem where the speaker would quit. Ended up the 1/4″ speaker jack was loose on the PCB. Fixed that, listened for buzzing tubes and replaced those and reinforced the power tube holder with adhesive backed felt, now the BJ is solid and I can’t wait to the next rehearsal. Your site made it possible. Thanks Bill!

  12. sscottmca says:

    Tube feedback vibration: I’m having a problem with the new lower octave unlocked after the mod. The under medium load with bass at 50% the tubes rattle and create a feedback loop. If I touch the power tubes, the rattle stops. What’s the best method to support the EL84’s to keep them from getting punished from the speakers newly found bass?

  13. jferns says:

    Thanks Bill, just one clarification – what type of capacitor should I use,ceramic, mylar….?

  14. jferns says:

    Thank you Bill for the info and taking the time to reply. Probably overlooked the details in excitement as I have been wanting to make one for a long time. Best regards.

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