Tracking Down Hum in a Blues Junior

Blues Juniors are not inherently hum-prone, but tube amps in general are more likely to hum than solid-state amps. Getting hum down to the bare minimum can be a challenge. Here are some steps to take to find the cause and reduce or eliminate it.

The likeliest cause of hum is 60Hz hum that’s induced from the heater (filaments) to the cathodes in the preamplifier. Does the hum go away when you press the Fat switch? That’s a sure sign that the tube is worn out. Replace it.

You can also move your 12AX7s around; put the quietest one in V1.

Note: If you have no idea what you’re doing and you can’t tell one tube from another, the short ones are 12AX7s, commonly called preamp tubes, although only one of them (V1) is actually a preamp. The two tall ones are the output tubes, EL84s. If you put an EL84 into a 12AX7 socket, there’s a very good chance that you will fry the circuit board. You’ll know because of the bright blue flash and The Bad Smell, followed by a blown fuse and other collateral damage to the amp.

V2 and V3 are not preamplifiers and they are less likely to induce hum into the amp. So you put a hummy tube from V1 into V2 or V3 and it may work OK.

You can perform the following process of elimination:

1. Remove V1. Turn the amp on. Did the hum go away? If yes, see above. If not,
2. Remove V2. Turn the amp on. Did the hum go away? If yes, replace V2.

The amp will almost surely be silent if you remove V3. It’s seldom the source of hum, but try a fresh tube in there anyway to see if it helps. You should always have at least one known-good 12AX7 and a matched pair of EL84s in your kit.

Your output tubes are a matched pair for an important reason. They’re arranged in a “push-pull” configuration, which allows each tube to rest a bit while the other works. The phase inverter (V3) makes mirror images of the signal, the music you’re playing, and sends it to each tube. The push-pull arrangement causes any noise or hum that appears in both tubes at the same time (not mirror images) to be canceled out–a pretty neat trick. So cathode hum in the output tubes is generally canceled pretty effectively. The more closely balanced the tubes are, the better the hum canceling works. Tubes wear differently as they age, so old tubes may not be as effective in canceling hum.

Beyond Tubes
Hum can come from many other places. Bad filter capacitors (those big, gray things) are a source of hum. If your amp is going on 20 years old, you should probably replace them. And if they are leaking, like the ones shown on this page, http://billmaudio.com/wp/?page_id=267, replace them. If one is bad, replace them all; they’re from the same batch.

Hum can also be induced. Your power transformer converts the AC line or mains voltage into the higher and lower voltages needed by the amplifier. The line voltage constantly magnetizes and demagnetizes the steel core of the transformer. A significant amount of magnetism leaks out, which is why you hear a loud, low hum when you bring your guitar close to the amp. The pickups respond to the 60Hz field around the power transformer.

The output transformer works on the same principle–a high voltage from the output tubes is converted into a low voltage suitable for the speaker. The output transformer sits inside the large magnetic field around the power transformer, but it’s oriented in a different direction and shielded with a core band (around the laminated steel plates) to prevent the 60Hz AC from “infecting” the output transformer and coming out the speaker.

Sometimes, however, the PT’s field overwhelms the OT’s defenses and some hum gets in. Amazingly enough, the push-pull configuration of the output tubes helps to fight some of this hum. Try this: Turn on a cold amp with your ear right up against the speaker. You will hear the 60Hz hum of the power transformer through the output transformer and into the speaker. As the amp warms up, however, the low hum should be devoured by the push-pull of the output tubes and will be replaced by a faint buzz that’s actually twice the pitch, 120Hz. You’re hearing the “sound” of the high voltage supply, which has a volt or two of 120Hz buzz on top of the 320 volts being fed to the output tubes. The tubes cancel most of it, and the Billm power supply stiffening mod reduces it further.

In my experiments, I’ve found that the amount of hum that gets coupled from the power transformer to the output transformer can be greatly reduced with a simple sheet metal shield:

The shield sets up a counter-polarized magnetic field that actually cancels the emanations from the power transformer. You adjust the angle for minimum hum. The shield is about 6 inches by 2 inches. The metal does not touch either transformer. Using a wider piece of metal does not work any better. The vertical portion should be as high as the output transformer; higher doesn’t help. Drill a hole or cut a notch for the screw. This works for either the stock transformer or upgrade transformers.

That big magnetic field from the power transformer can actually induce a 60Hz vibration in the chassis itself. If you can feel the chassis or the whole cabinet vibrating when the amp is on, sorry! You got a noisy one. You might be able to remove it and tighten the bolts, but it’ll probably be just as noisy when you reinstall it. Mojo Musical Supply makes a Blues Junior replacement transformer that’s slightly higher quality than the stock one. You could replace your noisy, vibration-prone PT with a better one, but that’s a lot of money for a rather minor hum. Play louder!

Bad grounds in the amp, ungrounded or miswired electrical outlets, and other problems can also cause hum. You’ll probably need an experienced technician to track down grounding problems in the amp. Get an outlet tester from the hardware store or Radio Shack to tell you whether your outlets are properly wired and grounded.

Offsetting the filament voltage with a positive DC voltage often helps, too. It prevents the negative swings from the heaters from impressing hum on the cathodes. It’s easier to steal a DC voltage or build a small DC supply than to rectify and filter the heaters. In my experiments, I was surprised to find that bridge rectifiers that can handle the current run pretty hot and place an additional load on the power transformer. I’ll add some details about this when I get a chance.

30 Comments

  1. Wil says:

    Hello Bill,
    I made the basic mods and I’m waiting for the TO20 to arrive but I have a problem with my blues junior !
    When I play some bass like a A on the 6th string, I have something buzzing in the amp. It sounds like a spring that vibrates. It’s pretty annoying because it can clearly be heard, almost as loud as the note itself ! I’m sure it’s not some fret buzzing.
    Maybe a bad tube then ? But I don’t want to change all the tubes in the amp without being sure the problem comes from them.
    Once again, thanks bill for all your work on the Blues Jr. !

    • bill says:

      It’s probably phase inverter oscillation. Your TO20 should come with a 100pF cap and instructions for installation.

  2. dyork423 says:

    Hi Bill,

    A couple months ago I ordered a few mod kits from you and have been very happy with the results except for a high frequency hum that is always present, even with no cable plugged into the amp. The only thing that affects the hum is the new presence control (hum gets louder as I turn up the presence). I’ve gone through all your suggestions for tracking down hum and nothing is working for me, although I haven’t tried installing the sheet metal shield. Do you have any suggestions? The mods I did were basic kit for green board, presence control, twin stack, and audio taper for master volume. Thanks so much.

  3. djheyhey24 says:

    I just recently replaced my tubes in my Blues jr, I used Gold Series GT’s, now when I turn the volume knob up there is a loud shhhhhhh noise it gets louder when you turn the volume up and softer when you turn it down, it is the only knob with this problem, you can crank the master and it is normal.

    It also has a nice AC hum all the time.

    Any ideas?

    • bill says:

      Does it hiss when you put the old tubes back in, particularly V1, the rightmost as you look from the back?

  4. djheyhey24 says:

    No it does the same thing now with the old tubes, I am thinking it is another problem, I have no mods, I had it converted to a head unit for a week or so and I put it back to a combo because I sold my 212 cab.

    • bill says:

      Does the hum go away if you pull V1? Does it go away if you engage the Fat switch? If so, V1 is probably defective, or there’s a grounding problem in the preamp circuitry.

  5. MaxR says:

    My Blues Jr III hums at ~250hz. The filter caps are fine. I’ve replaced the tubes, also. What would cause this hum?

    • bill says:

      250Hz is what, middle C? That’s not a hum! Something has to be oscillating. Does it go away when you pull V1? How about V2? How do you know the filter caps are good.

      If something is really messed up with the power supply grounds or if there’s high AC resistance (ESR) in the filter caps, it’s possible that you’d see a doubling of the 120Hz sawtooth on the plates. I’ve never seen that, but you never know.

  6. vicpassanisi says:

    when I remove v2 the hum goes away, so i replaced v2 with a new tube but the hum is ever present. what does this mean?

  7. rcooper says:

    I recently modded my Blues Jr with the basic mods, presence control, bias control. It made a huge difference and sounded great, almost no hum. But after around 20 hours of play time it developed a intermittent popping sound. The popping rate became faster and then stopped. The volume was reduced to a fraction of what it was before and made a quiet rumbling noise with the volumes cranked. I never smelled any smoke or saw any arcing. When i opened the amp up and the checked the bias, it was +12.6v! When I originally biased it, it was -2.6v. When checking the tubes, I removed V1 and my amp didn’t like that and blew the 2A fuse (and maybe the power tubes). What’s happening to amp?

    • bill says:

      It sounds like an output tube shorted internally.

      • bobbym says:

        I’m having a similar problem. Just did your basic mods and put in a fresh pair of new matched JJ EL84s. I also put in a new EHX 12AY7 in v1. Sounded great for the first day I played it after the mod but the 2nd I tried to play, no power light or sounding is coming through anymore. Took off the rear panel and sure enough the 2A 250V fuse blew out. I just put in a new fuse and as I turned the amp on again, the red light turned on for a second and then I saw a bright blue flash of light in the back of the amp and the fuse has blown out again. The EL84s were brand new after the mod and the output tube sockets are fine and tight. How can I fix?

        • bill says:

          Have you tried your old tubes? It’s possible that one of the new ones is defective and shorted internally. The problem is that it may have caused an arc on the back of the board. Once there’s an arced or burned place, the carbon becomes conductive and it keeps arcing.

          See here for repair techniques: http://billmaudio.com/wp/?page_id=204

  8. surfstrat says:

    Hi Bill,
    I run two blujus,one stock black and one tweed ltd, with a Palmer Y splitter between them. My problem is with the black one. Owned this from new, about three years old and has a loud humming, as soon as it warms up. I’ve tried your advice, to no avail. Strange thing is, when I turn the master vol fully off (I don’t push the gain above 5) you can still hear the guitar coming thru, although only quitely. The tweed one is completly silent with the master set to zero so this can’t be right, can it?
    Any advice would be greatly appreciated. Your site is great for lovers of this great little amp.
    Thanks

    • bill says:

      Have you replaced the tubes? Or at least cycled a known-good 12AX7 through the three sockets?

      The master volume “leak” may be coming through the reverb circuit.

      Since it’s only three years old and you bought it new, exercise your warranty!

  9. keef says:

    Hi Bill,

    My 2008 tweed BJr has had an annoying hum almost since the day I bought it. I would have returned it except I read somewhere that they all hum so I learned to live with it. It seems to be worse when using a guitar equipped with non wax-potted humbuckers but it is still there when using a lace sensor (Fender’s noiseless pickups) equiped guitar. I have tried turning lights/tv/appliances, etc. on/off but this does nothing. I put the amp on a dedicated circuit so I’m sure there is no A/C noise in the line plus I have it plugged into a Furman power conditioner. Turning on/off the fat switch does nothing – still present. I will try swapping tubes around next. The other weird thing is that I can hear a rattle (vibration noise) when I hit a “C” note on the G string. The same rattle sounds with any C note up and down the neck. Could it be that the frequency of the “C” notes are very close to the natural frequency of the tube supports or the chassis? It sounds like a mechanical noise. Is there anything I can do to dampen this vibration (if that is what it is)? I might add that this amp likely has less than 1000 hrs on it since new.

    Thanks for sharing your knowledge and time with everyone…

    • bill says:

      Feel around the tubes with a glove on while someone plays the offending notes. That way you can see if it’s a microphonic tube.

  10. keef says:

    Thanks Bill,

    I don’t have any helpers available right now so I simply tapped on the tubes with a wood chop stick while the amp was on and tubes were hot. It was immediately obvious that the first preamp tube (tube on far right looking at the back of the amp) made a spring like sound when tapped but no noise or stray tones came through the speaker. The others did not make any vibration sounds or noise when tapped. Even with the amp off the tube makes this sound. I think you nailed that part of my problem but I’ll swap the preamp tubes around and see what happens; thanks.

  11. keef says:

    I just took out V1 and turned the amp on. The noise (hum) was also less with this bad tube out of the picture. Also with my ear to the speaker I can hear some faint popping sounds (like pop corn). I will buy a new set of preamp tubes and hopefully this will fix her. Thanks again Bill

  12. keef says:

    Update,

    After some more troubleshooting it appears that the source of the rattle/vibration is the V1 tube socket (if this makes sense). No matter which tube is installed in the V1 socket, when you tapp on it you hear a rattle which sounds like a loose spring vibrating. This happens with the amp off. This sound is not present when tapping on the other tubes in V2 and V3. The tubes all fit snug in the sockets so I have no idea what could cause this. Bill, I would love to send this amp to you cuz I think it is demon possessed! 🙂

    • bill says:

      V1 will always have some audible vibration because it has the most gain — and two stages of it. Make sure that the four screws that hold the tube board to the chassis are tight. Always put your quietest tube (vibration or hum) in V1.

  13. whiteyguitar says:

    Hey Bill,
    Just got and installed the basic mod pack for my Blues Jr III and It sounded great for 10 Minutes, Now there is excessive Hum crackling Popping and it has a real High Pitched Feed Back when turned up to 4 Dirty or Clean with the Guitar Off or even unplugged, everything went off without a hitch, been soldering on PC boards for a long time so I’m sure I didn’t install the Mods wrong, not really sure what happened the amp is brand new 3 weeks old and I was digging it before the Mods just wanted more control of the EQ and to make the power tubes last longer and to get more Headroom.
    I’ve been playing thru the amp since I got it and none of these symptoms ever appeared,and Played on it all day yesterday and nothing wrong,and even played on it for an Hour before the Mods so I would have some reference to difference in tone.
    any help would be appreciated, Thanks

    • whiteyguitar says:

      well solved the Feed Back, Microphonic 12AX7 in V1, still getting some strange pops and crackles in the ac Hum not constant but every so often but other wise sounds great.

  14. Simon says:

    Hi Bill, thanks for you site. You helped me decide on the BJ.I got one for Christmas and I just love the sound.However, whenever I plug in it buzzes, this deadens if I touch the strings or chassis.only buzzes with a lead in the socket. I have added a 47 cap to filter. What should Ido now?

  15. sudhirprit says:

    Hi Bill sir
    I want to measure the humming produced in contactor used in starter which operated on 415 v ac. Is this possible using sensor ?

    • bill says:

      No, I do not think the probe will be useful for finding the hum in a contactor. A large relay usually hums and buzzes because of loose plates or windings in the electromagnet.