Pro Junior

I know that Pro Juniors have their fans, but I’m really not one of them. To me, it has all the distortion of an old tweed amp and none of the charm. The circuit design is kind of a mash-up of traditional Fender tweed preamp and modern output stage. Pro Juniors are very noise-prone and tend to be fussy about tubes. Hiss and hum are part of the territory, although some lucky folks have quiet, well-behaved PJs.

I have some mods on my old pages and I have some others forthcoming that help with the noise and such. Sorry to be a party pooper; it’s just not my ideal amp.


  1. forp says:

    hi bill,

    i bought a Pro junior 3 from a guy who won it new.He is a saleman so i cant use the warranty from that amp.( i know where he works and i asked the cashier , he really won it new)

    The pro junior was at fist too noisy in a quiet house ( but ok if people were talting loud , a movie playing, etc.)

    I then switch v1 for a tung-sol 12ax7 and v2 for an electro harmonix 12ay7.

    That dit help alot, but was still huming .

    i dit the wire twisting thing but got no more results.

    i bought an attenuator L-Pad mono 8 oms 100watts.

    with the setting at 75% attenuating a small hum remain but when my refregirator compressor is working( its a brand new fridge not noisy at all), per exemple, you may not notice the hum.

    The problem i still get, is, when recording a single guitar track without bass or drum there is still hum.

    I mostly use this amp for the clean sound, witch i love.

    in your old article abouth quieting a pro junior you added a second filter cap in parallel with the stock one(in fact you change the original and add another of the same value).

    I live in montreal and i cant find 47 uf 450v electrolitic cap.

    I have 6 questions

    If a put a 100 uf 450v cap( insead of another 47 uf) what will happen ?

    then would it be better to use two 22uf 450v cap ( in parallel with the stock 47uf ) ?

    would this single mod ( without doing the other you talk abouth in your article) make any difference in the hum that i will ear ?

    would changing the stocks sovtek eL84 for JJ power tubes help the noise issue ?

    i have reed on a forum thant the hum comes from r35 and r36 going bad, could it be true, knowing that some , new stock pro juniors 3 dont hum (i did try 2 in 2 stores that were way quieter than mine) ?

    • forp says:

      i forgot to ask ,

      after doing all the mods from your article abouth quieting a pro junior

      how much hum remained ?

      was it as quiet as if it was off ?

      could you still ear only the white noise ( like with modeling transistor amps) ?

      • bill says:

        As quiet as when it was off? Dreamer. 🙂

        It reduced the noise from obnoxious to tolerable.

    • bill says:

      If a put a 100 uf 450v cap( insead of another 47 uf) what will happen ?

      It’s a good solution.

      then would it be better to use two 22uf 450v cap ( in parallel with the stock 47uf ) ?

      In terms of performance, it’s the same.

      would this single mod ( without doing the other you talk abouth in your article) make any difference in the hum that i will ear ?

      A very slight difference.

      would changing the stocks sovtek eL84 for JJ power tubes help the noise issue ?

      Probably not.

      i have reed on a forum thant the hum comes from r35 and r36 going bad, could it be true, knowing that some , new stock pro juniors 3 dont hum (i did try 2 in 2 stores that were way quieter than mine) ?

      This is unlikely.

      • forp says:

        Thank you Bill, your the best
        ( i will perform this mod first and i plan to do the other mods from your article later)

  2. forp says:

    Hi bill,
    i was wondering if anybody ever did a test comparing the values a new pro junior that hum and one who don’t hum ( or as low as a normal blues junior).

    I am sure these do exist ( non humming pro jr) , i tried two in stores and if i look on youtube half of the video featuring a pro junior have a non-humming one.

    I do ask you that because i think there must be some caps too much off of the writen value or power transformer that do something wrong or i don’t know what in the bad ones…

    Reading all the blogs on the internet i realize the problem people gets is always the same : A constant hum that you can hear at any volume when the amp is on . the hum dont go up or down when you change the volume or the tone.It sound kind of like a microwave.

    so if we bring the values of the humming amp to the values of the non-humming amp , this could be the perfect mod to keep it stock.

    So, for now, I will try to change the preamp resistors for light blue ones as you did and keep dreaming

    I dont have the money to buy another pro junior and i dont understand enough how this amp work to do the tests

    Sorry for all those questions and comments

    • bill says:

      Most Pro Juniors hum because one of the preamp cathodes is not bypassed. You won’t find any measurable difference in components as a cause of the hum. Doing the heater offset mod on my site raises the DC voltage on the heater so that the cathode doesn’t pick up hum from the heater.

      PJs are very sensitive to the tubes installed and you can sometimes mix and match 12AX7s until you find a quiet pair.

      • forp says:

        Is your DC offset a better option for keeping the sound the same than the use of diodes or leds in the preamp (as you talked abouth on anoter forum) ?

        Can i buit the dc ofset on a new board pluging a wire where the r35 goes and having the r35 on the board with the two resistors and the cap ?

        If i want to try the dc ofset this way( if you say its possible), should i plug the gator clip on the buttom or the top leg of the bleeder resistor ?

        I found a better tube for v1 but any tube i put in v2 dont make more difference…

        Still , i somehow belive that a tech should gatter data on one that never hummed (They must have changed the tubes one day or another.. and they cant be that lucky…to never got a tube that made the amp hum)( maybe these amp are the one who are not meeting factory data…)

        Thank you bill for your help, sorry for all those questions

        I really wish to cure the problem without hurting the sound.

        • bill says:

          I don’t know if DC heater offset is more effective than LEDs for establishing cathode voltage. One drains AC to ground more readily, the other prevents the AC from imprinting the cathode.

          There’s no reason why you couldn’t build the DC offset circuit on a different board and tap into the PJ board. If you do it, don’t tap the voltage from the bleeder resistor. Go where the voltage is already lower and filtered, like the last filter cap. Remember that you have to lift the ground side of the balance resistors.

  3. forp says:

    Hi again Bill, im sorry to ask another question before you answer the first

    I have look at shematics to understand what you told me abouth ( the preamp cathode resistor not being bypassed and causing the hum ).

    i also red on a forum where you told us that r4 ( first stage cap) is bypassed but not r11 ( second stage cap).

    Could we simply add a capacitor over r11 ( the second stage capacitor)to bypass the cathode ?

    • bill says:

      Sure. Anything from .1uF on up will help conduct AC to ground. Larger values, up to 22uF, will extend the bass range.

  4. forp says:

    That worked ! , its been the easiest mod !

    i tried with the cap on and off , on a switch and with a 22uf 25 volts cap

    it seem to don’t lose anything and I get a little more treble.

    BUT with the volume on 10 or 12 i think its not as good as before , maybe its my imagination…

    Knowing that the hum was more almost unchanged with a .47uf and a little less high with a 2.2uf and finally almosty absent with a 22uf might help you to answer

    In the facts, Am i loosing frequencies doing this( bigger the cap = more low lost ) ?

    Why does the DC offset or the leds are more suitable than this mod ?

    • bill says:

      If the hum goes away when you have a fully bypassed cathode resistor on the second stage, it means that the tube is somewhat defective and is picking up hum from the filament.

      You’re gaining bass frequencies, not losing treble frequencies. The amp sounds worse on 10 or 12 because you’re feeding more bass into it than the output stage coupling caps, output transformer and/or speaker can handle.

      LEDs work because they are transparent to AC. If you really want to understand this stuff, get a copy of “Designing Tube Preamps for Guitar and Bass,” by Merlin Blencowe.

  5. forp says:

    Thanks bill , I will give a try to leds someday for sure

    And thank you for the book suggestion i was looking for one to teach me this

    For now, i have put a 12ay7 in v1 its way better , i loose some gain but with an overdrive pedal that suit me fine.

  6. schm0e says:

    I love the Dumble sound and I own an old Pro Junior. Is there any hope for me?

  7. HawkeyeKane says:

    Hey Bill,

    I admit up front that I haven’t perused your entire site here, but I have a question for you. Are any of the BJ mods you’ve come up with also applicable to the Pro Junior III? I’m particularly interested in the octal conversion & PT swap. I’m a harp player by trade and I wouldn’t mind having a BJIII or PJIII in my arsenal IF I could have them running on 6V6’s instead of EL84’s. Don’t get me wrong….I like some EL84 amps for harp, but 6V6/6L6 would be a better match for my particular tonal needs.

    • bill says:

      The octal kit is adaptable to the Pro Junior, but would require some changes in the instructions. The TP24 power transformer and a larger output transformer would work, too. It might be hard to find room for the bias board. but it might be necessary. I haven’t tested the TP24/6V6 conmbination in a PJ to see if there is enough bias voltage available from the simple (non-bias board) mod.

  8. dukeofwail says:

    Hi Bill, thanks for helping me out several years ago with PJ mods: adjustable bias, beefy TO22 transformer, pair of smaller 47uf caps, and Weber Sig 10 speaker. I am chasing a steady hum that clocks at 240Hz soon as the amp is on. Steady with V1 and V2 in or out, and no change with Vol or Tone. Bias at 8.5 watts. What can I look for next, please?

    • bill says:

      If it’s humming like that with V2 out, it’s a power supply problem. The weird part is the 240Hz. Heater hum is 60Hz and poorly filtered (blown caps) power supply output is 120Hz.

      Check the four diodes with a multimeter on diode-check setting. Check resistance to ground at the tails of the power supply resistors, the diodes, etc. It should be fractions of an ohm at most. Maybe replace the two 47uF 450V caps or replace them with a single 100uF cap. If you have access to a scope, take a look at the waveform at the output of the two 47uF caps, which is also B+, the voltage supplied to the center tap of the output transformer. It should be a simple, 120Hz sawtooth.

      Try another pair of EL84s, if you haven’t. If the cathodes are worn severely, they can hum strongly. You could be hearing a harmonic.

      • dukeofwail says:

        Weirdness, for sure!
        Using an iPhone app, front-of-speaker reads idle hum at 239.6 Hz.
        But BACK of speaker, with phone held inside to the speaker basket port is 175HZ. (I did it twice) What is that??

        • bill says:

          You’re getting standing waves and cancellation at the rear, because of the cabinet.

          Compare the tone from the speaker to a low B (A string, 2nd fret) on your guitar. That’s 123Hz. 240Hz is (duh) an octave higher, your B string. If you’re hearing the low note and the app is saying 240Hz, it’s hearing a harmonic.

  9. dukeofwail says:

    Great! Happy to check things mentioned in 2nd Paragraph. Tested with pair of EL84 Sovteks that show some burn in 3 patches in a row on each side, and with Fender GT’s with clean glass. No change in hum. Probably not power tubes, then?