Fender Pro Jr. Hiss and Hum

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jtaylor27, Aug 30, 2013.


  1. jtaylor27

    jtaylor27 Member

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    I bought a pro jr last year and have since basically lived with the hum and hiss and have minimized it, although it's still noticeable when recording (which is basically why I use the amp :().

    I have separated the wires as mentioned on many internet forums already. That did nothing for the noise for my particular amp.

    Today I tried moving the tubes around and noticed that it might be the v2 socket thats making all the noise. I've been experimenting with this by taking the same stock tube (GT 12AX7) and moving from V1 (absolutely no noise) to V2 (noticeable hum).

    Is this the symptom of a dirty tube socket or something else?
     
  2. TimmyP

    TimmyP Member

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    Some tubes have a propensity for coupling the heater (60Hz) to the signal. So far I think I've had this be a problem only V1 or V2 of JCM900s, but I expect it could happen in anything if the tube is bad enough. Also, check the components on the heater circuit: The artificial center tap (R35 and R36), and the diodes and resistor CR6, LD1, and R34.
     
  3. Blue Strat

    Blue Strat Member

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    Could be a dirty tube socket but that's more likely in older amps. If you haven't tried new EL84s yet you probably should.
     
  4. jtaylor27

    jtaylor27 Member

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    I also tried running with no preamp tubes at all and there was no noise. Would the power tubes make noise if they were bad, in this situation?

    So far the best i've gotten it calmed down to is with the stock GT 12AX7 in V1 and a EH 12AX7 in V2. There is more of a less obtrusive hiss than a hum.

    Two brand new EH 12AX7's is unbelievably hummy, and makes the amp totally unplayable.

    The amp's actual tone is fantastic, if only I could get these hiss and hums sorted out. The tone from the amp is easily one of the best i've heard and I've owned a Boogie Mark II, '77 Marshall JMP and a Fender Deluxe Reverb R.I.
     
  5. Blue Strat

    Blue Strat Member

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    No, you've pretty much ruled out power tubes. Dependency on different types of preamp tubes, unless some of them are just bad, is curious.

    Are you getting this hum and noise with nothing plugged into the amp? That's how you should be testing this.
     
  6. jtaylor27

    jtaylor27 Member

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    Yeah, it is there even when the volume is at 0 with nothing plugged in.

    I tried some detoxit in the V2 position and that did nothing at all for me, which makes me think it may have something to do with the wiring and or components connected along that part of the circuit maybe?

    I bought this amp off Ebay last year, and it came with presumably the stock Groove Tubes, which I still use the quietest one in V1. I bought brand new Electro Harmonix tubes a few months back, but that GT has to be in V1, otherwise the amp is unusable.

    With this setup the hiss isnt terrible out in the room but pretty noticeable when recording.
     
  7. Blue Strat

    Blue Strat Member

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    Hum definitely seems to be an internal issue. Maybe a cracked PC board trace or other bad connection. Hiss is usually tubes but could be any noisy component in the signal path.

    You're gonna need a tech to sort this out.
     
  8. jtaylor27

    jtaylor27 Member

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    Thanks Mike, I think so too.
     
  9. Billm

    Billm Member

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    Pro Juniors are notoriously sensitive to tubes. The second stage of V1 is unbypassed, so if there's any 60Hz imprinting on the cathode, you'll hear it. I have a mod on my site to put a DC bias on the heaters to eliminate hum transfer. I've tried DC filaments in a PJ, but found that even big diode bridges got surprisingly warm. I didn't like the idea of the extra load on the PT or the heat load on the amp in general.

    As for hiss, I've gone a little crazy trying to eliminate it from PJs, replacing essentially every resistor in the preamp with metal film. I've been able to reduce it, but not eliminate it. After all the resistors on the high and low sides of the tube stages and finding a really quiet 12AX7, it's tolerable.

    Life for me is better when I leave Pro Juniors out of it.
     
  10. DeaconBlues

    DeaconBlues Member

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    The preamp tube @ V2 seems to make a lot of difference in the noise level of the PJ.

    Fromel's Supreme Mod along with re-routing and twisting the heater wire goes a long way to reducing the inherent noises in a PJ. But, you still have to pay attention to the pre amp tubes. It's going to like some and hate others.

    I love these amps!

    http://fromelelectronics.com/modifications/Amps-Fender-Pro-Jr
     
  11. chervokas

    chervokas Member

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    +1....in my experience after unbundling and re-routing the wires it's just a matter of trying a bunch of different tubes in the preamp position until you find one that's quiet in there. It's not so much a matter of brand, you just need to try different tubes. But if the OP has moved tubes around and had noise with one particular tube whether it's in V1 or V2, sounds to me like a bad tube.

    The amp is always a little hissy, but I've never found it unacceptably so, especially for the price. It's not as hissy as those Roland JC series amps for example. I record often in my home demo studio with a PJ and I've never had a meaningful issue with hiss.
     
  12. insanecopilot

    insanecopilot Member

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    It is a bad design. Period..... I had a noisey one I got for cheap, and after the mods it still had too much hiss. I ended up making a point to point 18watt turret board . Using the same iron and speaker this amp rocks.
     
  13. Rob s

    Rob s Member

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    So its more a layout issue then?
     
  14. Jim Street

    Jim Street Member

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    I wanted to try a tone-tubby alnico 10 with it, but the speaker was too big, so i removed the whole power amp section and reattached it in back of the speaker shell. The hiss and humm was the lowest yet. The only explanation i have is that the magnet of the speaker is too close to the tubes, and when the amp is relocated up and behind the speaker enclosure, it helps. Alot. i also fojund that powering a 4x12 cab and detaching the on board speaker worked too.
     

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