Humming Twin Reverb - Help!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Fernando, Aug 13, 2005.


  1. Fernando

    Fernando Member

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    Hi guys. I have a brand new Twin Reverb Reissue (I purchased it 3 or 4 moonths ago). It sounds great and I love every aspect of it. Except for some hum I've been getting.

    With nothing plugged in, when I turn every knob all the way up to ten on both channels, it is like you could hear a running shower through a closed door, or a storm through a closed window. If I turn the volume of one of the channels to zero, the hum is half as loud. I noticed the reverb adds a lot of hiss when I turn it up. The treble knob makes a huge difference too. The mids knob adds some noise and the bass knob makes no difference. I also noticed that when I put my finger close to the bright switch on either channel, the hum increases and sounds almost like feedback noise. This is kinda funny cause I don't even have to touch the amp for that to happen, coming close to the switch with my fingers is enough. Switching it (the bright switch) on and off makes no difference in the noise, though. Playing live or even in a studio with my band, I can't hear the noise. But I figure it could be a hassle when recording.

    Is this normal? What could be wrong? Should I tweak the hum balance a little?

    Thanks!
     
  2. claptonisgood

    claptonisgood Member

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    given the condition that this happens (all controls pegged) i wouldn't worry about it too much...you are probably hearing the result of OEM (read: mediocre) tubes, particularly the preamp tubes. If you have some extra 12A_7 type tubes, you can experiment with tone/noise, particularly with the first two, V1 and V2, and, also V6.
     
  3. Fernando

    Fernando Member

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    So, swapping preamp tubes could make a difference regarding hum? What brand of preamp tubes would you recommend?
     
  4. claptonisgood

    claptonisgood Member

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    that is a loaded question!:) for new 12AX7/ECC83, i like the JJ's because they seem to be consistently high quality/low microphonics/low noise...some of the new Chinese seem good as well...if you want to try something different, try one of the new EH 12aU7 in V1,V2 or V6...less gain but very toneful (better yet, get a NOS GE or RCA, but, now we are getting carried away :eek: )...also, it won't cost anything, try swapping V1, V2 and V6 until you find the quietest combination order...


    oops! i corrected the above (i had stated V3 instead of V6)
     
  5. Fernando

    Fernando Member

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    Hey, man, I know it's a though question, but thanks for your answer. I'll experiment with different tubes. For the time being, I tried plugging the amp into diferent outlets and I'm also using a noise gate. turns out it is quite better. I recorded a track with the amp today and couldn't notice the hum in the recording. Guess I might have been a little obsessed. Thanks very much anyway!
     
  6. PacoCasanovas

    PacoCasanovas Member

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    mmh....first step: take out tube V6 and then turn the amp on....if you hear no hum...must be the preamp, if you still hear some.....then you have a poweramp/powersupply issue...wrong bias, or heating-voltage offset

    if its a preamp issue

    just take out V1 first....switch the amp on & listen....then change the tube! And if there is no difference, go for V2 (and so on!)

    greeetz
    Paco
     
  7. John Phillips

    John Phillips Member

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    Since the noise is dependent on the volume and tone settings, it's quite definitely coming from the very first gain stage on each channel. If it's a tube problem, it's V1 and V2 (furthest from the power tubes) or the common cathode resistor and cap they share.

    I'd definitely think about retubing it with better tubes, as has already been said.

    But - if you say putting you finger near to the input jacks and bright switch increases the hum and even brings the amp close to feeding back on itself, it sounds like you may have a slightly poor ground in the input stage. (Your body acts as a sort of aerial BTW, which is why you don't actually have to touch the amp.) On the original Twin Reverbs the grounding was done by a brass strip which ran along behind the controls and jacks and provided a very good ground - the RIs don't have this, they have PCB-mounted controls and jacks, and the noise may not be a 'fault' as much as a normal characteristic of these amps... it's a while since I worked on one (they're very reliable BTW) and I don't remember.

    Most people don't run them with all the knobs on 10 either... ;)
     
  8. claptonisgood

    claptonisgood Member

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    John brings up an excellent point...i have actually solved grounding/noise problems on more than a couple amps by re-tightening the input jacks...trouble-shooting 101 states, "always try the simplest, obvious fix first!"...
     

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