Noisy reverb in a SFTR

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by kwaping, Feb 13, 2015.

  1. kwaping

    kwaping Guitar payer Silver Supporting Member

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    My 1970 Twin just got back from a checkup. It has new power tubes but all original preamp tubes. It has a very low noise floor, except when I kick on the reverb with the effect dialed up past 5 or so. At that point, I get a fair amount of hiss.

    I tried deoxit on all of the reverb connections, which seemed to help a little, but there's still more noise than I prefer.

    Should I swap out the reverb tube next, or is this normal and I just have to get used to it?

    Thanks!
     
  2. zenas

    zenas Member

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    What did the tech have to say about it?

    Otherwise if the amp is ok with all new electrolytics. I'd try swapping bothe reverb tubes V-3 first and V-4 then. I'd try swapping the reverb pan and cables.
    Easy if you have another old Fender just stick them back to back and plug the reverb in I'd do that both ways to find out if it's the amp or the verb.
     
  3. kwaping

    kwaping Guitar payer Silver Supporting Member

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    He did say that the reverb pan was okay. I think I will try those tube swaps that you recommended. Should I care much about the quality of the tube I use, or just one that is functioning properly?
     
  4. zenas

    zenas Member

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    If you can get a NOS or a good used old tube for the 12at7 in V3 a 12au7 will work there too.
    Maybe the new 12at7s are ok but the old ones are pretty cheap so I've never tried the new ones. New 12ax7s seem ok though. (I really ain't to fussy)
     
  5. Pro Audio Service

    Pro Audio Service Member

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    1970 Twins are full of carbon resistors. As they age and get stressed from heat and vibration, they slowly break down and change value. They are one of the major factors in noise and hiss generation in tube amps.
     
  6. kwaping

    kwaping Guitar payer Silver Supporting Member

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    When the reverb is off, the old girl is quiet as a mouse. It's just when I kick it on that I get any sort of noise.
     
  7. Amp-Rat

    Amp-Rat Member

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    Exactly as stated, replace the out of spec carbon comp resistor(s) in the circuit and it should quiet right down.
     
  8. zenas

    zenas Member

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    Figure out if it's a bad resister before you replace it. I hear about how bad they are yet never need to replace them.
     
  9. kwaping

    kwaping Guitar payer Silver Supporting Member

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    I would assume my amp tech checked those? He's very good - Keith Reinegger at the Amp Lab in Campbell, CA. I'll give him a call right now to confirm.

    I appreciate the input, thanks!!
     

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