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SWR Bass 350 Preamp Question

Discussion in 'Amps and Cabs [BG]' started by Cirrus, Nov 20, 2000.


  1. Cirrus

    Cirrus Guest

    Apr 30, 2000
    Las Vegas, NV
    Hey kids, the SWR Bass 350 has a tube preamp. At what point do the tubes need to be swapped out? Will it be something obvious like it stops working, or will it just be a general loss in sound quality? I don't know the first thing about how this stuff works so any info is appreciated.

    Thanks!
    Peter
     
  2. Preamp tubes last for years, unlike their bigger brothers. You may not notice the tube going off, 'cos it will be gradual. Sometimes a pre tube will go microphonic (makes odd noises), but it's not really something ya have to worry about, as a rule. You can, however, alter the sound of your amp a bit with different brands of tube. Generally, the more expensive the tube, the better the quality.
     
  3. Cirrus

    Cirrus Guest

    Apr 30, 2000
    Las Vegas, NV
    Marty,
    Thanks a bunch for the info, I definitely appreciate it.

    Peter
     
  4. Bobb Nagel

    Bobb Nagel

    Aug 30, 2000
    Chicago area
    I have been using a Bass 350 for about 3 years. I've had to replace the tube twice, both times because it had gone "microphonic". The head sits in a rack atop my Goliath cabs, so I suppose the vibrations may be shortening the life of the tube, I dunno. Anyway, rapping your knuckles on the face of the amp usually quiets the crackling and popping til you can get to the store for a tube. By the way Sovtek tube was 8 bucks.

    Generally speaking, it was a rare occasion when I had to change the tube(s) prior to this amp.

    Hope this helps a little.
     
  5. ServiceSWR

    ServiceSWR

    May 24, 2000
    Hi, I am the service department manager at SWR and indeed the preamp tubes generaly do not need to be changed that often. Maybe every 2 years. Every tube that is made varies in quality even if brand new and the elements to which the amp endures effects the lifespan of the tube. Every tubethat is installed in one of our amps is tested and if any signs of microphonics or filiment noise is rejected.

    To see if your amp's tube is still good, have your amp connected to your speaker cabinet, no instrument plugged in, and turn your gain up to about 12 o'clock and your master volume up to the point where hissing is coming from the speakers. Tap the front of the amp with your finger and if you hear a glassy bell like sound the tube needs to be replaced. If you hear a scratchy rattle the filiment is loose and the tube needs to be replaced. If you don't hear anything then it is still good.

     
  6. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    Yo ServiceSWR,

    Are we allowed to ask you questions directly or are you just checking out the boards for kicks?

    Regarding the preamp tube thing, does it matter what brand you stick in the preamp? I own a used SM400 and recently had to change the pre tube (the original tube = 10 years old!). The local shop only had a MesaBoogie 12AX7, I liked the sound but I don't know how much of that was because of the newness of the tube.

    Question is, are there adjustments that need to be made when switching tubes? Or do I just plug and play?
     
  7. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.

    First, let me me say welcome to T.B., S-SWR
    Second, What Ixhulu asked. :)

    I have a few questions about my 4004 which I absolutely love. Definitely not for everybody but was my choice after looking very closely at everything in its power and price range.

    Hope to see you on T.B. often. I assume that you are a musician as well as a technician, but even if you're not, I'm sure you'll find a warm welcome here.

    Pkr2



     
  8. ServiceSWR

    ServiceSWR

    May 24, 2000
    We use Groove Tubes 12AX7a tubes. The 'a' denominates this tube as Groove Tubes' top of the line 12AX7. Most 12AX7 tubes, though are the same and are rated the same power wise. You may see some labeled 12AX7R and 12 AX7C standing for Russian and Chinese and they supposedly have their "sound"...it takes some Eric Johnson type ears to tell the difference...

    No adjustments are needed.