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Pre-tubes

Discussion in 'Amps and Cabs [BG]' started by i_got_a_mohawk, May 8, 2005.


  1. If a tube amp doesnt sound as good as you think it should, does that mean the pre tubes are gone, or your expecting too much of a tonal colouration?

    Is there anyway to test the preamp tubes ?
     
  2. Masher88

    Masher88 Believe in absurdities and you commit atrocities

    May 7, 2005
    Cleveland, OH
    I'd like to add a question to Mohawk's earlier question: My Mesa 400+ is making like a feedback-ish low humming when I stop playing. It goes away when I hit the Standby switch to "standby"...then I turn the thing back on and it's ok till I play more. It's not really load or anything...just annoying. Is this also the Preamp tubes? Or are the power tubes shot? As Mowhawk asked...is there a quick test on tubes...short of osilliscopes!
     
  3. I get alot of hum on mine too, but as soon as i turn the standby back on, so i think its my bass

    Also, just out of interest, when you hit to standby does it cut the sound instantly, there is still a signal on mine for a few seconds afterwards, i hope thats normal
     
  4. My Ampeg V-4B does the same, and I believe my guitarist's Marshall does so as well. The sound doesn't cut right away, it takes between 3 and 5 seconds for me. An SVT-II has 6 poweramp tubes I believe, so it may take longer, but it should be normal.
     
  5. Kelly Lee

    Kelly Lee Yeah, I'm a guy! Supporting Member

    Feb 17, 2004
    Marana, AZ, USA
    I would venture to say the noise is from the tubes. As to which, pre or power, I don't have a clue.

    On switching to standby, my QuatraValve (four 6550's) takes a good 30 seconds to kill the output. Luckily no noise/hum problem though.
     
  6. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    The standby switch cuts power to the output tubes. The filter caps can hold the voltage up for a while, but 30 seconds seems like a long time. It could be that your output tubes are lightly biased and not drawing much current when no signal is present. This isn’t necessarily a problem although you may have a little more crossover distortion than the amp was designed to have.

    It’s very likely that the howl is from a bad tube. All tubes are sensitive to mechanical vibrations, some more so that others. One symptom when a tube goes bad is that it becomes more microphonic. This tube can then react with the speakers and form an oscillator, which is obviously very undesirable. An easy way to spot the bad tube is to lightly tap on each tube with your fingernail. All tubes will give you a slight ring, but the bad one will be much worse. In all likelihood this will start up the howl if it isn’t doing it already. If the howl is too loud for this to method to work, then you’ll have to swap the tubes out one at a time until you find the problem one. Always power-down between swaps, giving a minute for the voltages to bleed off. This is the same method I use to find a tube that I suspect is killing tone.

    - John
     
  7. I know about the howling, last band practice one of the guitarists tubes in his 5150 went microphonic, SCRRRRRRRRRRREAAAAAAAAACHHH, was not nice, but can pre tubes being bad cause it to just not sound as good ?
     
  8. Well different kinds of tubes will sound a little different from each other... I guess it depends on what you consider to sound good or bad.

    If you have some really old ones in there, swapping them for some strategically chosen new ones may give you some more punch or warmth or whatever you're looking for.
     
  9. Well my SVTII sounds flat in comparison to my Firebass, so, yeah, i spose it shouldnt be like that, its better with the graphic EQ on the SVTII, but i recon it should probably sound better without the G-EQ on, maybe i just need to try it with an ampeg cab :)
     
  10. Masher88

    Masher88 Believe in absurdities and you commit atrocities

    May 7, 2005
    Cleveland, OH
    Do I tap the tubes while the amp is on and connected to a cab? Or when amp is powered off? Anybody know of a good tube dealer? ;)
     
  11. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    Amp on with cabinet (always run a tube amp with cabinet connected).

    - John