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How to change preamp tubes in an Ampeg SVT-4 Pro?

Discussion in 'Amps and Cabs [BG]' started by Fender32, Oct 13, 2005.

  1. Fender32


    Jun 23, 2005
    Kent, England
    Two questions about this;

    1. Are there any safety implications with removing and changing preamp tubes myself? (The booklet about tubes, which came with the amp, warns about high voltage discharges from tubes, but is that only from power tubes)?

    2. The original 12AX7's in my amp have some kind of silver paint on the outside of the glass, where it fits into the drilled metal bracket. My new 12AX7's don't have this external coating. Is it important? Does it help to eliminate vibration, or soak up heat, or what is it for? Any ideas?

    Any other tips on changing SVT-4 preamp tubes, or which brands and models are best, would be much appreciated. :)
  2. Make sure that the AC power cord is disconnected before opening the chassis cover. Avoid touching the circuit boards with your hands or anything metallic.

    If you want to be extra cautious, wear thin gloves and only use 1 hand to touch the amp. That way you won't be in any danger of completing a circuit with your body. The preamp tubes in the SVT-4 don't run on the very high voltages that power tubes use, but you can still get a good jolt if you accidentally discharge a large capacitor.

    As far as the appearance of the tubes, the coating doesn't affect the way the tube operates in the circuit. Look at the Groove Tubes website for a good description of the different sound qualities of the members of the 12AX family.

    Rick B.
  3. Fender32


    Jun 23, 2005
    Kent, England
    :) Thanks Rick,

    You may well have saved me a nasty jolt there!

    Re: Groove Tubes. Some forums I've read haved slagged them off, claiming that they're no better than the Russian made Sovteks. The tubes I'm installing are also Russian (branded "Electro Harmonix 12AX7 EH Made in Russia"). Any opinions about whether there's much of an audible difference between these and US built EH tubes, or is it just snobbery on the part of the critics?

    I've never had US tubes in the amp, so I don't really have anything to compare to.
  4. Most of the russian tubes are probably made in the same factory, with slight differences in structure and quality control

    I have a set of JJ's in my amp, they are nice :) , there will be a difference in different tubes, but not a huge amount, biggest differences will be how quickly it breaks up and how well it drives, most important thing should be reliablility imo tho

    JJ's have pretty much the best of all worlds, if you ever want to change out your new groove tubes 12ax7 ;)