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Tube Amp Load and Ampeg System Selector Q

Discussion in 'Amps and Cabs [BG]' started by Fealach, Sep 12, 2004.

  1. Fealach

    Fealach Guest

    Apr 23, 2003
    Gone to a better place
    Wondering if anyone is familiar with this device, or one of the tube amp experts, could help.

    I picked up an Ampeg System Selector really cheap. If it doesn't safely do what I want, I can sell it to my guitarist who uses all SS heads (Yeah, I've got dozens of tubes, he has none. It's just guitar, it doesn't have to sound that good.Heh.) or take it apart and use the casing for a project.

    It's the device used by Ampeg dealers to switch heads and cabs. Several jacks on the back that are connected to amps' speaker out jacks, several jacks for connecting to cabinets' input jacks, two selector switch knobs on front.

    Amp 1 and 2 jacks on the back are labelled as being "20 ohm fixed load," and something to the effect of connect tubes amps to these, the load will keep them from harm while the amp is parked and another is being used.

    If I used it, I'd want to switch between my SVT2 PRO and 400+, using the same set of cabs, which make a 2 ohm load. Apparently I "daisy chain" my cabs so that plugging one speaker cord into an amp would result in all the cabs getting sound, then plug this into the cab 1 jack on the selector, plug one of the Mesa's 2 Ohm outputs into amp 1 jack on the selector, and set the SVT to 2 Ohms, plugging one of its outputs into amp 2 jack on the selector.

    I'm not going to even dream of attempting this unless I have absolutely no doubt about its safety. As I understand, the idea is that the 20 Ohm load is enough to keep safe an amp parked on idle, as it were. 20 is a bit different from 2, though. Is this safe for the amp? If not, would it work if the amp was on standby? Or with a 4 Ohm load? The Mesa manual says "slight mismatches are not a major problem but may shorten tube life." It also says that the effects send of the 400+ can be used as a preamp out, if Effects Blend is at 10 and Master is at 0 no signal drives the power section so it is safe to run with no load at all. So is the Boogie really safe with no load, Master at 0? I usually have effects blend at 10 for my compressor/gate, so this would be easy to do to switch amps. Then there's the Ampeg...

    I'd call tech support but I never have any luck, and I don't have all day to sit around hoping Boogie will call me back.

  2. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Well, one thing I know for sure is "never run a tube amp without a load". That would surely be deadly. The general rule for tube amps is anywhere between half and double the rated load impedance. Less ohms is better than more, for a tube amp.

    Tube amps are very different from solid state amps. If your gadget was made for use with SS amps, I wouldn't use it with a tube amp. Especially not a big one like the 400. 20 ohms is outside the "safe" range (electrically speaking) for a tube amp, it sounds more like a dummy load for a solid state amp.
  3. notanaggie

    notanaggie Guest

    Sep 30, 2003
    The 20 ohms is no doubt enough load to prevent most tube amps from going unstable.

    Remember that an 8 ohm speaker can be 50 ohms or even more at system resonance. So a tube amp must be able to handle that much without getting unstable. You would not want to try to play it into a 50 ohm load, but sitting idle with 20 ohms load sounds perfectly OK for any quality tube amp.

    Since Ampeg makes both tube and SS units, the 20 ohm load inputs are for the tube amps and the others for SS. The 20 is high enough not to affect the normal load excessively.

    I see that as a good safety feature, showing thought and good engineering.

    Apparently you are seeing it as a dangerous problem.

    What's up with THAT?

    It will be OK with SS amps, in fact I'd maybe disconnect the 20 ohms if I were going to use it as a giggable selector switch with SS amps only.

    SS amps need no load.

    None of those switch units ought to be switched while playing thru them, the switches probably can carry but not break that kind of current.
  4. Fealach

    Fealach Guest

    Apr 23, 2003
    Gone to a better place
    notanaggie, as I read it, your post is in line with what I had been thinking, I just wanted some opinions. The 2 inputs in question are marked "tube" 1 and 2 and "20 ohm load," now that I examine it closer. I'd imagined I'd use it to switch tube amps by having them in standby, switching, then taking the active amp off standby. Just wasn't sure, if I had both amps set up to run a 4 ohm load, and 4 ohm cabs plugged in, if 20 would be safe. It had been my understanding that tube amps need a load, while SS do not.

    Moot point now, I have (hopefully) sold two tube amps and another has imploded, leaving me with one.

    Thanks for the information.