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Impedence and power output- tube vs. SS

Discussion in 'Amps and Cabs [BG]' started by emblymouse, Nov 6, 2006.


  1. emblymouse

    emblymouse exempt Supporting Member

    Jan 22, 2006
    W'Sconsin
    On my Pignose tube head there is an 8 ohm-4ohm switch, but as far as I can tell the power output remains the same regardless of the load. I think the Traynor YBA 200 has a similar switch.
    Why are these tube heads different than most other amps that put out more watts when the load is increased? Can someone explain the theory behind this?
     
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If you're just changing the switch position the output won't sound different. You also have to add another cab.
     
  3. Solid state amplifiers are directly coupled to the speakers and their output follows ohm's law. Theoretically the output voltage of the amp will stay the same at different speaker impedances but lower impedances draw more current, thus more power (Power=Voltage x Current). Tube amps have to be coupled to speaker loads via a transformer. Tubes are high voltage/low current devices and speakers are low voltage/high current devices. The transformer steps the voltage from the tubes down to a level the speaker can use. The transformer matches the amp to the load. Transformers have the property of being able to reflect impedances, so a certain impedance on the speaker side translates to another impedance on the other side. This is goverened by the ratio of the number of turns of wire on each side of the transformer. The number of turns on the tube side is constant, but the speaker side can have a couple of taps where an output is taken at less than the full number of turns. This allows you to connect different load impedances while keeping the impedance presented to the power tubes constant. This allows the amp to produce the same output power into different loads. I really hope I was able to clearly explain this. :)
     
  4. emblymouse

    emblymouse exempt Supporting Member

    Jan 22, 2006
    W'Sconsin
    Yes, thanks, I can visualize that. So there is a fundamental difference in current flow between tubes and SS. I guess I always knew that but never grasped the specifics. So...
    if my SS amp is rated at 200W @ 8 ohms, and 300W at 4 ohms,does that mean I'm 'getting more' out of my 200W SS amp vs. my 200W tube amp with the same 4ohm load? More volume amd power? Is this an inherent advantage to SS amp design? Or does it all come out in the wash because tube amps appear to be louder than SS at the same wattage rating?
     

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