On Tube amps, does speaker impedance affect total output power?

Discussion in 'Amps and Cabs [BG]' started by reel big bassist, Jan 4, 2001.

  1. reel big bassist

    reel big bassist

    Mar 27, 2000
    Hello Everyone,
    I have a question about total output power and speaker impedance,specificly on tube amps.

    On the back of my solid state amp
    it reads: 200 watts@8 ohms, 350 watts@4 ohms.
    So if you have a 4 ohm cab 350 watts will be put into it.
    And if you have an 8 ohm cab 200 watts will be put into it.

    On every tube amp spec sheet I've seen, it seems like speaker impedance does not affect total output power.
    For example, the specs concerning power output for the SVT reads as follows: 300 watts at 2 ohms, 300 watts at 4 ohms. On tube amps, does speaker impedance affect total output power?
    This has been confusing me for quite some time.

    Greg P

    P.S. I got the specs for the SVT on ampeg.com

  2. Hey!
    Speaker impedence does not affect total output power on tube amps. Unlike solid state amps tube amps have a transformer on the speaker outputs. The transformer has different output taps which enable the amp to produce the same output power into different impedences. Some amps have switches to change impedences. The SVT doesn't, I'll explain: The OP transformer on the SVT has 4 Ohm and 2 Ohm taps. The main speaker out is on the 4 ohm tap. When you plug something into the ext speaker out, the 4 ohm tap is bypassed and the 2 ohm tap is used. The SVT was designed to drive two 4 ohm 810 cabs! It's fairly important to match impedences, but it probably won't do much harm to run an 8 ohm cab off a 4 ohm output tap. However it is VERY important NOT to turn a tube amp on without a speaker connected. If the output transformer sees an open circuit (no load) it will eventually self destruct! Hope this helps and feel free to ask any questions, cheers!
  3. reel big bassist

    reel big bassist

    Mar 27, 2000
    Thanks for your reply Spacegoat, it really cleared things up. I have another question.

    So on tube amps, their is a transformer on the speaker outputs. Would it be possible to put a transformer on the speaker outputs of a solid-state amp? It seems like a major plus to me. You could acheive high power output levels, without building up lots of heat from low impedances. I know this would add a lot of weight to the amp, but I think it's worth it. You wouldn't have to worry about the impedance of your cabinet, because you can get the same output power from different impedances. You'd just have to make sure your impedances match. Would this be possible?

    Another question. How many transformers does a tube amp have? I have heard that the output transformer colors the sound, is this true? I have also heard that the output transfomer is made of iron. Would it be possible to make it out of a lighter metal, such as titanium or tin. That way the amp wouldn't weigh so much. Would making it out of titanium or tin make the amp sound bad?

    Greg P
  4. Christopher


    Apr 28, 2000
    New York, NY
    Tube amps have two transformers: one between the power supply and the amp stage, and one between the amp stage and the speakers, the purpose of this being that tube amps work best at voltages roughly between those other two components.

    As for why solid state amps typically don't have two transformers, you've hit the nail on the head. Audio-quality transformers typically are heavy, bulky and expensive. The simple fact is that solid states don't need another transformer. Many if not most high-quality solid state amps are designed to work at 2 ohms without overheating, and I think the only time you have to worry about the variable power output noticeably affecting your sound is if the impedances of your cabs are grossly mismatched.

    I'm no metallurgist, but I don't think tin or titanium are particularly good conductors; not as good as iron, anyway.
  5. The iron is necessary for magnetism to be "conducted" around the transformer to magnetically couple 2 coils of wire together. Ever seen the old electromagnet experiment with a coil of wire, a battery, and a nail? Same principals at work in a transformer, except one coil has current turning on and off through it and it causes the other coil to generate a current that turns off and on the same way. Tubes work at high voltages/low currents, like 500V with 100 milliamps, and speakers work at low voltages/high currents, like 50V with 11 Amps. The name "transformer" comes from the fact that it transforms one voltage to another.

    Magnets don't stick to aluminum, I don't know if they stick to titanium, if they do, it's probably no better than iron at a lot more cost. They do make toroidal (donut shaped) transformers out of ceramic/magnetic materials. So that is one other option. Here they are, click on Van der Veen icon to see one:

    The technical term is that the transformer core needs to be "ferrous", which means it contains iron (chemical symbol FE). You can make transformers with air cores, but they are inefficient. Iron transformers are like 90% efficient (put in 100 Watts, get out 90 Watts), and have been since they were first invented 100 years ago. They are pretty dang cool to be such an old idea.

  6. Well, the lads have summed this up! Any amp worth its salt will work efficiently down to 2 ohms. I guess you could put a transformer on the outputs of a solid state amp if you really wanted to. But I think amp designers basically traded equal power at several impedences for the reduction in weight and cost (high quality large format transformers are VERY expensive) that a transformerless output gives. Plus with the power of many of todays amps you'd need some big-ass output transformers! The output transformer in my SVT is about a 6 inch cube and weighs about 20 lb, for 300 watts. Imagine the size and weight of an output transformer in a 5000 watt Crown!!! Youch, my back hurts just thinking about it! What Throbbinnut (great handle BTW:D) said about iron is absolutely true. The mass and quality of the iron in a transformer can have very noticable affects on the sound of an amp. A firend of mine put a cheap mass produced op transformer in a 50 watt Bassman he had and it sounded crap. When he found an original one from another amp and installed it we couldn't believe the difference. With the cheapo it sounded thin and jangly, with the good one, it was full and clear. Check out http://www.ampage.com there's a load of useful stuff on tube amps over there. I'm glad I was of some use to you :D cheers!

    [Edited by Spacegoat on 01-05-2001 at 11:59 PM]
  7. so this means you can use cobalt or nickel as well instead of iron..

    just wanted to add that for no reason...

  8. Amicus


    May 4, 2000
    Spacegoat, the link you posted goes to Ampage Homepage Services: "the best web page design services on the net." Nothing on tubes as far as I can tell. Would you mind posting with a correct url.

    BTW, this has been a useful thread, for me at least. Thanks.
  9. Oysterman


    Mar 30, 2000
    AFAIK, an iron core produces a stronger force field than any other metal. My two cents worth of physics knowledge...
  10. Sorry about the false URL, the glue must be getting to me... go to http://www.firebottle.com , there is a link to Ampage and a bunch of other tube sites.