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A techie question on power transformers

Discussion in 'Amps and Cabs [BG]' started by Benjamin Strange, Aug 27, 2003.

  1. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    I've heard a little talk here about guitar power amps vs. bass and pa power amps. Some say that the transformers are wound differently, and you shouldn't use a guitar power amp for bass. Is this similar to how pickups are wound for bass and guitar?

    What would be the dissadvantage of using a guitar power amp vs. bass or pa, if any? I've got a Boogie Coliseum 300, which sounds great, but I'm wondering if I am missing out on anything?

    If I am I may scoop up a Strategy 400 instead. Comments?
  2. You're not talking about the difference between POWER transformers; you're talking about the differnce between OUTPUT transformers. They're not the same thing. Power transformers take 110 volts at 60 Hz (in the US anyway) and step it up to whatever high voltage the tubes or transistors require and additionally also supply filament and/or low voltage bus lines. Both tube and solid state amps use power transformers.

    The difference in output trannies comes from their physical size, specifically core mass. They take the high voltage, high impedance, low current flow from the tubes and turn it into a low voltage, high current, low impedance flow that can be directly connected to a speaker. The amount of iron in their core determines determines their inductive potential, measured in Henries, which is the ability to swing large amounts of current. The larger the core, the better it handles low frequencies, because lows have a larger current content than higher frequencies. A tube amp which is designed to produce large accurate amounts of low frequncy content will have a larger output transformer.

    The older output transformers used in vintage guitar amps were usually made from then-existing specs for radio and hifi operation and as such have good low frequency response. In the 1980's, faced with rising costs and falling profits, most guitar amp companies began cutting costs by using smaller output transformers. They wound them only to reproduce the sonic spectrum of the guitar, forsaking lows below 80 Hz, which made them lighter, smaller and most importantly, cheaper.

    There is a Coliseum 300 on eBay right now.


    the BIG tranny is the power tranny. The smaller output transformer is tucked in front. In contrast, the output iron of an SVT is nearly equal in size to its power tranny.
  3. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Thanks. I understand the difference between power and output transformers; I used the wrong term there.

    My Coliseums 300 is rated at 300 watts, the output transformers are pretty large, and it's heavy as hell (about 50 lbs.). It was made sometime in the mid nineties, specifically to work with the Triaxis guitar pre-amp. Do you think that the trannies are wound specifically to guitar frequencies? It's the same wattage as an SVT, yet the SVT is almost twice the weight. Does the SVT have something that the Coliseum does not (besides pream and chassis)?

    I've always thought that my rig sounds great, but it doesn't have thunderous low end (even thought the lows are good). The Stategy 400 has similar specs and weight, yet was marketed for guitar and bass in the '80's. Does it stand to reason that the trannies in the Strat will handle lows better than the Coliseum, or am I splitting hairs here?
  4. The other 30 lbs of weight in an SVT is in the core of the output transformer. A 50 lb bass amp is pretty lightweight for six 6550A's, the tubes the Coliseum uses. It would work for bass, but Boogie is notorious for skimping on OT's. That's why nu-metal guitar players can crank the bass knob wide open on their Triple Rectifiers and not hang up speakers, even when drastically detuned.

    A cheap OT will have no problems from 80 Hz up, but below that, the heating caused by the increased impedance of the windings caused by the core not transfering high current properly (core saturation) will act as a mechanical high pass filter and roll off frequencies below that at a steep slope. Highs are not a problem until you get above 2Khz, and then the windings must be interleaved, which is easy to do. Guitar and bass amps generally have really good high frequncy performance. If you like a low-mid heavy sound, you may not notice any loss of lows, but it won't be a hi-fi or clean tone.

    If you have a tech buddy with a tone generator, scope, and voltmeter, you can MEASURE the ouput variations and see how well it handles lows. Hook the amp to a dummy load and put the voltmeter in parallel with the dummy load. Set the tone generator for an amplitude just below clipping of the amp on the scope at 1 kHz and measure the voltage. Square that number and divide by the resistance of the load and you'll get the max clean power rating of the amp. Observe any voltage variation as you lower the frequency of the tone generator for successively lower freqs and you'll see exactly how well it handles lows.
  5. Petebass


    Dec 22, 2002
    QLD Australia
    Psycho, I could listen to you talk like this all day. I love these threads :)