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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: OTL and Tube Power Rating Q posted by weinstro on July 13, 2001 at 06:50:29:
Quick and dirty.Tube amps, whether Class A, Class AB, push-pull or single ended all use output transformers because tubes generate lots of voltage but small currents -- exactly the opposite of what dynamic loudspeakers want. The transformer changes the high voltage, low current output of the tubes into the low voltage, high current output needed by the speaker.
The exception is tube OTL (output transformerless) amps, which strap together a bunch of tubes to develop sufficient current to drive the speaker directly, without a transformer. OTLs may or may not be Class A, which refers to the way the amp is biased.
Older, conventional (non-OTL) tube amps (like the venerable Dyna Stereo 70) had multiple taps on their output transformers. You connected your speaker to the tap that matched your speaker (4, 8 or 16 ohm). More or less, that allowed the amp to develop the same power into 4, 8 or 16 ohms. That seems to have fallen out of fashion today, as most tube amps seem to have only one tap on their output transformer, which is optimised for 8 ohms. As a result, they usually develop less power into 4 or 16 ohms.
OTLs typically develop more watts into higher impedences because tubes, not being very good at supplying current, are happier driving the higher impedence, which reduces the current demand and allows them to attain a higher voltage.
The limiting factor on power rating for tube amps is the power capacity of the output tube. Very high power tube amps require lots of output tubes to share the load, which means more heat, size and expense.
Transistor amps do the same thing to achieve higher power (add more transistors in the output stage); but transistors are much smaller than tubes and throw off a lot less heat.
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Follow Ups
- Re: OTL and Tube Power Rating Q - Bruce from DC 18:35:33 07/13/01 (1)
- Re: OTL and Tube Power Rating Q - Bill Rew 19:35:43 07/13/01 (0)
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