Quote:
Originally Posted by Low Hertz
I am looking to replace my current solid state unit.
I see some tube units on EBAY, made in China. A friend tells me they would be good if the tubes were all replaced. I have read reports that the units
are cheaply made, with bad solder connections and such.
I also am a professional musician, and can not tolerate distortion, even
if it's the natural harmonics associated with tubes. I remember hearing
a Fisher or Scott unit years ago, and the distortion was horrible to me.
I want a well made unit that does not require modifications to keep for the rest of my life.
I can't afford vintage McIntosh, though they are clean and free of distortion. Obviously, one is paying for the name.
As a musician, I have always preferred the sound of warm tube amplifiers, but most prefer the sound of acoustic instruments.
Thank you
Bob C
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May I give you a short tutorial on vacuum-tube high fidelity amplifiers? This discussion applies only to a class A push-pull amplifier, not one of those single-ended, no-feedback things that were a fad a few years ago. The most critical component in a tube amp is the output transformer. It must pass the audio range, 20Hz to 20 KHz with minimal loss at either end of the audio spectrum. That means Tthe transformer must be physically large, or low frequency saturation (and resulting distortion) will result. It must have high quality, 29-gauge laminations, stacked 100% interleaved. The primary turns must be tapped exactly in the center of the winding, and the two halves should be physically symmetrical, so that two halves are of equal resistance. The primary and secondary windings must be sufficiently interleaved or high frequency loss will result. Now the tubes: The output tubes must be capable of the power expected of the amplifier. The tubes should be selected as a matched set. There should be a bias adjustment for the output tubes, as well as a balance adjustment. The idea is to operate the tubes within ratings, and make sure that the two tubes have equal quiescent current, to prevent saturation of the output transformer with resulting distortion. If the output tubes are not matched, there could be balanced quiescent current, but unbalanced dynamic current, resulting in output transformer saturation and distortion when in use. The second reason for having bias and balance adjustments is to keep proper bias and balance as the output tubes age, which they will. The amplifier should have about 20 dB of negative feedback from speaker output to input. The first truly practical high fidelity tube amp design was the "Williamson," (not a brand, but a circuit design) followed by the "Ultra-linear" design.
What I have tried to do is to take the magic out of tube amps, and look a the engineering.