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From: Mike Curtis
Date: Tue, 25 Jul 1995 02:02:28 -0700 (PDT)
Subject: Re: new tube amps



A little on vacuum tubes (or valves as the "warm beer" crowd calls them :-)

The way tubes work is, the heater (often called the "filament" - it's the thing that glows red in the center of the tube) heats the cathode, causing it to emit electrons. The grid, a kind of screen thingie near the cathode, is electrically charged with your signal. Because electrons repel each
other, this signal affects current that's emitted from the cathode. This affected current flows to the plate, where it is collected, run through a transformer, and into your speaker.

New tubes have nice fresh cathodes. As tubes age, the cathode emits less electrons. This causes the tube to go "flat"; that is, it puts out less power. This condition is aggravated by running the tubes "hot", such as running them distorted, or at 50% power (maximum plate dissipation occurs at 50% power.) This is most pronounced in push pull class B or AB amplifiers.
Single output tube amps (or amps that run the tubes in parallel, if there are any audio amps of this design in use today) run class A, and are not nearly as affected by overdriving.

If you like the sound of new tubes, they'll sound best new. If you like the sound of flat tubes, old tubes will make your socks go up and down. If you like 'em in between - well, I think you get the drift...

-- mike