Here's an interesting harp tidbit. There's a long standing debate as to what Little Walter actually played in his classic blues song, "Juke". The opening lick, repeated 8 times, is 2 ~3 4 5> {6> 6>}? The last 2 notes definitely have a 6 blow, but people also hear notes an octave below, the 3 blow, and even more commonly an octave above, the 9 blow. The argument arises as to whether Little Walter played a 6>&9> octave or a 3>&6> octave or a plain 6>. An intriguing theory that I subscribe to is that Walter played the straight 6 blow, no octaves, and the higher and lower octaves are a result of tube microphonics. The tube amps of the day were not as good as more modern tube amps (late 50's and newer), and this theory has some merit. People have set up tube amps and been able to duplicate the Little Walter sound using tube microphonics, where straight octaves just don't quite sound right.
Preamp tubes are used as the drivers for the power tubes. The 12AX7's will give more power and distortion than the cleaner sounding 12AU7's. 12AT7's are even cleaner and brighter.
The power tubes are the larger ones inside your amp, and do the job of converting the signal from the preamp tubes into the power needed to drive your speakers. There are several types of power tubes available, each offering different performance characteristics. For example, EL-34 power tubes tend to distort quickly and easily and give a creamier sound with a looser low end. These tubes are mainly manufactured in Europe. 6550's are manufactured in the USA, and they stay cleaner even up to full power. When they do distort, it's a "heavy metal" type of dirty distortion, with lots of power. 6L6 tubes have a very good dynamic range and give the traditional "American Rock" sounds, while 6V6 tubes produce a creamy sound with nicer distortion. 5881's can also be described as having a softer less brittle sound than 6L6's, which is normally considered better for harp.
Here's a great tip that may even save you money. Used power tubes tend to lose harshness and have decreased treble response, which is great for harp! You may be able to get them cheap or free from guitar players or music stores that have replaced power tubes that have become too weak for guitar player's taste.
Amps also have rectifier tubes, or solid state rectifier as in the stock Fender Bassman Reissue (RI). Some typical tube replacements for solid state rectifiers are GZ34, 5U4, and 5Y3. The rectifier controls the power (wattage) your amp can produce, The tube replacements shown provide progressivly less wattage.
When looking for tubes, the best ones to get are New Old Stock, or NOS tubes. These are tubes that are old stock, made when tubes was it! But they're new in that they've never been used before. You can order NOS tubes from http://www.angela.com/.
Here are some preamp tube substitutions for the 12AX7 tubes:
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The preamp tube closest to the power tubes typically drives the power tubes, and changes to this tube generally have the most impact on the amplifier output. Different combinations of tubes can be used to produce different sounds, and give more or less headroom to feedback. The best advice is, as always, to try it out yourself and decide what you like for the music you want to play.
