1. Amps Use Three Categories of Tubes
Up until the late 1950s, all guitar amplifiers used three different types of tubes: preamp tubes, which add gain at the front end of the circuit; power (or output) tubes, which create the power needed to drive a speaker via an output transformer; and a rectifier tube, which converts AC line voltage into the DC voltage that tubes use to amplify an audio signal. Many amps, larger designs in particular, use solid-state diodes to convert AC voltage to DC. Even so, since the rectifier isn’t in the signal path, these are still considered “all-tube amps” if the preamp and output tubes still perform all amplifications duties.
2. Replacing Preamp Tubes
To replace worn preamp tubes, or try out different makes, simply select a new tube of the correct type, gently wiggle the old one loose, line up the pins of the new one, and gently push it into the socket. It’s a good idea to purchase pre-tested preamp tubes, and to acquire a guarantee if possible, but these tubes do not need to be “matched.” Some are certainly better than others, though, and different makes of even the same type can often sound a little different, too, so you can try different preamp tubes to discover which sonic characteristics you prefer.
3. Tweak ’em To Mod Your Gain Structure
If your amp carries 12AX7 preamp tubes, as the vast majority do today, and its breakup tone is a little more harsh or fizzy than you’d prefer, you can try a 12AY7 in the first gain stage (usually the first preamp tube position, but check your owner’s manual) like many tweed Fenders used, to drop the input gain and smooth out the sound. A 5751 will work too, and drop the gain a little less (one of Stevie Ray Vaughan’s favorite tricks). Or try a 5751 in the phase inverter position in an amp that usually carries a 12AX7 there to reduce some of the “splat” as the signal hits the output tubes.
4. Output Tube Replacement Requires More Thought
Many amps need to be re-biased when their output tubes are replaced; a job for a pro, unless you have the skills to do so safely yourself. This applies to most “class AB” amps using 6L6, 6V6, or EL34 tubes, such as Fender’s Twin Reverb and Deluxe Reverb, Marshall’s JMP50 and JCM800, and similar models. In order to bias an amp correctly—which sets the optimum operating voltage for the tubes, since even tubes of the same type will vary slightly—matched pairs or quads should be installed. Slightly mismatched tubes will work, but their bias levels will be somewhat unbalanced.
5. For Class A, Hey, Just Pop ’em In
Cathode-biased amps, on the other hand, which are often advertised as being “class A,” don’t need re-biasing when output tubes are replaced. This includes amps based on the Marshall “18-watter,” Vox AC15 and AC30, and Fender tweed Deluxe templates, as well as plenty of larger designs such as the Matchless Chieftain, which uses the bigger EL34s. Matched tubes might still help these amps sound smoother, with a firmer low end in particular, but it’s easier to get away with installing slightly mismatched output tubes in cathode-biased amps.