Running a classic Marshall amp, unsurprisingly, requires some classic amp parts now and then:
This time I noticed a purple glow in the back, and I thought, hey, that’s kind of cool… A little research tells us that the purple glow is ionized air that has leaked into our vacuum, so it looks like we are going to need a set of power amplifier tubes after all.
The tubes in question are 6CA7 (RETMA) or EL34 (Mullard-Philips); they have a nominal heater voltage of 6.3V, and a decently-matched set of four can produce something like 100 watts.
Marshall had been using EL34s all along, but vacuum tubes were becoming harder to come by, and in the 1980s Marshall introduced a line of amps based on the easier-to-find 6550 tube; a decade later, though, former Soviet-bloc countries began building tubes, and Marshall returned to the EL34. These tubes are usually sold in matched pairs, at prices ranging from under $30 to nearly $200.
The replacement tubes selected for this amp were Svetlanas, about which a dealer says: “For guitar players that need an EL34 tube and play classic rock music we don’t think you can find a better tube.” Now that’s heartening.