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Discovering Octal Preamp Tubes for Guitars Amps

March 12th, 2018

Little Walter model 50 HeadFor most players using tube guitar amplifiers, the ubiquitous 12AX7 may be the only preamp tube they have ever used. It’s the highest gain of the 9-pin dual triode preamp tubes, although the lower gain 12AY7 and 12AT7 show up in various functions such as phase inverters and reverb drivers. There is also the pentode EF86 9-pin tube that has been used by Dr. Z., Bad Cat and others. But it’s a safe bet that the 12AX7 is king of guitar preamp tubes.

Early guitar amplifiers from Fender, Gibson, and others used what are known as “octal” preamp tubes (such as the 6SL7, 6SN7 and 6SC7), which use the same 8-pin base as the 6V6 and 6L6. This tube pre-dated the development of the 12AX7, and is essentially what there was for early guitar amp builders to use.

When the 9-pin designs came along, builders quickly changed over to them for the simple reason that it helped make louder, tighter amplifiers at lower cost. Distortion was not a feature back then, it was a problem, and the 12AX7 was part of the solution. Early amp builders were not obsessing over the “sound” of a particular tube. The goal was amplification, plain and simple.

Octal preamp amplifiers are still around, but they represent a tiny sliver of overall amplifier production. While there are a handful of amp builders using them, BC Audio did a lot to put them back on the map. The BC Audio products won great praise in the guitar magazines, especially for their great crunch tones.

Octal tubes have lower gain factors than the 12AX7, run at lower voltages, and are felt to have a warmer more complex tone. While a 12AX7 amplifier with two 6L6 power tubes will produce about 50 watts, an octal design with the same power tubes will be in the mid-30’s.

We’ve recently become exposed to octal guitar amplifiers through our association with Little Walter tube amps. We’ve also had the ability to directly compare the Little Walter 50 octal head with their “59” head (50 watts, nine-pin 12AX7 preamp).

While the 50 and 59 are not the same amplifier with different preamp tubes, they do highlight the characteristics of the two platforms. The octal 50 has a more organic feel, a slightly softer attack, and a smooth top end. It’s not spongy and loose in the way of a small Tweed, but it’s remarkably tactile. It’s got a enough clean headroom to work in almost any band situation, and distortion pedals produce a detailed linear crunch that is devoid of sharp peaks or emphasized frequencies.

The 59 has a more familiar feel, and while it’s not a “blackface” amplifier, the attack and response is more in line with a mid-power 12AX7 American-style amplifier. The 59’s cleans have more sparkle than the 50, and the low end is big, rich and percussive. Distortion pedal response is not as smooth as the 50, and there is more high end sizzle with the 59.

Without choosing sides, octal amplifiers offer the guitar player a demonstrably different feel and tone. With mainstream production dedicated towards 12AX7-based designs, octal amplifiers are decidedly more expensive, and limited to “boutique” builders. However, for a  player looking for more a more dynamic and tactile response without resorting to the limited headroom and squashed attack of a low powered amp, octal amplifiers offer a solid alternative.