Greetings fellow Quakelings. I’m here to hip you to some cool amplifier stuff, tackling topics both common and technical. An amalgam of information, if you will, wherein we get a better understanding of that which makes our instruments and pedals louder. This month we're in the Valve Zone, focusing on Preamp Tubes. Let’s delve into some common preamp tubes and their compatible peers.
9-Pin / Noval Tube Name
|Amplification Factor (mu)||Plate Resistance (rp)||Sonic / Tonal Attributes|
|12ax7 / 7025 / ECC83||100||62.5k||Classic preamp tube sound; most output|
|5751||70||58k||Smoother; good mids and bottom|
|12at7 / 6201 / ECC81||60||10.9k||Cleaner/clearer than 12ax7; less output|
|12ay7 / 6072||45||25k||Warmer, with good lows. Much less output than 12ax7|
|12av7 / 5965||40||4.8k||Low mu and rp - most often used as phase inverters|
|12au7||20||7.7k||Similar to 12ax7, much less output|
|8-Pin / Octal Tube Name|
|6SL7 / 5691||70||44k||Similar to 12ax7; fuller sounding, but noisier|
|6SN7 / 5692||20||7.7k||Musical and sweet with extended bandwith. Low output.|
Here is a list of all 9-pin / Noval tubes and 8-pin / Octal tubes with compatible pinouts. They are all Dual Triodes, meaning there are 2 gain stages in each tube. The amplification factor, or mu, gives you the potential output or gain of the tube. Also included is my tonal opinion of the tube to consult when swapping. All info here should give you a reference point so that you can swap them into your amp's preamp section and see for yourself what they have to offer. The only caveat is that there is a filament or Heater Current specification for each tube and they vary. So, if your amp has 12ax7s in it and you go to another tube with a higher Heater Current rating, you may tax your amp. It shouldn’t harm your amp, though, and if you want to check the Heater Current rating it’s at the top of most of tube data sheets. Other than that, just swap and listen. Bear in mind that if you already have 12ax7’s in your amp and you try another tube you will lose some output.
Here are some scenarios that are ideal for experimenting:
1. Your high gain amp is just a little too edgy and crisp.
Try a 5751 in the place of a 12ax7. Voila! Darker and smoother overall.
2. My old Fender needs a kick in the pants.
Try subbing a 12ax7 in place of a 12at7.
The other bit of info on this chart is the Plate Resistance, or rp. This value is important to understanding the signal amplification of the tube and how it will interact with other tubes or circuitry downstream within your amplifier. To not drop too much nerdy stuff on you, it dictates how the tubes transfer voltage to the rest of the circuit. More on that in a future column.
When selecting an amplifier, what demands, both functionally and sonically, are you looking for?
'til next time,
Joe Golden is a circuit builder and amp designer at EarthQuaker Devices. His touring and recording credits include Bernie Worrel (Parliament/Funkadelic, Talking Heads), Chrissy Hynde, and the Dirty Dozen Brass Band. He currently plays guitar in the Mark Leach band and is a Fender certified amp technician. Joe lives in Akron, Ohio.