Primer of Tubes, Pt. 1

By Fringe Staff

I want to thank those who noticed the Gear Geek column was missing from the August issue, and called me about it. We have my recent hand/wrist surgery to thank for the interruption. It’s that kind of feedback that assures me you are reading Gear Geek.

This month will be the part one of a primer on tubes. If you are like most musicians, you have already decided that there is nothing quite like the sound of a warmed up, slightly overdriven vacuum tube amplifier. But, do you really know the basic characteristics of the tube amp and the tubes? Do you know what the numbers stamped on the side of the tube represent? If you needed to substitute a tube, would you know what you could use?

We know the typical all-tube amplifier is separated into two sections. These would be the Preamp section and the Power Amp section. We will focus in on the preamp.

Typically the preamp circuit tubes are the smaller tubes. You will typically find anywhere from one to three preamp tubes per channel. Each tube contains (2) individual circuits that allow for a variety of configurations. Usually this means (3-4) gain stages per channel and maybe something for spare. Preamp tubes usually use a 12 volt heater, hence the first number stamped on the side, as in 12AX7. In an amplifier with a real spring reverb, you will also find a driver tube. This will usually be the last preamp tube in the series. In most cases, it will also be a 12 volt preamp tube (such as a 12AT7 or 12AY7), but with much less gain than the signal tubes.

Here’s a little field trouble-shooting tip! If you have an amplifier as I described above

(With a driver tube and a spring reverb unit) and think that you are having problems with your preamp section, here is a possible solution. But, first, remember that tube amplifiers hold voltage in the large filter capacitors and it is enough voltage to KILL YOU!

Now, shut off and unplug your amp. If you can identify the tubes by position, normally the driver tube will be the last small tube (V4 in many cases). Pull the driver tube. If you find a 12(XX) 7 number on the driver tube and the other preamp signal tubes (V1-V3) are also 12(XX) 7 tubes, you now have an interchangeable test tube in a pinch. Your amplifier should run just fine without the driver tube installed. Obviously, the best way to go about this is to identify the tubes in advance and have a known good set or at least a test tube with you.

Note: (XX) in the tube number 12(XX) 7 means there could be a variety of letters inserted. The letter variations identify the gain of the tube. For example a 12AX7 has more gain than a 12AT7 or 12AY7.

In the next Gear Geek, I’ll break down all of the preamp tubes in order of gain, the optimum substitutions for each and optimum combinations in typical amplifiers.

If you have any specific questions email Bob at cs-rocinstruments@sbcglobal.net.