The least understood tube in your vintage harp amp is the phase inverter. What the heck does it do?
In amps with two or more power tubes the phase inverter (PI) splits the music – after the preamp gain and tone stack EQ – into two out-of-phase signals. Each of your pairs of power tubes gets one of the signals to each tube, so that one tube pushes the sound and the other pulls it. If you have a master volume amp there is another gain stage after the phase inverter, but that is a different topic.
Amps with only one power tube don’t need a phase inverter. One of the main differences you hear between single ended amps and Class AB amps (with multiple power tubes) is the phase inverter. Stick with me here while I explain.
The common theory you hear spouted over and over by harp players is that we want to reduce the preamp gain in our amps so that we get to that sweet spot of power tube distortion before we hit the feedback threshold. Heck, I used the believe that myself, but I've come to learn that it means a lot more in small single ended amps.
In Class AB harp amps the power tubes are almost never driven hard enough to distort much. What you hear is the phase inverter breaking up! The power tubes just amplify that and pass it along. It can be a very sweet tone with a nice organic breakup.
The phase inverter circuit is not a gain stage, so the difference in amplitude you hear from a 12AX7, say, to a 12AU7 is not as great in the PI socket as it would be in the preamp socket. The goal in the choice of PI tube is no so much to quiet the amp down as it is to get a nice warm dynamic distortion when you hit the amp hard.
In general, my advice is to leave the phase inverter tube alone. Many amp makers now use all 12AX7 tubes in the preamp section (first and second gain stages and PI tube) to keep things simple and cheap. Older Fender amps used a 12AT7 as the PI. (Note: Never use a 12AT7 tube in any circuit for which it was not specifically designed. The AT7 is a terrible tone generator and very different from other tubes in the 12A?7 family.)
But, you CAN improve the tone of your amp by plugging in a 5751 in the PI socket. As you may know, this is my favorite input tube for harp amps. It is an industrial grade 12AX7 with slightly less gain and a warmer tone. Going all the way to a low-gain 12AU7 will give you more clean headroom, which is not really what you want in the phase inverter.
Or, you can throw out everything you just read and try it yourself. The phase inverter does not have to be biased or anything like that, so this is pretty much plug ‘n’ play. The PI tube is usually the small tube closest to the bigger power tubes. In a Fender Blues Junior, for example, it is the third small tube. Start counting at the tube closest to where you plug in your mic.
I’d encourage you to try the 12DW7 trick as well: very interesting results.
Remember to turn your amp off when swapping tubes and BE CAREFUL! Getting zapped can ruin your day. But understanding the role of the phase inverter tube in your vintage-style harp amp can help you get where you want to be tone-wise.
NOTE: Coming up soon I will have a video tone comparison of three different input tubes in the VHT Special 6 amp: The stock 12AX7, an NOS JAN Philips 5751, and an NOS 5965 tube.