The Phase Splitter Valve

I am often asked by customers “What is the phase splitter valve and is it special or different from the other valves in my valve guitar amplifier?”

Hopefully this short article will explain what the phase splitter valve and clear up any questions about the phase splitter valve you may have (but were too afraid to ask!).

Before explaining the phase splitter valve in detail, let’s do a quick overview of the types of valve in your amplifier. There are two basic types – the preamplifier valves (of which the phase splitter valve is one) and the power output valves which do all the heavy lifting.

The phase splitter valve is part of the preamplifier chain. In fact it is the very last valve in that chain just prior to the output valves. The phase splitter valve takes the preamplified signal, conditions it in a special way (which I will explain) to create the drive signal for the power valves.

What Does The Phase Splitter Valve Actually do?

First you need to know that your guitar amplifier will either have two, or four output valves.

If it has two, then they work as a pair; one ‘pushing’ the loudspeaker cone, the other ‘pulling’ it. In technical terms, one valve handles one half of the sine wave (let’s say the top half) and the other valve handles the other half of the sine wave (say the bottom half). You may have heard of ‘push pull’ amplification and this is what’s going on here.

99% of all valve guitar amplifiers have this ‘push-pull’ arrangement of valves driven by the phase splitter valve.

Before talking about the phase splitter valve which precedes these output valves, let’s just briefly explain why an amplifier may have four output tubes instead of two.

In a 4-valve set up, the valves are just doubled up to give twice the power. E.g. a 2 valve amplifier will typically be 50W, whereas a 4 valve amplifier will be 100W. So two valves ‘push’ and the other two ‘pull’. All are driven by the phase splitter valve.

So now we come on to the phase splitter valve. The phase splitter circuit has the specific job of taking the preamplified signal (think of it as a clean sine wave for the moment) and splitting it into TWO sine waves which are 180 degrees out of phase with each other. This is called a split phase signal. When one wave is going UP the other is going DOWN. Think of it as producing the original signal, and a mirror image of the original signal.

It is these ‘out of phase’ signals which are needed to drive the output tubes (causing one to ‘pull’ while the other is ‘pushing’.)

Is The Phase Splitter Valve a Different Type?

This is where most confusion arises. Most preamplifer valves are 12AX7 or ECC83 (same thing). The phase splitter valve is also commonly a 12AX7 (ECC83). Bottom line is you can use any make/model of ECC83 as your phase splitter valve, it doesn’t have to be ‘special’.

However, because the ECC83 is, in fact, two identical valves in one glass envelope, some purists insist on using a selected valve as their phase splitter valve. These selected valves have been picked so that the gain of each valve half is identical. The theory here is you don’t want to be (say) ‘pushing’ slightly more than you are ‘pulling’ due to the different gains. The audible result of that, by the way, would be a very slightly distorted signal. (Distortion? On a guitar amplifier? We can’t have that!!)).

Phase Splitter Valve Take Home

So now you know what the phase splitter valve does. The take home is that you don’t really need a special selected valve in this position. By all means go to the extra expense of fitting one but you’re most unlikely to hear the slightest difference.

Sometimes the phase splitter valve is an ECC81 or ECC82. These are simply lower gain versions of the ECC83 and all the above discussion applies.


You can buy valves suitable for phase splitter here


Need more in depth technical info on the phase splitter valve?

Try this YouTube video.:


Posted by Stuart on May 3, 2015