Hi, My name is Nic and I run Voc Rock Guitars,
I thought it might be helpful for me to write a short paragraph and guide on what a Balanced pre amp tube is, where to fit it and the benefits of using a balanced tube in a particular position in a valve amp versus a standard tube.
In short the Phase inverter sometimes called a phase splitter or power tube driver is exactly that. It is the preamp tube, often last in the preamp line of tubes and the one that directly drives the power output tubes in a valve amp. In most guitar tube equipment we find that the phase invertor or power tube driver pre amp tube is either an ECC83 (12AX7) or an ECC81 (12AT7). Both of these types of valves have 2 parts to their make up or 2 anodes. In most amps the circuit is designed in such a way that one part or side of the Phase invertor tube drives one side of the power tube section whilst the other drives the other side (one side pushes current whilst the other pulls and vice versa. If a standard 'non balanced' Preamp tube is used in this last position the output tubes will not be driven evenly which will result in some loss of power output and possibly added unwanted distortion as well as working one side of the output valve section harder than the other. Using a balanced tube in almost all other positions in your amp will reveal little or no benefit since in other positions in your amp the 2 parts of the valve are often used for different functions and work independently of each other.
A balanced tube is A STANDARD PRE AMP TUBE THAT HAS BEEN SPECIALLY SELECTED from a tested batch that has both internal parts particularly equal and balanced as in both elements in the pre amp tube behave in the same way in terms of Gain and drive capability.
We always supply one balanced tube with all our re-valve kits or when we consider someone has purchased power and pre amp tubes together. We also mark the balanced tube on the outside of the box and also on the base of the valves between the pins. It is perfectly fine to use a balanced tube in any position in an amplifier but the largest benefit will be in using it as an output tube driver or in the phase invertor position of your amp.
The driver tube or phase splitter is quite often found to be the last one in a line of pre amp tubes and usually nearest the power tubes. Very often furthest away from your input jack.
DOES USING A BALANCED TUBE IN DRIVER POSITION HAVE BENEFIT- Fact or Fiction
In our humble opinion, Yes. Truth is that if one side of the power cycle of the output signal is running into one of the power rails so as to run out of steam earlier than the opposite signal cycle then you will get uneven distortion and less power output. Imagine a sine wave with the positive hump having a flat bit on top due to running into the positive power supply rail limit and the neg cycle of the sine wave still looking as it should. That is what you get when you drive the output section unevenly. You can actually see the effect of using an unbalanced tube as phase inverter or output tube driver and then really driving the output stage. We have seen the result with an oscilliscope, not nice. That is why we always provide a balanced tube with all our kits. Sure let your ears be the judge. We have done that as well as measured the effects and believe that using a balanced selected tube as phase inverter does really make a difference. After all why take all the trouble of using carefully matched output tubes so they perform equally, draw the same current when biased etc and then driving them with an unbalanced pre amp tube.
Hopefully this short guide has been helpful in explaining the reasons and benefits of fitting a balanced tube in the driver position of a valve amplifier.