Dual triode pre-amp valves like the ECC83 12AX7 have two separate valves inside one glass envelope. When the characteristics on each triode are the same it is considered to be a balanced valve. These are generally used in the phase inverter/splitter position (usually the last preamp valve before the power-amp valves).
PHASE INVERTER CIRCUIT
The phase inverter circuit feeds the audio signal into the power-amp valves and in almost all guitar amps is designed unbalanced (not symmetrical) and the components used in the amp circuit will generally have tolerances of 5% and often more in older amps. So by design it is inherently not balanced.
MY HUMBLE OPINION
So what does a balanced valve do in the phase inverter position, it probably just locks in these imbalances. After testing 1,000's of pre-amp valves and doing audible tests on guitar amps with balanced and unbalanced valves we have yet to hear any audible difference by putting these in the phase inverter position. Yes some balanced valves sounded better, but not as a rule just like unbalanced ones sounded good to. One thing we have noticed and only generally is that they are slightly less noisy.
Guitar amps are not hi-fi amps where they are looking to balance the whole circuit. Guitar amps are about tone and their circuits are flawed, but this is what makes them sound so good to our ears. It seems that many ideas have come over from the hi-fi fraternity (and needlessly though) and then perpetuated by valve vendors.
The inherent imbalance produced in the Phase Inverter results in more harmonic complexity (adding 2nd order harmonic overtones), resulting in a sonically pleasing end result. Remove the imperfections and you make your amp sterile.
My advice is to use your ears and do a bit of valve rolling in the pre-amp section and see what sound you prefer and come to you own conclusion.
You may ask why do i sell balanced triodes pre-amp valves, i'm always getting asked for them for the phase inverter position so i provide them.
Balanced triodes in a hi-fi amp? Well that could be a different story ;-)
I hope this small guide has been a help and it is only my opinion on a subject that has many differing opinions.
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