Amp Valve Balanced

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What is a Phase inverter Amp Valve

The pre amp valve that drives the power tubes in a valve amp is often referred to as the Phase Inverter (PI) or the Phase Splitter.  Its primary role is to drive the power tubes.  A common set up is to use a double Triode pre amp tube like the ECC83 aka 12AX7 for this purpose.  The 12AX7 has 2 internal elements (triodes) which act individually from each other.  Valve Amp design is such that one half of this pre amp tube is designated to drive the left hand pair or single of the power tube section and the other half of the same preamp tube is designated the job of driving the right hand pair or single power tube (s).

A balanced selected preamp tube differs from a regular one in as much as the internal triodes have been deemed to have exactly the same drive, gain and characteristics as each other so internals are exactly matched.  This is generally not the case with most tubes and is not usually any cause for concern since each half within a preamp valve is often designated a different task within the amp to perform so does not need to be closely matched.  The benefit of using a balanced selected valve in the power tube driving or phase inverter position of a valve amp is that the power tubes are driven more evenly.

Where do I put the Balanced selected Phase inverter Amp Valve

Amp design is such that the signal path follows through starting from the input jack socket and finishing at your loud speaker so the first preamp valve in your amp (often labelled V1 and is sometimes fitted with a metal shielding cap) is NOT the phase inverter.  The Phase inverter sits right at the end of the signal chain just before and nearest the power tubes.  So if your amp has 3 preamp valves fitted the general rule is to fit the PI valve in V3 furthest from the input jack and nearest to the power tubes.  A common mistake is to fit the PI in V1 (the tube with the shielding fitted).  This is not correct.