5f6a Presence Control no Joy

[Structo]

Long Tail refers to the phase inverter.

That is the type of circuit it is.

The other type that Fender used was the Cathodyne phase inverter.
(think 5E3 Deluxe)

[The New Steve H]

Yeah, but what's long about it?

[Fischerman]

Merlin mentions why in his LTP article (albeit the DC version) on his site.

From that article:

The cathodes of the two triodes are connected together, and one grid is usually grounded at AC. The first triode acts as both a normal inverting gain stage, and as a cathode follower. The signal on the cathode drives the second triode which is non-inverting. The circuit produces perfectly balanced outputs when the anode loads are equal and the cathodes are connected to a constant current sink. If the total current through the two triodes is constant, increasing the current through the first MUST cause the current through the second to decrease by exactly the same amount, and vice versa. Perfect balance is preferred for hifi, but for a guitar amp we can easily replace the constant current sink with a large 'tail' resistor instead. The larger the resistor the more it will approximate a constant current sink and the better the balance, although it cannot be too large or it will reduce maximum output signal swing. Because of this large 'tail' resistor (or current sink) the differential pair is affectionately known as the "long tailed pair". There are several versions of the long tailed pair (also known as the Schmitt inverter), and the one requiring the least components is this DC coupled version.

[Firestorm]

I can't find Alan Blumlein's original patent (it's British) so I don't know if referring to a large value cathode resistor as a "long tail" was common in the thirties, or just Blumlein's pet name for it. It might be that that: he had another circuit called the "garter" because it resembled a garter belt.

[The New Steve H]

Meanwhile, I have succeeded in hijacking my own thread.

[tubeswell]

Yeah, but what's long about it?

The cathode (bias) resistor is in series with another (bigger) resistor before it hits the ground return. This bigger resistor is the 'tail' of the gain stage and so someone called it a 'long tail' and so we get to where we are today.