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Interstage Coupling

The output voltage of a tube stage is much higher than the bias needed for the next stage, we should eliminate the DC and let only the AC arrive to the next stage.

Transformer coupling

It is no more used a lot, quality transformers are rare and so are their manufacturers.
A transformer cannot transform DC, only the AC can reach the secondary. Another advantage is the possibility to use a transformer that elevates the AC voltage, in this case the first stage should have a low output impedance and the second one a high one, which is usually the case. As the transformer is driven by a DC current, there should be an air gap in its magnetic circuit to avoid the saturation of its core which would give poor dynamics and distorsion.

Capacitor-Resistor Coupling

The DC is stopped by the capacitor placed serially with the input.
It's the most common type in use.
The quality of this cap should be very high, the signal passing through it.
The resistor value fixes the input impedance of the second stage (the grid has a nearly infinite impedance, at least for the tubes without grid current).
This circuit is a voltage divider where the first resistor is the cap's reactance and the second the input impedance of the second stage.

Coupling Capacitor Calculation

where C = coupling cap
          Rg = grid resistor
          Fmin = lower frequency to pass

Direct Coupling

The less components the signal goes through, the better our ears feel ...
Thus the idea to suppress any coupling element.

To achieve this, we must increase the cathode voltage of the second stage to keep a good bias voltage for the second stage.
That is to say, we must increase the second stage supply voltage and increase the cathode resistor so that the voltage drop across it corresponds to the first stage output voltage minus the bias voltage needed for the second stage. For the direct heated triodes, the heater voltage is polarised to achieve this same condition.

The Loftin-White schematics use this principle.
Example with 2A3 tube.

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