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Power cascode

A cascode amplifier normally use a pair of small power triodes to obtain high levels of gain and sensibility.

The two triodes is usually of the same type. High transconductance triodes are more recommended, but types as ECC82 is normally used.

The circuit use one triode working as signal amplifier in the input and other triode as common base as output.( figure 1) This common base stage have a good current gain and a unitary voltage gain. But the power gain is reasonable.

Thinking about this configuration, I verified a good quality for use in power amplifiers: the stage have high impedance in input and output.

My idea is to use this circuit as class A2 output , but the circuit can work as  class A1 output , too.

I go to explain the use in A2 class, but for A1 class the circuit principle is seemed.

I made some simple modifications in the circuit: ( figure 2)

1)I changed  the lower triode for a russian high transcondutance pentode( the 6P15P type) for obtain more gain;

2)I used one blue led to fix the grid polarization of lower pentode;

3)As output tube( up triode) I used a indirect heater beam TV triode( 6HZ5) with good results, but the use of a direct heater triode( a 300B in A1 class or the 811A in A2 class) is possible;

4) A fine polarization point is with 2/3Vbb in the lower triode  plate and 1/3Vbb in the up triode.(Vbb = supply voltage)

Is possible to use a high transcondutance triode in the lower position, but it need support a large cathode current( 50mA or more). Two triodes in parallel can work fine supplying more current to up triode.

After various tests and a good sonic quality, I have two schematics above: one with the lower pentode in pentode mode and other in triode mode.( figure 3 and 4)

I prefer the triode mode, and in this case the power is reduced, and you can need a pre-amplifier stage, but with  a bigger  power supply voltage( 500V, per example), the power is very good.

A great advantage of this circuit is that the coupling between the input tube and the output tube is direct, without capacitors.

Another advantage is that the signal passes of first tube to second, of the plate of down tube for the cathode of up tube, without suffering great limitations from excursion of the signal.

I like of the Power Cascode sound for A2 stages. This stage have a best sound when compared a Cathode follower stage, for my ears.

I did not make tests with  a indirect triode In the up position, but I believe possible.  

For more informations, contact me.