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Mixed topology amplifiers
The amplifier shown above not only mixes classes of operation, but also topologies. The first pair of EL34s work as Class-A triodes, while the second pair works as Class-B ultra-linear pentodes. This amplifier would put out 60 watts of power, with 10 watts coming from the Class-A stage and 50 watts coming from the Class-B stage. The Class-B pair could just as easily been configured as pure pentodes, which might yield some benefit from its increased output impedance. In other words, having the Class-B stage produce a high output impedance allows the low-output-impedance triode pair to dominate the output, as the triode's low rp will buck any extraneous contribution from the pentode pair by increasing or decreasing the triode's conduction in response. (As an aside, even within one output stage made up of only two output tubes, mixed mode configurations are possible. For example, in the amplifier below, we see an amplifier that is configured as an ultra-linear at low frequencies, but becomes a triode configured amplifier at high frequencies.)
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