Remember, small signal amplification is not restricted to small tubes; a 300B could be, and maybe should be, used as a line amplifier tube.   If a 12AX7 based line stage amplifier cannot adequately drive a 20k input impedance solid-state amplifier, then replacing the 12AX7 with a 5687 and increasing the current flow through the line stage by tenfold is one possible answer; another is to use an SRPP circuit with a 12AX7, as the SRPP will deliver twice the current than the single-ended circuit would with the same idle current.
Once again, ask yourself this question: Is This Trip Really Necessary? If a severe amount of current is required or if the power supply cannot be any further burdened, then certainly the SRPP topology may prove handy. The quick test is to take the desired peak output voltage into the load and divide it by the load impedance to find the needed peak current into the load. Now if this value of peak current is less than one fifth the idle current of a single-ended circuit, such as the Cathode Follower or the Grounded Cathode amplifier, stick to these circuits. On the other hand, if the peak current is equal to twice the idle current, considered using an SRPP stage instead.

Over-Designing
In tube audio work, we must avoid the trap of over-designing; in other words, trying to get more out of a circuit than is possible. The gain of an SRPP circuit is given by:
G =               -mu(rp + muRak)

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2rp + (mu + 1)Rak + (rp + Rak)rp/Rl
and output impedance by:
Zo =         rp(rp + Rak)

2rp + (mu + 1)Rak.

Basically, the formulae for the SRPP tell us that the larger the value of resistor Rak, the lower the output impedance and the higher the gain. A 12AX7 based SRPP circuit with a Rak resistor of 100k has spectacular specifications on paper, but can output only a few mV into a Grado headphone. What went wrong?

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