The Valve Wizard

The SRPP is an elegantly simple circuit, and is really a small output transformerless (OTL) amplifier. It was widely used in TV circuits for delivering significant current into heavy capacitative loads. It was first patented in 1940 by Henry Clough of Marconi, and has returned in may guises since. Interestingly though, it was not referred to as the SRPP until quite recently and instead had a variety of other names like 'bootstrap follower' and 'shunt regulated amplifier'; no one seems to know when the SRPP name first took hold, but it looks like some time in the 1980s. To read more about the SRPP, see my published article: The Optimised SRPP Amp.

Sadly, the SRPP is not actually much use for guitar, as it is really only capable of driving relatively fixed loads (not tone stacks!) and it does not clip pleasantly at all. One useful place to use it might be as a reverb driver.

The circuit uses two triodes, usually in the same envelope. Each triode is biased the same, so half the HT is dropped across each one. The lower triode acts as a common-cathode gain stage with an active load, and the upper triode acts as common-anode gain stage with an idenatical active load. This is about as close to a complementary transistor pair as valves get!
The reason the circuit is push-pull and not single ended is that the signal reaching the bottom triode causes the signal on the grid of the top triode to be in anti-phase with it. When the top triode conducts more, driving current into the output coupling capacitor, the other conducts less. When the top triode conducts less, charge stored in the capacitor is returned and flows down into the lower triode.
Unlike the mu-follower, the output should be taken only from the cathode of the upper triode.

Since the circuit is really a small power amplifier, high current valves are preferred, but the following example uses an ECC83 (12AX7) with an HT of 300V

This circuit is often used as an output transformerless (OTL) power output stage, so high current valves are preferred, but let's see how well (or badly) and ECC83 does. For perfect balance, V2 ought to have an anode resistor equal to Rk if the cathode is bypassed, or 2Rk if the cathode is unbypassed. However, the unbalance is very small for high-mu valves.

The two cathode resistors should equal*:
Rk1 = Rk2 = (2Rl + ra) / mu
Where Rl is the following load resistance. If we were driving a 10k load, say;
Rk1 = Rk2 = (20k + 65k) / 100
= 850 ohms.
820 ohms is the nearest standard.

It is usual to add a cathode bypass capacitor to the lower cathode. Leaving it out will hardly affect the gain in this case, but it would increase the anode impedance which makes the stage more susceptible to noise and increases output impedance. Since the frequency response will be hardly afftected, there is little point calculating the bypass capacitor's value carefully, any value greater than 1uF should do.

The quiescent current is given by:
Iq = HT / (2ra + 2muRk)
Iq = 300 / (130k + 2*100*820)
1 mA
And since the SRPP can only operate in Class A, the peak current delivered into the load is 2Iq per triode, making 4mAp-p in total, or about 1.4mArms- which is not as spectacular as we might have hoped. The maximum undistorted voltage across the load must therefore be 4mA * 10k = 40Vp-p, or 20mW. The maximum input signal before clipping is simply 2Iq * Rk, which is about 1.6Vp-p (so the circuit must therefore have a gain of 25).

Alternatively, an LED or diodes could be used to bias the lower triode, which would negate the need for a bypass capacitor, but only if they provide the same bias voltage as Rk2, for optimum performance (in this case 0.82V).
Normal rules apply for the grid-leak resistor, and 1Meg is usual.

If both triodes are identical and biased the same, thel output impedance will be:
Zout = (ra + 2Rk)[ra+Rk(mu+2)] / [2ra + 2Rk (mu + 2)]
Zout = (65000+2*820) [65000 + 820 * 102]/ [2 * 65000 + 2* 820 *102)
Zout = 32.7k
This is about half the normal value for regular unbypassed ECC83 gain stage.
But of course, this figure is of limited use, since the circuit is still only capable of driving maximum current into the optimum load impedance it was designed for.

Heater considerations: Because the cathode of the upper triode will be at roughly half HT, the heater supply will probably need to be elevated to avoid exceeding the valve's maximum heater-cathode potential- always check the data sheet.