A few months ago I bought some 4P1L tubes. They're Russian, directly-heated pentodes with a plate dissipation of ~7.5W. With octal bases, they're readily available at reasonable prices.
I always had a fancy for building a hybrid with the solid state at the front of the amp. The usual thing is to use a MOSFET as the power stage for a tube front end to avoid the need for an expensive, heavy transformer. I just like unusual juxtapositions of components. I had no idea what I was going to do with the tubes. Gradually, as I looked at the arrangement of the heater circuit, some ideas emerged.
I called it The Heretic because it runs against the grain of most SS/tube designs, most are a compromise designed to bring the benefits of tube amplification without incurring the cost implied by a transformer. I've also chosen to use a SMPS and SMT components in many instances. Both of these are frequently avoided by traditional tube enthusiasts. It's a push-pull design, all the excitement these days in low-power tube amps is for single-ended, so it cuts across the expected in more ways than one.
The 4P1Ls have centre-tapped filaments. I decided to tie these points to ground and make use of a low-power DC SMPS for the heater supply. I picked a 5V Meanwell supply for this. They only cost U$10~15. Each end of the filament is driven by a constant-current source built with an LM317. Since the supply required is only 2.1V there's plenty of overhead to keep the LM317s in regulation.
Power supplies are often a problem with tube amps, so nibbling away at the requirements like this can sometimes help provide a solution.
If the filaments are to be grounded then a bias supply will be required. Obviously, if there's going to be an opamp SS front end then +/- LT supplies will be required. The negative rail of this will conveniently provide bias for the tubes. If the grids are biassed to -8V then the tubes will run ~45mA. With 150V B+, this gives a plate dissipation of 6.75W, a little high, but we can make this variable.
Lastly, a gain stage and phase splitter is required. A non-inverting opamp drives one tube and the input of an inverting opamp, which drives the second tube. The non-inverting opamp provides a high input impedance, a shunt resistor of 1M defines the impedance at this point and a nod to keeping RF out of the amp is made by the inclusion of a small cap here. The non-inverting opamp is arranged with a high gain to limit its bandwidth and the global feedback loop from the transformer is closed round its inverting terminal.
Two back-to-back transformers provide isolated supplies of 150V and +/-18V regulated.
...here you can see the low-power distortion which is vanishingly small... ...0.001%THD for 7.5mW in 8 ohms.
...and the distortion @ 3.125W into 8 ohms, which is not a huge power to extract from these tubes, but I think a well-chosen transformer will provide more than this. All-in-all it looks pretty good (0.2% THD), it's just a shame about the capacitor coupling.
Below, the FR.
Nicely accommodates the audio spectrum and gives no immediate concern about stability, gain is <1 @ 180 degrees
I don't know when I can get around to building this, I don't have the energy for doing much apart from working a keyboard and mouse ATM, although I don't seem to be short of ideas. The 4P1Ls are highly thought of in terms of their linearity, they're triode-strapped in this instance, directly-heated tubes and push a lot of the right buttons for many enthusiasts, and the amplifier will almost certainly drive headphones perfectly adequately in the bass with Hammond 125D universal transformers, about €23 ea. All-in-all with the intended power tx's, not too expensive. The simulation here uses a NE5532 model, but an LM4562 would probably be substituted.