SRPP With Regenerative Feedback and Current Stabilizing Resistors

Remember when I last looked at the SRPP Amplifier circuit? We discussed the problem with degenerative feedback and how the circuit can run without cathode bypass capacitors (less gain, more stability) or with cathode bypass capacitors (More gain, less stability). Well, I got some feedback from one of the readers, and as I suspected, I got into the “what if” argument. So here we go. Remember that my first two observations were based on the SRPP running with and without degenerative feedback and controlled with the addition of a cathode bypass capacitor.

One of my readers pointed out correctly that you can run the circuit somewhere in between the two situations by providing regenerative feedback and using a better current-managing circuit. Yes, this is true, and here is a circuit that does exactly that.

I will explain, once again how the circuit works. When you first apply power to the circuit, V1 and V2 conduct. The heater elements heat the cathode, and electrons “bubble off” of the cathode and flow to the grid and then to ground; this sets up what is called a reverse voltage on V2’s grid, the flow of current and the heating of V1’s cathode creates the same effect on the grid of V1not exactly negative voltage but a voltage more negative than the voltage at ?The voltage divider of R6 and R5. Both grids impede current flow in both tubes. This flow of current eventually reaches a quiescence point. At this quiescence point, the tube is running but in a steady state. The steady state is when all the voltages have settled, and the tubes are now biased for functionality.

If you remember the first circuit, only Rk V2 and Rak V1 performed current limiting; now, R1, R4, and R5 perform that function. In addition, R1 is bypassed with C1 (increased gain), and the grid of V1 receives regenerative feedback, which adds to the gain and provides some stability. This circuit will provide gain and stability while being more efficient power-wise because less voltage is required to provide a larger gain.

Ask yourself, is this circuit perfect? No, it is not. A few more things can be added to provide even higher gain and more stability, but this circuit can provide an excellent gain with substantial stability.

I want everybody to remember that the circuit is not complicated or complex. It is versatile, and many amplifier builders use variations of this circuit to build their amplifiers. This leads to many debates on how this circuit should be built (with or without bypass capacitors, with additional stabilization circuitry, and with or without additional resistors forming a bias network).

Have fun trying a few configurations; if you have a simulator, try Spice programming and build your tubes. I built these using a combination of Spice and Private AI. The tubes turned out well (except for the U label instead of a V. Well, I never said I was perfect!).

I hope this helps your build project.