Reply to a question on the SRPP Amplifier.

I recently received a question about an article about the Series (Shunt) Regulated Push-Pull (SRPP) amplifier. This question concerned the age-old issue of degenerative feedback: Should Rk Have a cathode bypass capacitor, and will adding a cathode bypass capacitor improve the sound? The answer to this question is a double-edged sword.

Here is the original question:

Let’s talk about what happens without the cathode bypass capacitor:

Without the cathode bypass capacitor, Rk is crucial in limiting the current flow through the tube and providing the triode grid with a negative going bias voltage. When sound is introduced at the junction of R1 and R2, it triggers a grid fluctuation, causing a voltage fluctuation throughout the tube. However, a signal is generated across the resistance of Rk, which is approximately 180 degrees out of phase with the output signal (the first answer to this was correct but was not inclusive of the overall effect). This out-of-phase signal limits the output signal on a plate of V2, thereby reducing the signal voltage on the grid of V1. In this scenario, the degenerative feedback stabilizes the tube arrangement by controlling (limiting) the signal voltage. The result is a distortion-free sound, albeit at a lower signal voltage.

Let’s talk about what happens with the cathode bypass capacitor:

Without the cathode bypass capacitor, Rk is crucial in limiting the current flow through the tube and providing the triode grid with a negative going bias voltage. When sound is introduced at the junction of R1 and R2, it triggers a grid fluctuation, causing a voltage fluctuation throughout the tube. However, a signal is generated across the resistance of Rk, which is approximately 180 degrees out of phase with the output signal. (the first answer to this was correct but was not inclusive of the overall effect). This out-of-phase signal limits the output signal on a plate of V2,, thereby reducing the signal voltage on the grid of V1. In this scenario, the degenerative feedback stabilizes the tube arrangement by controlling (limiting) the signal voltage. When a cathode bypass capacitor is added, the degenerative feedback is removed, and due to this, the signal gain of the stage is increased; this adds more voltage to the signal voltage, thus increasing the amplitude of the voltage (higher sound level). However, with the higher sound level, you also increase the possibility of the wave distorting as the input signal voltage increases (destabilization). However, the amplitude of the signal from the amplifier does increase, with the possibility of distortion at higher signal inputs.

What I have done in experimentation using this type of amplifier stage:

I have sometimes added a switchable capacitor to allow me to choose the mode I want to work with. It has worked out great. I have even gotten good results. I have the capacitor for some types of music, but it is not for other kinds of music. It works either way. Do you think this is a solution? Well, it is one of many; I have had to ensure that when the capacitor is not used, it is shorted to ground with a 100K ohm resistor (load to discharge the capacitor). When used, it disconnects from the resistor to the ground and is connected in-circuit before turning the amp on. Yes, it works, but you must fully explain some Harry Potter, voodoo, and Santeria stuff to your customer or the user. I ran some tests with an NI Multisim Simulator. Just as I expected, when the cap (100uf) is left out the overall gain is 3.75, and the tube is stable, if I add the cap (100uf) the gain goes to about 6 but there is a chance of instability. Granted, as long as you keep the signal in the input range's linear region, you will be good. (If none of this makes sense, either put the capacitor in (gain of 6) or leave it out(gain of 3) )