Part of a blog series Tube Amplifier Circuits Explained
Let’s look one more time at the entire schematic, I will point out a few final elements, and you should have a complete understanding of this circuit.
You will see as the B+ comes to the driver stage there is one more RC filter, using a 3.9k resistor and 33uF capacitor. This provides just a bit more filtering for the more sensitive driver stage, and also plays a role in decoupling the driver stage from the rest of the power supply that is feeding the output stage. Remember the B+ voltage goes to both the output stage (through the output transformer) as well as to the driver stage. By adding a capacitor between these, we can help to send any AC noise to ground to keep the input stage power supply as clean as possible.
There is also a 2.7k resistor leading into the grid of each tube. This is a “grid-stopper” and is used to help avoid stability problems. It combines with the internal capacitance of the tube from grid to cathode to act as an RC filter, small enough that it does not impact audio frequencies, but large enough to stop any very high frequency interference that can create instability that you may not be able to hear but could lead to oscillation and damage to the tubes. Don’t worry too much about this resistor—it could be 1k, 2k, etc. Some designs do not even use them, but I think it’s a good idea.
You also see the input signal entering the circuit and going through a 100k potentiometer, which is acting as a voltage divider to attenuate the signal depending on the position of the volume knob. 100k is the amount of resistance this potentiometer uses. When the knob is turned to one end of its range, the signal goes straight to ground. At the other end of the extreme, there is 100k of resistance, allowing nearly the full signal to be applied to the grid. And in between has some variable amount of resistance that can reduce the level of the signal to some degree.
That’s it! We have walked through the entire circuit and I hope you feel you’ve had enough overview to understand each part of it. I don’t know about you, but I’m the type of person who needs to go over it a few times to really get it. Knowledge is powerful, and it’s great fun to learn. I hope you find this useful and perhaps you’ll want to keep learning more!
Invitation for feedback and questions:
I am continually trying to improve this educational content. Could you be so kind, if you find things that seem off or needing better explanation, to submit comments or send me an e-mail? Thanks!