Vacuum Tube Amp in SPICE

You can download LTSpice IV  and this zipped file. Install Spice and extract the zipped file to the folder of your choice. For example I used a folder labeled \amp.

The circuit to examine

This will give us access to a functioning circuit to examine.

Load the schematic in LTSpice and run it. Close the program and click on the diagram will place the voltages as I have done. The teacher says first adjust the DC biases and then the AC. Look at the suffix on the voltages. The grid voltages are in pV. (10^-12)

Take a close look at U1. The screen grid and plate are close to B+. The grid is basically 0. This is normal for the circuit.

Possibly the most common problem with the old electronics is the paper capacitor aging and leaking current. I simulated that problem here by placing a 10 meg resistor across the coupling cap. You can see the grid is high positive and the plate and screen grid are drawing high current. (the series resistor are dropping more voltage and reducing the plate and screen grid voltages.

U2 plate voltage was effected too.

U1 voltage is riding 6.8 volt reference. Most likely it plate is glowing a cherry red.

I "fixed" my leaky capacitor and the grid is at 0V.

U1 grid is swing positive and negative. (The 0Volt reference it should have)

This is what U1 plate voltage and current should look like.

Be aware the screen grid can drive a tetrode into cut-off. I increased the value of R3 which reduced the screen grid voltage. If you remove the screen grid voltage it will turn off the plate circuit.

You can download the program and test the circuit actions with different faults. This is intended as an introduction to troubleshooting with more to come. Whether you have only a DVM or a shop full of test equipment you can find faults and fix them with some practice and understanding of how the circuits function. Some people suggest the shotgun approach. This can be used but be careful about changing parts in the tuning circuit and in reflex circuits. You can change bypass and coupling components and expect no problems.