The Output Stage

Our output tube of choice is the glorious 300B.

I used the indispensable SE Amp CAD software to design both our output and driver stages. SE AmpCAD was written by John Broskie of Tube CAD Journal. John has also created many other very useful programs. The famous Aikido linestage kits are available at the Tube CAD site as well.

This is the SE Amp CAD screen with our parameters. I decided to use cathode bias for the output tube because it is self-adjusting and I didn't want to have to make periodic adjustments. I left the cathode resistor unbypassed, which helps reduce distortion.

The Silk S-325F output transformers were not in SE AmpCAD's transformer list, so I created a new record and entered the specifications.

Here are the grid curves for the 300B tube. The program plotted our load line, as well as Vmax, Imax Wp and other curves based on the parameters we entered on the left side of the screen.

Beneath the curves you can see the results of the key calculations performed by the program based on our input. Having the software do all the calculations is so great! It gives us the freedom to experiment and to try as many 'What If' scenarios as we want.

Shown here are the calculated grid and plate traces for a 1KHz sine wave...

Below is a scope trace of a 1 KHz sine wave at the amplifier's speaker terminals. 2.83V is equivalent to 1 Watt RMS into 8 Ohms. Notice how well the actual waveform corresponds to the SE Tube CAD trace shown above.