Power Supply

The power supply was fairly novel. The original power supply was thought to be DC, so it needed to be converted to AC using an LM386 as a square wave oscillator at 1kHz. Then, the AC would be run through 70V speaker transformers and rectified to obtain the B+ voltage. In this way, relatively few transformers would be used since the frequency of the AC was high. However, the power supply turned out to be AC. It was a Wal Mart transformer, taken from an old set of computer speakers.

Using a voltage multiplier circuit was considered in order to achieve the B+ voltage, but due to limitations in the number of capacitors this idea was dropped. Instead, two 10-watt 70V transformers were used. Such transformers are used to match 70V amplifiers over long distances. After testing the circuit, it was found to have 50 volts of AC output from the 13.5 volt AC supply. In order to increase the voltage, two transformers were connected in series to achieve 100V of AC. To further increase the level of voltage, a voltage doubler circuit was added to achieve a voltage of 230V DC. Adding two 100-ohm resistors to create a center tap in the 13.5-volt AC supply, grounding the center tap and then wiring the tubes in series pairs, ensured the heating supply was powered. The wiring was done due to the constraint that each tube require 6.3V.

Input Stage

Phase Splitter

Power Stage and Final Amplifier