Conventional MC-MM Pre Amplifier

The circuit is (more or less) conventional, with a common emitter stage constant current source modulation. When the series is a mirror that is reproduced for both positive and negative supply voltage. Net output voltage again a low DC offset, but may be slightly higher than previous models, because of the lack of power to 0V DC bias to obtain a correct output.

Benefit from a 2.2k resistor from the collector of the output device (parallel, 1.1k) and a 15 ohm resistor (MC or moving coil position) optionally controlled in series with a 150 ohm (MM Moving Magnet or position).

This results in a theoretical advantage 74 (37dB) in the MC position or 7.6 (17.6 dB) for the MM setting. In practice, one can expect very little profit to be less than these figures, especially for MC setting.

I can not comment on the relative noise performance of both pre-amp, but because they are both on a design by renowned audio designer, I would hope that the noise either be a problem with the circuit.

Marshall JCM900 Preamp Schematic

I did not include technical data for this preamp, it's just a scheme to guide repair. May be useful for those who have a Marshall JCM900 guitar preamp .

300B Single End Vacuum Tube Amplifier

Current (new) production 300B tubes like the Electro-Harmonix 300B and the JJ 300B sound wonderful at a fraction of the Western Electric 300B price. A direct coupled driver stage and a low mu triode array are the specific reasons why this design works so well. Good output transformers are the key to good performance. Magnequest FS-030 or XE-60S are good choices.

The schematic for the 300B Single-Ended Vacuum Tube Amplifier is shown below. Using audio output transformers with a primary impedance of about 3000 to 3500 ohms you can expect about 8W of output with at about 2.5% THD.