the leonardo set amp

The driver tube are heated with DC. Although it can be argued whether this is really necessary, I chose to use the same kind of heater supply as for the power tube filaments, see Fig. 6.

My prototype amplifier uses a dropping resistor in place of the voltage regulator. It works very well and is still in service. To avoid the need to empirically find the correct value for the dropping resistor, however, I decided to include the regulator for subsequent versions of the amplifier. The same supply is used to heat both driver tubes.

A common-mode filter is included also here. Since the current is much lower than in the power tube filaments, a larger inductance is used.

As previously mentioned the output tube is controlled using fixed grid bias. The bias voltage supply which is fairly straightforward is also depicted by Fig. 6.

In order to achieve a very quiet supply, an L-C-R-C filter is used after rectification. The dropping resistor offers an opportunity to adjust the output voltage, although that has proven unnecessary.

A bleeder resistor is added to ensure that the current through the choke is always higher than its required minimum value. The two adjustable bias voltages are then provided by 15-turn potentiometers.

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