Circuit Project Electronic

This is the schematic diagram of the “Li’l 4×4′s” main amplifier circuitry. This completes the circuit if you’re only interested in line-level input sensitivity.
The first big difference in this design is that the output stage is fixed-biased rather than cathode-biased. This gives excellent control over the bias point, and also does not “rob” any range from the tubes’ allowable voltage swing. It also allows us to operate the tubes in Class AB1 mode without any bias shifts as either tube is driven close to cut-off.

The use of Class AB1 mode essentially doubles our output power, compared to the KT-92 design; where the KT-92 barely achieves 2 watts before exhibiting noticeable flattening of the input waveform, the “Li’l 4×4″ puts out a solid four watts before clipping (using 30A5/HL94′s). What’s more, the total power dissipation of the tubes is actually reduced! The KT-92 runs at about 65 milliamperes, with a plate-to-cathode voltage of about 87 volts, for a dissipation of 5.7 watts. The Li’l 4×4 runs a plate-to-cathode voltage of around 180 volts, at 20 milliamperes quiescent, for a dissipation of 3.6 watts. At full output power plate dissipation increases to about 5.4 watts.

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Read More Source:

http://www.dogstar.dantimax.dk/tubestuf/4x4howit.htm

Thank you.