The MiniBlok II Push-Pull Amp

Posted on Feb 6, 2014

As in the original single-ended design, the push-pull MiniBlok II uses the relatively inexpensive 13EM7 `dissimilar dual triode` (or its variants the 10EM7 or 13EM7/15EA7). Using the same tube with essentially the same operating point makes for a more equitable comparison between the single-ended and push-pull topologies. However, it should be fairly obvious that two of these are required for the push-pull version. As before,

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one of the small triode sections is used as our preamplifier. The component values are, in fact, exactly the same. The two power triode sections form our push-pull output stage, again with essentially the same operating point. That leaves one small voltage amplifier triode, which will form our phase-inverter for the "paraphase splitter". As before, there are essentially three separate power supplies required: The first ("A" supply) is to heat the filament of the tubes, the second provides the high voltage for the plate circuits (the "B" supply), and the third provides a portion of the negative grid bias for the power amplifier ("C" supply). Incoming power from the 120 VAC power line is switched by on-off switch S1, and thence applied to step-down transformer T1. Unlike the replacement-grade transformer used in MiniBlok, however, MiniBlok II uses a 24VAC, 40VA "Class 2" transformer. This kind of transformer is commonly used to provide low-voltage AC in house wiring, and is supplied with a large mounting nut suitable for installing in standard circuit boxes. While these are more expensive than the usual run of experimenter transformers, they are of higher quality and therefore capable of supplying more power for a given physical size. Although a 24V, 40VA unit is not much larger than the 12. 6 volt at 2 ampere unit used for T1 in MiniBlok, it is capable of nearly twice the output power. As before, the output from T1 supplies...

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