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The YJS is for amplifiers with fixed bias power tubes. Fixed bias power tubes have their cathode connected directly to ground and a negative DC voltage applied to their control grid. The negative bias voltage is usually adjustable by way of a bias pot.
The YJC is for amplifiers with cathode biased power tubes. (This is often referred to as self-bias or auto-bias because it does not require a bias pot adjustment when tubes are changed). Cathode biased power tubes will have a resistor and bypass capacitor connected in parallel from their cathode to ground, as seen with most preamp 12AX7 triodes. The ground wire is used to bypass the amp's original cathode bias components so that the Yellow Jacket® converter's internal bias components are used instead.
The YJ20 is like a YJC that does not drop the plate and screen voltages. It is meant for use in lower power output amps (20W or less) when no power reduction is desired. It comes with a ground wire for cathode biased amps.
The YJUni is a direct pin to pin converter with no internal voltage dropping or bias components. It is meant for use in lower power output amps (20W or less) when no power reduction is desired and you would prefer to use the amp's original cathode bias components.
The YJ7591 is for amplifiers that use 7591 power tubes. Only the triode version will provide a power reduction. It comes with a ground wire for cathode biased amps.