I hate to argue with the collective knowledge here but this is what THD says about their Product:
"Converts your amp to Class A EL84 operation.
The YLSD is an adapter that permits the use of an EL84 power tube in place of a 6L6, EL34, or 6550 in grid-bias amps. There are no adjustments to make and no modifications necessary. You simply plug the Yellow Jacket into the amp's output tube sockets, plug the provided EL84s into the Yellow Jacket, turn on the amp, and play.
THD's Yellow Jacket converters not only rearrange the pin locations of the tubes, but also provide the necessary current limiting on the screens and cathode as well as blocking the amplifier's grid-bias voltage, configuring the EL84 in a Class-A, self-adjusting cathode-bias circuit.
When the Yellow Jacket Converters are installed in larger amplifiers intended for 6L6s or EL34 output tubes, the high plate and screen voltages are reduced as well as current limited to protect the EL84 and to drop output power. When a pair of Yellow Jacket are used in a Marshall 50W or similar amplifier, the overall power drops to about 20W and takes on more of the character of an old Vox AC30. The distortion is smoother and more even and the output is both substantially reduced and more compressed due to the Class-A nature of the Yellow Jackets. In a 100W amp like a Twin Reverb or Marshall 100, either 2 or 4 Yellow Jackets can be used. If only 2 are used and the other 2 sockets are left empty, the output is dropped to about 20 watts. With 4 in place, the amp puts out about 40W and takes on a whole new warmth and richness.
Will they hurt your amp? No! Yellow Jacket converters are safe for all amplifiers and transformers. As all Yellow Jacket converters internally regulates voltage and current they put no extra strain on amplifiers' power and output transformers or internal components. Using the Yellow Jacket in higher powered amplifiers actually reduces the strain on the transformers and internal components because it reduces the overall wattage of the system. Furthermore, the heater (or filament) current of the EL84 is lower than that of the 6L6, EL34 or 7591, so the Yellow Jacket will reduce the strain on the filament windings of the power transformer as well."
I'm thinking this is about what Simul-Class is in the old Mark Series.
Sounds like a good idea to me.
