Very much the same as the "yellowjacket", except about triple the price.
However... some noted fluff in their ad-copy should be pointed out.
(Groove Tube stated)
An adapter which allows class A/B amplifiers such as Marshall and Fender, to replace their output tubes with the EL-84 tube, and operate in class A. This reduces the power of the amplifier. In the case of a 100 watt amp, the output will be about 40 watts. In a typical 50 watt amp, the power will be about 20-25 watts.
-CB- notes that plate dissipation for a quartet of EL84's in Class A operation is 30 watts, and a pair can dissipate 15 watts.
These are used in pairs or quads, not individually. These are NOT for use in Class A amplifiers. For Class A amplifiers, please use the 928, not this 928-PR.
These will reduce the load on your power transformer, and allow your amp to reach distortion, with a nice AC-30 or EL-84 sound and feel at levels that are lower and more friendly to recording and venues where your amp will not feel, play, or distort at the amplifier's normal higher sound levels.
-CB- notes that the load on the power transformer will be, for a two tube 50 watt amp, the same as running that amp at about 30watts output.... no matter how loud you play...because even at soft volume, class A operation has constant transformer load. So, if you're playing softer with the Substitube, you're still using the transformers (all of them) as if you were running normally about #7 or so on teh dial.
(end of their ad-copy)