Do you mean a THD Hot Plate? I've never heard of a Yellow Jacket. Is that an attenuator too? No, it's a slightly smart octal-to-9 pin adaptor which allows use of EL84/6BQ5 in an amp designed for 6L6's or EL34's. Their propoganda says you don't have to rebias as it converts the amp to cathode bias, blocks grid bias voltage and limits screen and cathode current, but I wonder if the plate-to-cathode voltage may be a bit too high in some cases. I doubt it would be a useful item in a MusicMan!!!

(not only do they work @ 700V, they're cathode-driven) Check out:
http://thdelectronics.com/products/index.html