Not sure how I feel about these, lets just take the tube out of tube amps.
Quality Tube Guitar Amplifiers
RetroValves are 100% analog vacuum tube replacement devices offering the warm, fat, punchy sound traditionally associated with only glass vacuum tubes.
RetroValves are pin-for-pin compatible with 12AX7 tubes in nearly any guitar amplifier, offering instant modification of preamp gain characteristics.
RetroValves are very low noise and have no potential for the microphonics sometimes characteristic of vacuum tubes. And because RetroValves operate with much less heat and consume less power, they reduce stress on the amplifier’s electronics.
Use RetroValves alongside vacuum tubes, or replace vacuum tubes in any “cocktail” combination. They operate in any 12AX7 socket, in nearly any guitar amplifier.
RetroValves are so cool and advanced, they’re protected by US patents: 7,286,060; 7,408,401; and 7,482,863; as provided under license from ROBERTS AUDIO TECHNOLOGIES.
And, they’re so robust, JCA warrants RetroValves to be free from defects in material and workmanship affecting form, fit, and function for the lifetime of the product. RetroValves are only recommended and warranted for use in guitar amplifier products.
AMBER RetroValves are nominally the same gain as a standard 12AX7 and perform as you would expect a 12AX7 would.
BLUE RetroValves are significantly lower gain, effectively “cooling off” the preamp gain.
RED RetroValves are very high gain, allowing the preamp to achieve gain characteristics not possible with standard vacuum tubes.
Not tried these myself but they have been around a while. AMT also do something very similar.