The THD Yellow Jacket® Converter

THD Yellow Jacket

The entire line of Yellow Jacket® Converters have been sold to CE Distribution and are now available exclusively through them.


Yellow Jacket® Converters are a type of specialized adapter which permit the use of EL84/6BQ5 power tubes in place of 6V6, 6L6, EL34, and 7591 types.

How do they work?
They give you simple Class-A operation from any amp.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, while configuring the EL84 in a Class-A, Self-adjusiting cathode-bias circuit. In other words, there are no adjustments to make and no modifications necesary, you simply plug the Yellow Jackets® into the amp's output tube sockets, (in cathode-bias amplifiers) screw the ground wires under one of the output tube socket mounting screws, plug the provided EL84's into the Yellow Jackets®, turn on the amp and play.

How will they change my amplifier?
It's like getting a whole new amplifier! In Champ and Deluxe type amplifiers, the Yellow Jackets® output is smooth, strong and more even than with the stock 6V6s. When the Yellow jackets® Converters are installed inlarger 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 Jackets® are used in a Marshall 50 watt or similar amplifier, the overall power drops to about 20 watts 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 100 watt amp like a Twin Reverb or Marshall 100, either two or four Yellow Jackets® can be used. If only two are used and the other two sockets are left empy, the output is dropped to about 20 watts. With four in place, the amp puts out about 40 watts and takes on a whole new warmth and richness.

In the cases of the YJ7591 converters, the overall ouput power remains the same, but the sound becomes fuller and the extinct 7591 are replaced with common and inexpensive EL84 tubes. In old Ampeg Reverb Rockets and the like, you experience a warmth and sweetness that you could never get with the now unavailable 7591 tubes.

Will they hurt my amp?
No! Yellow Jacket® converters are safe for all amplifiers and transformers. As all Yellow Jacket® Converters internally limit voltage and current they put no extra strain on amplifiers' power and output transformers or internal components. Using the Yellow Jackets® 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.

How do I get one?
First, figure out the type of power tubes your amp uses (call us or the amp's manufacturer if you're not sure). Then call your local music store (many dealers who don't carry the THD amplifier line do carry the Yellow Jacket.) If they are out of stock, call your closest THD dealer, or call us here at the factory. We will be happy to answer any questions you might have on any of our products.

Shipping dimensions: 7x5x5, 1lb

Full product specs. (PDF)
Product manual. (PDF)


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