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THD: Yello Jackets

THD official product page for Yellow Jacket tube convertor sockets

Michael posted to alt.guitar.amps:

I just bought a pair of THD Yellow Jacket tube convertor sockets with EL84s [too tall!]. I plan to upload the instructions. My ancient Kay amp with 2 6V6's has been cutting up and down in volume; I think the tubes are totally mismatched and are shot to hell from my amp-cranking experiments with a speaker isolation cabinet.

These replace common tubes such as 6V6, 6L6, or 6550, by an EL84 pair in class A operation. Can I only plug in a single EL84?

THD home page

If you start with a Marshall with 4 power tubes, you can pull all 4, and put in two Yellow Jackets with EL84s, resulting in about 1/4 power -- 1/5, the instructions say (20%). A 100 watt Marshall becomes a 20 watt Marshall.

I've been doing so much online research. First I did a lot of hardware research, in the days of the DigiTech 2101 and the Harry Kolbe Silent Speaker R/L load. Then I got rid of practically all my gear, just for psychological reasons -- to clear the drawing board and start with a completely fresh mental slate, and to do pure literature research to figure out what pieces are available and how to put them together in an elegant way. That was about 1996.

Now I have researched all the possibilities and I'm looking forward to buying gear and experimenting. I figure I'm going to have to invest in a variety of speakers, cabs, amps, power-tube products, and effects. Looks like an oversized custom flight case could be $600, to form a giant isolation box (doghouse), or mini isolation booth. I also live now in a house that has a cement-block darkroom. And now, totally unlike 1996, there is a fair selection of tube power amps in the 0-10 watt range. And now WeberVST has an established line of great speakers, and MP3 and hard drive recording technology is mature, and everyone's one the Web. And digital cameras are established.

And ToneFrenzy.com clearly has the start of a great thing going, though they *suck* at the moment at miking amps. So I'm feeling eager to upload some samples that will impress people. I might actually solicit .wav files from good guitarists as input data for my power-tube processing transforms.

When I was cranking my Kay and melting down the old, mismatched power tubes, I certainly proved one thing: the "re-amp" approach with a short looping clip kicks *ass* for dialing-in a tone. I sure grok the studio logic of always recording the straight guitar as well as the amp tone, during the original take. It is incredibly powerful, infinitely powerful, being able to run this source signal later, using a computer or whatever, through any black-box processors you want. A miked cab driven by a power tube is merely another abstract I/O transform... especially I'm reporting the *leisurely* sense. No more nightmare of trying to strum and turn knobs at the same time!

ToneFrenzy.com has also discovered the immense power of this approach, for mass-production comparison of effects. It *really helps* to have a consistent self-playing guitar signal so you can use both hands to dial in a Tone, compare mics, compare speakers, etc. You can record the result, say, of micing each speaker model, save a library of those tones to hard drive in an organized database, then A/B any Tone, all via WinAmp -- all driven by the same input signal.

I want to buy and try things like the Black Jack tube convertors which enable plugging a certain spec'd preamp tube into a power-tube socket.

User comments about THD Yello Jackets (6V6) - Submitted Feb. 1997. $154 street. Excerpts:

Yello Jackets convert your Class AB power section to cathode-biased Class A using EL-84 tubes. It works exactly as advertised. I installed a matched duet in a 1972 Fender Deluxe Reverb that had been blueprinted to Blackface specs. I bought this amp used, with the original (RCA, GE) tubes, so I had a good benchmark for evaluating the Yello Jackets. I really love the sound of this amp with the Yello Jackets more than I did with the GE 6V6s. I like a detailed surgical tone. The 6V6s always seemed to splatter a bit. Yello Jackets have greatly tightened up the bass and yet the high end is not strident. I had feared that the brighter EL84s night sound harsh with the Fender front end. The midrange signature is now much more forgiving of my pedals (MESA V-Twin, TS-808 and DM-2 analog delays). I think the Yello Jackets are an improvement over the somewhat trashy Fender output stage. I might buy these for other amps. I am curious as to how one of these would sound with the THD Uni-Tube amp. This could make a 10 watt class A amp. My Yello Jackets have good reliability. I ordered these direct from THD, and they answered several of my questions in great detail before I placed the order.

See also

thdunivalve.htm - THD UniValve head with 1 power tube, integrated Hot Plate, and Line Out
thdhotplate.htm - THD Hot Plate power attenuator with volume fine-adjust, Line Out jack with Level control, and high-level Bass and Treble boost switches


THD Official Email Information about Yellow Jacket Convertors

THD Electronics, Ltd. announces its newest line of products: the Yellow Jacket Converters. Yellow Jackets are a type of specialized adapters which permit the use of EL84/6BQ5 power tubes in place of 6V6, 6L6, EL34 and 7591 types.

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-adjusting cathode-bias circuit. In other words, there are no adjustments to make and no modifications necessary, 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 EL84s into the Yellow Jackets, turn the amp on and play.

Like Getting A Whole New Amplifier - In Champ and Deluxe type amplifiers, the YJS's output is smooth, strong and more even than with the stock 6V6s. When the YJS Converters are installed in larger amplifiers intended for 6L6 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 YJSs 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 YJSs can be used. If only two are used and the other two sockets are left empty, 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 output 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.

No Adjustments or Modifications, Change At Will - As the Yellow Jackets ignore the amplifier's grid bias voltage, it is possible to take them in and out at will to adjust you power level and sound for the individual gig or session. For an interesting blend of sounds, you can even exchange two of the output tubes in a four tube amp for Yellow Jacket Converters and leave the other two as 6L6s or EL34s. This blends the sound of both and lets you run a combination of Class-A and Class-AB.

Safe For All Amplifiers and Transformers - 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 YJSs 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 6V6, 6L6, EL34 or 7591, so the Yellow Jacket will reduce the strain on the filament windings of the power transformer as well.

THD's Yellow Jackets in the Press

Guitar World "... the next best thing to getting yourself a new amp." May, 1996

"What do you do if you're tired of the way your amp sounds, but don't have the bucks to buy a new one or to have some high-priced amp surgeon modify its innards?

"Fortunately, THD has come up with the Yellow Jacket, a cost-effective, non-permanent amp modification that allows you to replace your existing power tubes with EL84s without rebiasing the amp. The concept is similar to that of 12AX7 preamp modifications like the Soldano Hot Mod, where you just plug an adapter and tube into the socket and you're ready to go. But unlike preamp adapters, which simply boost your amp's gain, the Yellow Jackets significantly alter the tonal and response characteristics of your amp.

"The Yellow Jackets are available in four different versions: a set of four adapters for EL34- or 5881-based 100-watt amps; a pair for EL34- or 5881-based 50-watt amps; a pair for 6V6-based amps; and a pair for 7591-based amps. We tried the Yellow Jackets in a 100-watt Marshall Plexi reissue. Because four EL84s provide about 50 watts of output, the amp wasn't quite as loud. The treble response was enhanced, the midrange somewhat thicker, and the amp produced rich, glassy distortion and sparkling clean tones similar to those of a Vox AC30. The Yellow Jackets transformed the Marshall into a totally different amp. Best of all, it took less than five minutes to install the adapters.

"THD's Yellow Jackets are the next best thing to getting yourself a new amp. Costing about the same as a good preamp or distortion pedal, they're the way to go if you want to explore new tones on a budget."

Vintage Guitar "... too hip!" May, 1996

"THD Electronics, Seattle, has a device called the Yellow Jacket, which enables you to switch the output tube of your amplifier by using an external socket adapter that plugs into an amp's existing tube socket. These adapters come in several versions; they essentially allow you to use EL-84 output tubes in place of 6V6s, 6L6s, EL-34s and the extinct 7591 vintage Ampeg tube. No re-biasing is required because it automatically makes your amp go into simple Class 'A' operation. It's especially nifty if you can't find good (or the correct type) tubes for your current amp, or if you simply want to change its sound. List price is $60 each, tube included - too hip! You can also use two of these things and leave the stock tubes in two of the amp's own sockets (in a 100-watt Marshall, for example), giving you a combination of both Class 'A' and Class 'AB.'"

Guitar Player "... increase amplifier flexibility" May, 1996

"THD's Yellow Jacket converters ($60 each) increase amplifier flexibility by allowing you to run EL84/6BQ5 output tubes in any amp that normally uses 6V6s, 6L6s, EL34s or 7591s. They simply pop into the tube sockets, instantly reconfiguring the EL84s to Class A self-adjusting cathode bias. An EL84 is included with each converter."

THD Electronics, Ltd.
1925 8th Ave.
Seattle, WA 98101
(206) 292-5500

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