Fender Amp Circuit Modification

Ibanez JD9 Jet Driver Pedal Review

Ibanez is about as major a player in the electric guitar market as there is—dishing out an array of products ranging from artsy hollow-bodies and “super-Strats” to amplifiers and cabinets. But they’re arguably still best known for their little green stompbox dubbed the Tube Screamer (TS) that Stevie Ray Vaughan put on the map, and which has been used by about a billion guitarists since. Ibanez has reissued TS pedals for years, and its popularity has spawned clones, DIY kits, and modifications in an attempt to capture that long sought after blues crunch. Though the Ibanez Tube Screamer earned its rightful place in the blues-rock canon for its sweet overdrive capabilities, many players look to push their TS-9s, 10s, and 808s beyond their inherent gain capabilities. How could do you retain the sweet sound of a TS and catapult an already breaking-up Marshall into more frenzied territory? Many companies have already addressed this issue. And with the JD9 Jet Driver, Ibanez throws its hat in the ring. The JD9 marks the first true-bypass entry in the company's 9 Series of pedals (along with the simultaneously released BB9). The pedal  has four simple controls: Drive, Volume, Mid, and Tone. The Drive knob allows you to adjust the gain, ranging from a medium overdrive to a hard punch. The Volume knob controls the output level—a full counterclockwise rotation will shut off the output while a full clockwise rotation will actually give the pedal a fairly significant boost from your base volume level. The Mid control is the most obvious—and welcome—addition to what you’d consider a standard TS circuit. My previous experiences with an Ibanez TS9 reissue found me using an EQ pedal to deal with the sometimes jammed midrange, which crowded up an already busy pedalboard. The JD9’s Mid control has a very adjustable sweep, however. The Tone knob enables you to very effectively adjust the bass/treble response. Ibanez has been using these compact pedal casings for quite some time now—and that’s because they work, they’re efficient, and they’re sturdy as hell. In fact, I can’t image how you’d break one of these boxes short of strapping the housing with small explosives. My previous TS9s, which share the same housing, have endured years of road-wear with little to show except a loss of paint, and the occasional tightening of the nut below the knobs.

Fender Concert Rivera Mod

On channel two I moved the tone stack after the first preamp tube V2A, dropped plated voltage on that tube to dial in distortion-little to fuzz-then shape it through the tone stack out a pot to a full wired (parallel) triode (I liked the 12AU7). It is the other half of the second preamp stage V4B, of the same tube as V4A, and was not used-not wired. I took advantage of that and copied the circuit for the input preamp tube V1. Same configuration. This allows one to amplify the chosen shaped distortion cleanly until really overdriven (if desired). You can get almost as clean as channel one and anywhere in between to Ozzy fuzz without it sounding muddy. I also added a fixed bias adjustment pot to dial in the correct bias voltage in addition to the stock bias balance pot. (the one I used in 100K with a 4.7K wired parallel, that's all I had of that type of pot) I then added a switch to change from the fixed bias to cathode bias. Cathode bias has more sustain, breakup, compression, sag and less gain. It is used in most lower power amps up to 18-20 watts. Good for blues, classic rock and situations where you don't want so much bite with every hard stroke. Fixed bias is in most higher powered amps and is better for up front picking, heavy metal, jazz, country, more gain and pick response. The only thing I dispute here is tripling the value of the front end bypass cap. The stock value of 22uF yields a -3dB point damn close to the bottom of the audio spectrum. Even Fender bass amps used this value. Marshall used 320uF in their tube bass amps, which was way overkill. With guitar, you won't start to notice any rolloff until you get down to less than 4.7uF or so. Cathode bias with such a high B+ (475V) can be a problem; 'no-signal' plate dissipation can be rather high (have you checked this) and high signal levels result in 'squish' - the cathode voltage rising and causing crossover distortion, which in this case tends to take big bites out of the waveform, resulting in a thin overdriven tone. Have you scoped the output signal? Jfrond: I used that value based on what I read about the values used. Because the tube is wired in parallel the normal value would be 22uf@25v, but wired full it needs to be doubled for the parallel circuit to 47uf@50v. I chose the 68uf@160v because a higher value is supposed to provide a cleaner response and is used on bass amps and acoustic amps. I did not want distortion at this point, only to amplify the incoming signal.

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