Fender Replacement Tubes

Fender® Tube Amps come with Groove tubes installed. The tubes are color coded which means you don't have to re-bias the power output tubes if they are replaced with the same color. GT tests each tube for power output, gain, and quality variance. These tubes are then matched to other tubes with the same parameters. This is especially important when replacing two or more power output tubes. If you replace only a single power output tube in an amp that has two tubes then your amp will probably not sound or perform the same. Many amps operate in class A/B which requires both tubes to have match characteristics. Another parameter to consider is that each tube response differently, even if it was made in the same factory at the same time. The amount of ease of which the amp goes into distortion is another major factor. This cannot be determined by looking at the tube but only by testing it. Using a matched set of tubes as opposed to an unmatched set, will increase the amps output power and tube life as well as have a more even tone.

Preamp

Fender GT12AX7A Tube
The Fender GT12AX7A Tube can also be named ECC83 or 7025. It's known for a rounder and warmer sound. It's one of the more popular tubes used. It's more than likely used as a preamp tube but can be used for EQ, and even reverb driver. This tube isn't as powerful as the Sovtek version but has a warmer rounder tone. This tube is commonly used in guitar amps as a reverb driver or phase inverter. It is also used in many microphone preamps mainly because it's quieter specs. Fender amps made from the early 1960's (blackfaced fenders). It can be used as a preamp tube but has less power than the 12AX7A versions. It can be used in place of a 12AX7 tube but again it will be quieter and less powerful. You should be certain that your amp can used this as a direct substitution. This tube has more headroom and is cleaner if used in the preamp stage. It will change the way your amp will sound and how easily it goes into distortion. If the 12AT7 is used in place of a 12AX7 in a blackface fender then the amp will have more of a tweed era tube sound. In a Marshall you'll get a cleaner but less drive type of sound if used in the final preamp stage output.

Groove Tube GT-12AU7 Preamp Tube
Groove Tube GT-12AU7 Preamp Tube - This tube is used in the preamp stages of the most recent Fender Pro Series amps like the Pro Reverb, Twin, and Concert amps. If the tube is used as the 2nd stage preamp driver circuit (right before the power tubes) you'll get a less harsh sound. The fender amps listed used a 12AY7 of which this tube can be a direct replacement/substitution. It's not a powerful but will allow a greater amount of tone variation before the ultimate crunch signal is reached.

Output

Fender GT6550 Tube
This tube is more commonly found in Ampeg and Marshall Amps as well as Fender and other tube amps. It is also known as KT88. The 6550 tubes have a boxy or woody sound to them, unlike an EL34 which is easier to distort and has a more fluid/salty sound. If you plan on using this tube in a guitar amp for rock music then a lower distortion number from 2-3 or so. The lower number means it distorts sooner. It has a slightly more dark tone with a harsher midrange when cranked up.

Fender 6V6 Tube
Fender 6V6 Tube - This tube is used in many blackfaced fenders and well as vintage Gibson amps. The earlier Fender Champ amp uses the 6V6 as do the Fender Princeton, Deluxe models. This tube is somewhat reliable and is available in matched sets: duets, and quartets. It has a good clean tone and has a more noticeable compression sound when driven hard.

Fender EL84 Single Amp Tube
The Fender EL84 tube is becoming one of the more popular power tubes for amps that have fewer than 30 watts. It found in the Vox AC15 and AC30 as well as Vox type of sounding amps like the Marshall, and Badcat amps as well as some Gibsons and lower powered Marshall amps. It has a darker sound than other power amp tubes but has great mid and more articulate high-end without becoming too harsh sounding. It's also may be labeled as 6BQ5.