One way you can affordably tailor your tube amp’s sound is by trying out different preamp tubes. This is possible because unlike power tubes which require expert bias adjustment, preamp tubes are self-biasing in the circuit. Because of this you can learn to swap out your amplifier’s preamp tubes in only minutes without specialized tools. What difference can changing your preamp tubes really make? Well, you can customize how much gain the stage has available, select a particular tube for the way it colors your equalization, the location of a high or low gain tube in the amp’s signal can change your distortion and tone character, and you can even choose a tube for more subtle features like ultra-low noise, durability or extra headroom. As it happens, quite a wide range of variations can be accomplished by replacing a part that usually costs less than $20.
Most preamp tubes in common use are dual-triodes that require a 12V filament voltage (in order of gain from highest to lowest): 12AX7, 5751(military grade 12AX7), 12AT7, 12AY7, 12AV7 & 12AU7. Other designations are used for European and specialty tubes but fortunately the supplier will usually tell you if they are compatible with your 12AX7 tube socket. Sometimes there will be additional letters tagged onto the end to indicate additional variations of the same basic tube design with more gain, higher fidelity, etc.
Tube Gain Most Useful Substitutes______________________
12AX7 100 5751 - 7025 - 12AT7 - 12AY7
5751 70 12AX7 - 12AT7 - 12AY7
12AT7 60 5751 - 12AY7
12AY7 45 5751 - 12AT7 - 12AV7
12AV7 41 12AY7 - 12AU7
12AU7 19 12AV7
There are also variations in the number of preamp tubes used by amp manufacturers, but generally speaking the tube farthest from the power tubes is your first tube stage and/or clean channel amp, with an additional dual-triode tube for the high gain channel coming next. These two stages are the most sensitive to changes in preamp tubes. Some designs have a tube-driven reverb or effects loop. These tubes are selected for low-noise, headroom and reliability and you should replace them with the same class of tube. The tube closest to your power tubes is not being used the same as the rest of the preamp tubes, but rather this is the phase-inverter that splits the signal for your power amplifier. Again, choose a high quality replacement of the same type. Your amplifier’s service manual should indicate the function of each of your preamp tubes, but usually the first couple slots make the biggest difference.
How do you know which tube is right for your sound? Well that’s the fun part: it is an experiment of trial-and-error. Of course a few million opinions are as close as your ‘local’ internet tube amp forum (and the tube suppliers will usually try to describe them to you) but in the end it’s your particular amp and your ears that are the only reliable authority. To make things even more interesting these tubes also vary from one tube to the next, of the same make and model. Often times the more expensive tubes are companies trying to narrow the variations through testing the tubes.
Most avid guitarists have a box on a shelf full of old guitar pickups and pedals they accumulated over the process of searching for their tone. A box of preamp tubes would be similar, but which has the best sound? This is a matter of opinion and your how your amp responds to the different tubes. Invest some time learning what each of the major brands/models sound like to your ears and remember they will sound different in different positions in the amp. So the next time you want to use tubes to change your amp voicing you will have a better idea what works for you in your amp. In the end remember if it sounds good it is good. Fortunately preamp tubes are cheap and even one that is not your favorite makes a good emergency backup to keep in your gig bag for a dark and stormy night. You should wrap it in bubble wrap or leave it in a padded tube container if you put it in your bag. They are made of glass and reaching into your gig bag and grabbing a handful of broken glass can end a show.
Electrical safety warning: Do not attempt to service your amplifier without training in electronic safety! Even when unplugged some parts of the circuit can store and discharge high voltages which can cause serious injury or death. Refer to your manufacturer’s service manual for tube replacement procedures.
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Even if you’ve matched your bass head and cab properly impedance wise and set your amp for clean sound, sending simply too much power to your bass cab can result in blown speakers. This often happens when you are using a rig you are unfamiliar with, as we tend to know the limitations of our own equipment and have chosen that setup for a reason. Borrowing another bassist’s amp or using a backline rig only to blow it up is definitely not a great feeling.