If you must swap different power tubes, we strongly suggest you have a technician add a heater transformer to handle extra load. It is very dangerous to blindly plug different tube types into a guitar amp. If your amp was meant for 6V6s, do not try 6L6s, as this will overload the power transformer. If your amp was intended for 6L6s, do not try 6V6s as you might damage the tubes. Beware of technicians who tell you otherwise! Class A operation of such an amp requires operating the tubes at lower plate voltage and higher plate current - this is a major modification. So long as one tube from each pair is on one side of the transformer, and the other is on the other side, the amp will balance.Īll Fender push-pull tube amps operate in Class AB. Matched pairs are recommended to keep the amplifier balanced from side to side–if your amp uses 4 or more output tubes, you can use groups of matched pairs, and you do not necessarily need to buy a matched quartet, sextet or octet. Bias should be adjusted when tubes are replaced. Most push-pull Fenders have a tube bias adjustment inside, which varies the idling current in all the power tubes at once. This gives more power and more efficiency. When one tube is decreasing in voltage, the other is increasing. Larger amps had two or four power tubes in PUSH-PULL, which work like a see-saw. Small amps like the Champ had one power tube and no bias adjustment. They drive the output transformer, then the speakers. The power tubes in almost all Fender tube amps were either 6V6s or 6L6s. Some earlier Fender tube amp models used a 12AX7 or 7025 for the phase inverter, and you should use a good quality tube for such amps (12AT7s will usually work as well). If the heater shorts to a cathode in this tube, the amp will not work properly. You find this circuit in early Deluxe, Vibrolux and Tremolux models.) This tube need not be especially quiet, but its heater cathode insulation must be very good.
(Some early models used a “paraphase” inverter, which was less critical of tube quality. Most Fender phase inverters used a very crude circuit called a “long-tailed pair,” which also provided some voltage gain.
#Tweed bandmaster 5e7 driver
It was usually a 12AT7, for the same reasons as the reverb driver above. It makes the “see-saw” drive voltages that properly drive the output tubes. The last preamp tube before the power tubes is the “phase inverter,” seen only in push-pull amps (those having 2 or 4 output tubes). This tube need not be quiet any inexpensive 12AX7 type (with adequate gain to oscillate) will work. If the amp has tremolo or vibrato, it will have at least one extra tube, usually a 12AX7. Both terms mean the same thing - an oscillator makes a low speed signal that varies the gain of the guitar signal. However, it should be a 12AX7 with plenty of gain.įender had tremolo or vibrato on some amps. Because this tube amplifies the signal from a reverb spring (which is inherently microphonic), a really quiet tube is not needed. Reverb amps will also have an additional 12AX7 to reamplify the output from the spring. Some early models used 12AX7s or 7025s for this job, however we still recommend using a 12AT7 here. This tube need not be low noise or nonmicrophonic. This is severe duty and will use up the tube’s cathode quickly. If the amp has reverb, there will usually be a 12AT7 to drive the reverb spring. A low noise, low microphony tube is recommended here. It usually drives the tone control “stack,” depending on the model. The first preamp tube (or the first 2 tubes, in 2 channel amps) provides the first gain of the guitar signal.
Replacement Tube Packages are available for Early Fender Tweed tube amps.īy Eric Barbour – Former Vacuum Tube Valley senior editor
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