Early Fender Tweed Amps
Replacement Tube Packages are available for Fender tube amps.
By Eric Barbour - Former Vacuum Tube Valley senior editor
What the tubes do in most Fender tube amps
The first preamp tube (or the first 2 tubes, in 2 channel amps) provides the first gain of the guitar signal. It usually drives the tone control "stack," depending on the model. A low noise, low microphony tube is recommended here.
If the amp has reverb, there will usually be a 12AT7 to drive the reverb spring. This is severe duty and will use up the tube's cathode quickly. This tube need not be low noise or nonmicrophonic. Some early models used 12AX7s or 7025s for this job, however we still recommend using a 12AT7 here. Reverb amps will also have an additional 12AX7 to reamplify the output from the spring. Because this tube amplifies the signal from a reverb spring (which is inherently microphonic), a really quiet tube is not needed. However, it should be a 12AX7 with plenty of gain.
Fender had tremolo or vibrato on some amps. Both terms mean the same thing -- an oscillator makes a low speed signal that varies the gain of the guitar signal. If the amp has tremolo or vibrato, it will have at least one extra tube, usually a 12AX7. This tube need not be quiet; any inexpensive 12AX7 type (with adequate gain to oscillate) will work. 12AT7s might also work.
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). It makes the "see-saw" drive voltages that properly drive the output tubes. It was usually a 12AT7, for the same reasons as the reverb driver above. Most Fender phase inverters used a very crude circuit called a "long-tailed pair," which also provided some voltage gain. (Some early models used a "paraphase" inverter, which was less critical of tube quality. 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. If the heater shorts to a cathode in this tube, the amp will not work properly. 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).
The power tubes in almost all Fender tube amps were either 6V6s or 6L6s. They drive the output transformer, then the speakers.
Small amps like the Champ had one power tube and no bias adjustment.
Larger amps had two or four power tubes in PUSH-PULL, which work like a see-saw. When one tube is decreasing in voltage, the other is increasing. This gives more power and more efficiency. Most push-pull Fenders have a tube bias adjustment inside, which varies the idling current in all the power tubes at once. Bias should be adjusted when tubes are replaced. 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. 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.
All Fender push-pull tube amps operate in Class AB. Class A operation of such an amp requires operating the tubes at lower plate voltage and higher plate current --this is a major modification. Beware of technicians who tell you otherwise!
If your amp was intended for 6L6s, do not try 6V6s as you might damage the tubes. If your amp was meant for 6V6s, do not try 6L6s, as this will overload the power transformer. It is very dangerous to blindly plug different tube types into a guitar amp. If you must swap different power tubes, we strongly suggest you have a technician add a heater transformer to handle extra load. If you want to try 6V6s in a 6L6 amp, plate voltage and bias must be adjusted and a suitable output transformer should also be installed.
Many earlier Fender tube amps used rectifier tubes. These were almost always at the extreme left of the chassis, when looking at the rear of the amp. 5Y3s were seen in small tube amps, while large ones used 5AR4/GZ34s or 5U4s. Do not swap different rectifiers, as some consume more heater power than others or cannot handle the amp's plate voltages. Two rare Fender models used type 83 rectifiers, which are very scarce today. After 1966, most Fender tube amps switched to solid-state rectifiers, until recent Custom Shop reissue models reintroduced tube rectifiers.
By the way, in the past there were all kinds of suffixes on tubes--6L6, 6L6G, 6L6GA, 6L6GB, 6L6GC, etc. Some early Fender 6L6 amps, such as the early Bandmaster or Super models, can use any of these types. Later Tweed, Blackface and Silverface amps should not have 6L6, 6L6G, 6L6GA or 6L6GB installed, due to higher operating voltages and currents. In such later models, stick to higher rated types--6L6GC, 7581A, or KT66. Finally, 5881 meant a special high-quality tube in the past, but it no longer means much today. True NOS 5881s are scarce and costly. If you really want to know the differences, we recommend you consult past issues of Vacuum Tube Valley magazine. The exception is the re-issue Tung-Sol 5881. It is built to the original 5881 specs and is certainly high-quality.
In all cases, if you are not certain which amplifier version you have, it's best not to experiment! You might damage a vintage amplifier and its tubes. See an experienced technician, and get second opinions on major changes.
Click a Fender amp in this list to jump to its description below...
Tweed very early Champ
Tweed early Princeton
Tweed Harvard model 5F10
Tweed early 12AX7 Champs, Princeton models 5D2, 5E2, 5F2, and Harvard 6G10
Tweed Deluxe (early and 5C3 models)
Tweed Deluxe (5D3 and 5E3 models)
Tweed Vibrolux (5E11 and 5F11 models)
Tweed Tremolux (5E9-A and 5G9 models)
Tweed Bandmaster 5E7 and Super 5F4
Tweed Pro-Amp 5C5 and Super 5C4
Tweed Pro-Amp models 5D5, 5E5, 5E5A and Super models 5D4 and 5E4A
Tweed early Super and early Bassman
Tweed Bassman second version
Tweed Bassman models 5E6 and 5E6A
Tweed Bassman 5F6
Tweed Bassman 5F6A
Tweed Twin 5C8
Tweed Twin 5D8
Tweed Twin 5E8, 5E8A
Tweed Twin 5F8, 5F8A
Tweed very early Champ
Preamp tube is a 6SJ7 pentode, NOT a triode. Use types 6SJ7, 6SJ7GT, 6SE7, or 5693. Types 6SD7, 6SK7, 6SS7, or 6137 may be substituted in an emergency, but will give less gain and more distortion. DO NOT use other types. Be careful not to mix up the tubes--plug the right ones into the correct sockets.
Power tube (middle socket) is 6V6. Recommend using glass 6V6GTs. Current JJ/Tesla 6V6S and Electro-Harmonix 6V6EH are very good. NOS metal 6V6s run too hot. DO NOT use other types.
Rectifier tube is 5Y3GT. Avoid using NOS metal 5Y3s, only glass. You may also use 5V4 types or 5R4 types (if they fit physically). DO NOT use GZ34/5AR4 or 5U4. Inexpensive Russian 5Y3GTs are readily available but we have found they raise the plate voltage considerably and therefore do not recommend them in these vintage amps. Stick with NOS tubes such as the JAN-Philips 5Y3WGTA.
Tweed early Princeton
Preamp tube is a 6SC7 or 6SC7GT (in earliest versions) or 6SL7 (later version). Choice is limited to available NOS for 6SC7/SC7GT and to recent Russian-made 6SL7s. If you have a 6SC7 amp, do NOT attempt to substitute other types, as the 6SC7 has an unusual pinout. If you have a 6SL7 amp, use only types 6SL7, 5691, 6113, or 6188 in this socket.
Power tube (middle socket) is 6V6. Recommend using glass 6V6GTs. Current JJ/Tesla 6V6S and Electro-Harmonix 6V6EH are very good. NOS metal 6V6s run too hot. DO NOT use other types.
Rectifier tube is 5Y3GT. Avoid using NOS metal 5Y3s, only glass. You may also use 5V4 types or 5R4 types (if they fit physically). DO NOT use GZ34/5AR4 or 5U4. Inexpensive Russian 5Y3GTs are readily available but we have found they raise the plate voltage considerably and therefore do not recommend them in these vintage amps. Stick with NOS tubes such as the JAN-Philips 5Y3WGTA.
Tweed Harvard model 5F10
First tube is preamp, 6AT6 is specified. Only NOS is available. 6AQ6, 6AV6, 6BK6, 6BT6 may also be used, though the amp's sound may vary. Do not use other types.
Second tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. Some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6V6. Recommend using matched pair of glass 6V6GTs. Current JJ/Tesla 6V6S and Electro-Harmonix 6V6EH are very good. NOS metal 6V6s run too hot. DO NOT use other types.
Rectifier tube is 5Y3GT. Avoid using NOS metal 5Y3s, only glass. You may also use 5V4 types or 5R4 types (if they fit physically). DO NOT use GZ34/5AR4 or 5U4. Inexpensive Russian 5Y3GTs are readily available but we have found they raise the plate voltage considerably and therefore do not recommend them in these vintage amps. Stick with NOS tubes such as the JAN-Philips 5Y3WGTA.
Tweed early 12AX7 Champs, Princeton models 5D2, 5E2, 5F2, and Harvard 6G10
Preamp tube is a 12AX7. Any 12AX7 type will work here. Recommend a type with a coiled heater, such as NOS 7025s or ECC83s . Preferred Series 7025 and Tung-Sol 12AX7 are suitable. NOS 5751s recommended if lower gain is desired.
Power tube (middle socket) is 6V6. Recommend using glass 6V6GTs. Current JJ/Tesla 6V6S and Electro-Harmonix 6V6EH are very good. NOS metal 6V6s run too hot. DO NOT use other types
Rectifier tube is 5Y3GT. Avoid using NOS metal 5Y3s, only glass. You may also use 5V4 types or 5R4 types (if they fit physically). DO NOT use GZ34/5AR4 or 5U4. Inexpensive Russian 5Y3GTs are readily available but we have found they raise the plate voltage considerably and therefore do not recommend them in these vintage amps. Stick with NOS tubes such as the JAN-Philips 5Y3WGTA.
Tweed Deluxe (early and 5C3 models)
Preamp tubes are (2) x 6SC7 or 6SC7GT. First tube is preamp, second is phase inverter. Choice is limited to available NOS. Do NOT attempt to substitute other types as the 6SC7 has an unusual pinout.
Power tubes are a pair of 6V6. Recommend using matched pair of glass 6V6GTs. Current JJ/Tesla 6V6S and Electro-Harmonix 6V6EH are very good. NOS metal 6V6s run too hot. DO NOT use other types.
Rectifier tube is 5Y3GT. Avoid using NOS metal 5Y3s, only glass. You may also use 5V4 types or 5R4 types (if they fit physically). DO NOT use GZ34/5AR4 or 5U4. Inexpensive Russian 5Y3GTs are readily available but we have found they raise the plate voltage considerably and therefore do not recommend them in these vintage amps. Stick with NOS tubes such as the JAN-Philips 5Y3WGTA.
Tweed Deluxe (5D3 and 5E3 models)
First tube is preamp, 12AY7 is specified. 6072A may also be used. A low-noise, low-microphony 5751, 12AX7 or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Second tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6V6. Recommend using matched pair of glass 6V6GTs. Current JJ/Tesla 6V6S and Electro-Harmonix 6V6EH are very good. NOS metal 6V6s run too hot. DO NOT use other types.
Rectifier tube is 5Y3GT. Avoid using NOS metal 5Y3s, only glass. You may also use 5V4 types or 5R4 types (if they fit physically). DO NOT use GZ34/5AR4 or 5U4. Inexpensive Russian 5Y3GTs are readily available but we have found they raise the plate voltage considerably and therefore do not recommend them in these vintage amps. Stick with NOS tubes such as the JAN-Philips 5Y3WGTA.
Tweed Vibrolux (5E11 and 5F11 models)
First tube is preamp and tremolo (vibrato) oscillator, 12AX7 is specified. A low-noise, low-microphony 12AX7or 7025 is recommended.
Second tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6V6. Recommend using matched pair of glass 6V6GTs. Current JJ/Tesla 6V6S and Electro-Harmonix 6V6EH are very good. NOS metal 6V6s run too hot. DO NOT use other types.
Rectifier tube is 5Y3GT. Avoid using NOS metal 5Y3s, only glass. You may also use 5V4 types or 5R4 types (if they fit physically). DO NOT use GZ34/5AR4 or 5U4. Inexpensive Russian 5Y3GTs are readily available but we have found they raise the plate voltage considerably and therefore do not recommend them in these vintage amps. Stick with NOS tubes such as the JAN-Philips 5Y3WGTA.
Tweed Tremolux (5E9-A and 5G9 models)
First tube is preamp, 12AY7 is specified. 6072A may also be used. A low-noise, low-microphony 5751, 12AX7, or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Second tube is a 12AX7. This is the tremolo (vibrato) oscillator. Any 12AX7or 7025 is usable here--it need not be low-noise or low-hum, nor does it need to be tested for microphony.
Third tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6V6. Recommend using matched pair of glass 6V6GTs. Current JJ/Tesla 6V6S and Electro-Harmonix 6V6EH are very good. NOS metal 6V6s run too hot. DO NOT use other types.
Rectifier tube is 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed Bandmaster 5E7 and Super 5F4
First tube is preamp, 12AY7 is specified. 12AY7A may also be used. A low-noise, low-microphony 5751,12AX7, 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Second tube is a 12AX7. This is the tremolo (vibrato) oscillator. Any 12AX7 or 7025 is usable here--it need not be low-noise or low-hum, nor does it need to be tested for microphony.
Third tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6L6. Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tube is 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed Pro-Amp 5C5 and Super 5C4
First and second tubes are the two channel preamps, 6SC7 or 6SC7GT specified. Choice is limited to available NOS. Do NOT attempt to substitute other types! 6SC7 has an unusual pinout.
Third tube is the phase inverter. 6SC7 or 6SC7GT specified.
Power tubes are a pair of 6L6. Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tube is 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed Pro-Amp models 5D5, 5E5, 5E5A and Super models 5D4 and 5E4A
First and second tubes are channel preamp and driver, 12AY7 is specified. 6072A may also be used. A low-noise, low-microphony 5751, 12AX7 or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Third tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6L6. Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tube is 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed early Super and early Bassman
First and second tubes are channel preamp and driver. 6SJ7 pentode specified, NOT a triode. Use types 6SJ7, 6SJ7GT, 6SE7, or 5693. Types 6SD7, 6SK7, 6SS7, or 6137 may be substituted in an emergency, but will give less gain and more distortion. DO NOT use other types.
Third tube is the phase inverter. 6N7 specified. Only NOS is available. Types 6N7, 6N7G, 6N7GT usable. 6Y7G or 1635 may be used in an emergency, though amplifier performance may be degraded. Do NOT use other types.
Power tubes are a pair of 6L6. Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tube is 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed Bassman second version
Power tubes are a pair of 6L6. Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Third tube is the phase inverter. 6SL7 is specified. Choice is limited to available NOS and to recent Russian-made 6SL7s. Use only types 6SL7, 5691, 6113, or 6188 in this socket. Do NOT substitute other types.
Fourth tube is the preamp. 6SC7 or 6SC7GT specified. Choice is limited to available NOS. Do not attempt to substitute other types as the 6SC7 has an unusual pinout.
Rectifier tube is 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed Bassman models 5E6 and 5E6A
First and second tubes are channel preamps, 12AY7 is specified. 6072A may also be used. A low-noise, low-microphony 5751, 12AX7, 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Third tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6L6. Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tubes are a PAIR of 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed Bassman 5F6
First tube is preamp, 12AY7 is specified. 6072A may also be used. A low-noise, low-microphony 5751, 12AX7, or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Second tube is a 12AX7. This is the driver. Any 12AX7 or 7025 is usable here--it need not be low-noise or low-hum, nor does it need to be tested for microphony.
Third tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6L6 (5881s were originally specified). Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tube is type 83. This is a rare mercury-vapor rectifier. Only costly NOS is available. The similar type 83-V might work, although some technicians recommend against it. Warning: if an 83 rectifier is broken, there is a risk of mercury poisoning.
Tweed Bassman 5F6A
First tube is preamp, 12AY7 is specified. 6072A may also be used. A low- noise, low-microphony 5751, 12AX7, or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Second tube is a 12AX7. This is the driver. Any 12AX7 or 7025 is usable here--it need not be low-noise or low-hum, nor does it need to be tested for microphony.
Third tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6L6 (5881s were originally specified). Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tube is GZ34/5AR4. This tube should be of good quality. 5R4 types may also be used, although the amplifier will sound different. Do not attempt to use 5Y3, 5V4 or 5U4 types.
Tweed Twin 5C8
First and second tubes are the two channel preamps, 6SC7 or 6SC7GT specified. Choice is limited to available NOS. Do NOT attempt to substitute other types! 6SC7 has an unusual pinout.
Third tube is the mixer. A 6J5 is specified. Only NOS is available. Types 6J5, 6J5G, 6J5GT, 6C5, 6c5GT are suitable. Do not use types 6CA5GT, 6AE5GT, 6L5G, 6P5GT or others.
Fourth tube is the phase inverter. 6SC7 or 6SC7GT specified. Choice is limited to available NOS. Do NOT attempt to substitute other types! 6SC7 has an unusual pinout.
Power tubes are a pair of 6L6 (5881s were originally specified). Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tube is 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed Twin 5D8
First and second tubes are channel preamps, 12AY7 is specified. 6072A may also be used. A low-noise, low-microphony 5751, 12AX7, or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Third tube is the driver. 12AY7 is specified. 6072A may also be used. A 5751, 12AX7, 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Fourth tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6L6. Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tubes are a PAIR of 5Y3. Any 5Y3 type is acceptable. 5R4 and 5V4 types may be used in an emergency. DO NOT use GZ34/5AR4, or 5U4. Inexpensive Russian 5Y3GTs are readily available but we have found they raise the plate voltage considerably and therefore do not recommend them in these vintage amps. Stick with NOS tubes such as the JAN-Philips 5Y3WGTA.
Tweed Twin 5E8, 5E8A
First and second tubes are channel preamps, 12AY7 is specified. 6072A may also be used. A low-noise, low-microphony 5751, 12AX7 or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Third tube is the driver. 12AY7 is specified. 6072A may also be used. A 5751, 12AX7 or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Fourth tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a pair of 6L6. Recommend using matched pair of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tubes are a PAIR of 5U4. Any 5U4 type is acceptable. 5R4 types may be used in an emergency. DO NOT use GZ34/5AR4, 5V4 or 5Y3. Inexpensive Russian 5U4Gs are readily available and suitable.
Tweed Twin 5F8, 5F8A
First tube is channel preamp, 12AY7 is specified. 6072A may also be used. A low-noise, low-microphony 5751, 12AX7 or 7025 is also usable, though they will have more voltage gain and will change the sound of the amp considerably.
Second tube is the driver. 12AX7 specified. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable.
Third tube is a 12AX7. This is the phase inverter tube. This tube need not be low-noise or low-hum, nor does it need to be tested for microphony. It must have good quality heater insulation. 12AX7s or 7025s are suitable. Good NOS 12AT7s may also be used here. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.
Power tubes are a QUAD of 5881s (originally specified). Recommend using matched quad of 6L6GCs or NOS 6L6Gs, GAs, GBs, or WGB, or 5881s. Current Preferred Series 6L6GC and re-issue Tung-Sol 5881 are the best option in place of NOS tubes. Also acceptable are NOS JAN-Philips 6L6WGB, and NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types.
Rectifier tube type for the 5F8 is 83. This is a rare mercury-vapor rectifier that we no longer stock. The acceptable replacement for this tube is the 5Z3. The similar type 83-V might work, although some technicians recommend against it. Warning: if an 83 rectifier is broken, there is a risk of mercury poisoning so we no longer stock this product. The 5F8A circuit changed rectifier tubes to the GZ34/5AR4.