Marshall Amplifiers, Pre-1990

By Eric Barbour - Former Vacuum Tube Valley senior editor

What the tubes do in most Marshall tube amps

(excluding more recent "Master Volume" and JCM models):

The first preamp tube (or the first 2 tubes, in 2 channel amps) provide the first gain of the guitar signal. A low noise, low microphony 12AX7 or ECC83 tube is recommended here. 5751s are also usable, though voltage gain and loudness will be less. This is a matter of personal taste.

Most Marshall "lead" models have a second tube, which drives the tone controls. One of the triodes of this tube is usually operated as a cathode follower, which provides low impedance drive to the tone "stack". It may also be used in some 1960s lead models as a simple phase inverter. This means that triode's cathode operates more than 100 volts above ground. For this reason, a 12AX7/ECC83 with good quality heater- cathode insulation must be used. Surplus JAN 12AX7s tend to be superior to current production in this area.

Marshall put tremolo or vibrato on a few tube 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. Marshall rarely used tubes other than 12AX7s in preamp stages.

Reverb amps will 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 really needed. However, it should be a 12AX7 with plenty of gain. Marshall made very few amp models with reverb.

The last preamp tube before the power tubes is usually the "phase inverter". It makes the "see-saw" drive voltages that properly drive the output tubes. It was usually a 12AX7/ECC83. Early 200-watt models used a single 12AX7 triode as a phase inverter, followed with a 12AX7 used as a dual voltage-gain driver to drive the power tubes. Phase inverters in most Marshall models used a circuit (called a "long tailed pair") which also provided some voltage gain. 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.

The power tubes in most Marshall tube amps were EL34s. They drive the output transformer, then the speakers. Large 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. Most push-pull Marshall tube amps 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. This is not recommended for the average user, as the bias trimpot is inaccessible except by removing the chassis from the cabinet. 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. See a service technician about power tube replacement if you're unsure.

Marshall EL34 amps are very hard on EL34s. These tubes endure great stress on their screen grids at full playing volume. Marshall currently fits Svetlana EL34s to all its EL34 models, and recommends Svetlanas for retubing older amps. If you like to play very loud, and are having problems with EL34 reliability, there are only two reasonable courses of action: either pay for quality NOS EL34s, or else use beam tubes with aligned grids, such as 6550s or KT88s.

In the early 1980s, Marshall tube amps sold in the USA were equipped with 6550s instead of EL34s. These amps can accept either type of tube. This was a common modification done to various models in the 1980s and 1990s. Some hard-rock guitarists prefer the sound of 6550s (they break up at much higher volumes than EL34s). Also, traveling musicians have found that 6550s are more rugged than EL34s in guitar amps because the screen grids in 6550s endure less current draw (and thus, less heating) than EL34 screen grids. Be warned, guitar tube amps were not one of the intended applications of the EL34 (actually, no power tube was designed to take such a pounding). Consult a service technician!

A few early models, such as the JTM45 and 1962, used 6L6 type output tubes. They can also accept KT66s or 7581s. Marshall made a few very rare low-power models with ECL86s (10 watt Tremolo), EL84s (Artist 15, Artist 30, 18 Watt and 20 Watt models), and 6V6GTs (4001 Studio 15) as output tubes.

If your Marshall tube amp was intended for EL34s, do not try 6L6s, 6V6s, or similar types; you might damage the amp. These tube families are not recommended for use in a Marshall tube amp, as the output transformers are optimized to load EL34s and 6550s properly. It is very dangerous to blindly plug different tubes into a guitar amp.

If you have an early 200-watt model, such as the 1966, 1967 and 1978 "Major", the plate and screen voltage will be very high--these amps were designed with the original Genalex KT88 in mind. Many of today's KT88s cannot handle the high voltages. Recommend Svetlana KT88s, they seem to be the best currently available. Some later big amps, such as the model 2001 Bass (375 watts) and model 2000 Lead (250 watts) use either eight or six 6550 or KT88 types.

Recent JCM600, 800, 2000 and "Master Volume" amps have complex preamp circuits. Usually all the tubes therein are 12AX7/ECC83 types. The model 2001 uses two 12AT7s and three 12AX7s.

Marshall tube amps usually have solid-state rectifiers. Only a few early models, such as the original JTM45, used a tube rectifier. Solid-state rectifiers do not wear out as tubes do, nor can they be replaced by the user. The solid-state diodes used in older amps are rather primitive compared to modern devices--if you wish slightly better performance, suggest speaking to a service technician about replacing the diodes with modern fast-recovery epitaxial (FRED) diodes.

By the way, in the past EL34 /6CA7s did not come with suffixes. Modern tube dealers have invented names such as EL34B, EL34EH, EL34G, etc. for marketing purposes. These suffixes do not mean very much, in practical terms. Marshall currently likes the Svetlana EL34. If you really want to know the differences in different EL34s, we recommend you consult past issues of Vacuum Tube Valley magazine.

Click a Marshall tube amp in this list to jump to its description below...

Classic Models

Lead models 1959, 1959 Mk II, 1985, 1986, 1986 bass Mk II, 1987, 1987 lead Mk II, 1989, 1992, 1992 bass Mk II, 2203, 2204 50-watt, "Silver Jubilee" models 2550 through 2555

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the driver and cathode follower for the tone controls. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V3 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

Power tubes are 2 x EL34 in 50-watt models, 4 x EL34 in 100-watt models. The Mk II models were designed for 6550s, and can also accept KT88s or EL34s. Bias adjustment is MANDATORY.

Back to top

Lead models with tremolo 1959T, 1987T

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the tremolo oscillator. This position is not critical, though a 12AX7 with reasonably high gain is required. 12AT7s may not function here.

V3 is the driver and cathode follower for the tone controls. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V4 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

Power tubes are 2 x EL34 in 50-watt models, 4 x EL34 in 100-watt models. The Mk II models were designed for 6550s, and can also accept KT88s or EL34s. Bias adjustment is MANDATORY.

Back to top

200-watt models "Major", 1967, 1978

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the driver and phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position.

V3 is the driver for the power tubes. It is a 12AU7/ ECC82, NOT a 12AX7. A good quality 12AU7 (such as premium NOS) is recommended, as this tube handles high plate and signal voltages.

Power tubes are 4 x KT88s. These MUST be high quality and capable of handling high screen dissipation. Do NOT attempt to use EL34s, 6550s or 6L6s in these models. Bias adjustment is MANDATORY.

Back to top

200-watt 8-input model 1966 and PA variants of 1976 and "Major"

V1 and V2 are the first preamps. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V3 is the driver and phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V4 is the driver for the power tubes. It is a 12AU7/ ECC82, NOT a 12AX7. A good quality 12AU7 (such as premium NOS) is recommended, as this tube handles high plate and signal voltages.

Power tubes are 4 x KT88s. These MUST be high quality and capable of handling high screen dissipation. Do NOT attempt to use EL34s, 6550s or 6L6s in these models. Bias adjustment is MANDATORY.

Back to top

JTM45 early version

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 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.

V3 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 5881s. Current TAD 6L6GC-STR and WGC-STR, or Tung-Sol 5881 are acceptable, as are NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types. Bias adjustment is MANDATORY.

Rectifier tube is 5AR4/ GZ34. 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.

JTM45 later/reissue version

Back to top

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the driver and cathode follower for the tone controls. A low noise tube is not critical, however the heater- cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V3 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. 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 5881s. Current TAD 6L6GC-STR and WGC-STR, or Tung-Sol 5881 are acceptable, as are NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types. Bias adjustment is MANDATORY.

Rectifier tube is 5AR4/ GZ34. 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.

Back to top

JTM45 8-input PA version

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the driver and cathode follower for the tone controls. A low noise tube is not critical, however the heater- cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V3 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. 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 5881s. Current TAD 6L6GC-STR and WGC-STR, or Tung-Sol 5881 are acceptable, as are NOS types 7581A, KT66 or 350B. NOS metal 6L6s run too hot. DO NOT use other types. Bias adjustment is MANDATORY.

Rectifier tube is 5AR4/ GZ34. 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.

Back to top

18-watt and 20-watt amplifiers (lead, bass, and PA) models 1917, 1958, 1973, 1974, 2019, 2022, 2022T, 2061

V1 is the preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 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 EL84/ 6BQ5. Good quality versions are acceptable. There is no bias adjustment in these amplifiers.

A few early models used a 6CA4/ EZ81rectifier tube. Do not substitute other types.

Back to top

"Popular" model 1930

V1 is the preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 and V3 are the phase inverter and output tubes. They are 6GW8/ ECL86 triode/pentodes. Only NOS and recent Ei versions are available. Do NOT substitute other types. This amp does not have a bias adjustment.

Back to top

"Studio 15" model 4001

V1 is the preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 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 6V6GTs. NOS versions are recommended. Types 6EY6 and 6EZ5 are not recommended as their heaters draw more current than the power transformer is designed for. Bias is self biasing (cathode bias).

Back to top

"Artiste" models 2040, 2048, 2059, 2068, 2069

V1 and V2 are the first preamps. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V3 is the reverb driver and recovery amplifier. A low noise tube is not critical. This position is very hard on the cathode, due to high current. NOS JAN 12AX7s are recommended for this position.

V4 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

Power tubes are 2 x EL34 in 50-watt models, 4 x EL34 in 100-watt models 2059 and 2068. Bias adjustment is MANDATORY.

Back to top

"Specialist" model 2046

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V3 is the reverb driver and recovery amplifier. It is a 6GW8/ ECL86 triode/pentode. Only NOS and recent Ei versions are available. Do NOT substitute other types.

Power tubes are 2 x EL34 in 50-watt models, 4 x EL34 in 100-watt models 2059 and 2068. Bias adjustment is MANDATORY.

Back to top

Rackmount amp model 9005

V1 is the preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 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 2 x EL34. Bias adjustment is MANDATORY.

Back to top

JCM800 MODELS

Lead models 1959, 1986, 1986 bass, 1987, 1992 lead, 2103, 2104, 2203, 2204, 4010, 4103, 4104

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. JJ ECC83s and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the driver and cathode follower for the tone controls. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN ECC83/12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V3 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

Power tubes are 2 x EL34 or 6550 in 50-watt models, 4 x EL34 or 6550 in 100-watt models. Bias adjustment is MANDATORY.

Back to top

Lead models 2205, 2210, 4210, 4211, 4212

V1 is the first preamp. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the driver and cathode follower for the tone controls. A low noise tube is not critical, however the heater- cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V3 is the mix amplifier after the master volume control. A low noise tube is not needed here. Any 12AX7 of good quality will work, although guitarists picky about their tone may prefer a top-grade NOS version.

V4 is the reverb driver and recovery amplifier. A low noise tube is not critical. This position is very hard on the cathode, due to high current. NOS JAN 12AX7s are recommended for this position.

V5 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

Power tubes are 2 x EL34 in 50-watt models, 4 x EL34 in 100-watt models (2210, 4211). These models were designed for 6550s, and can also accept KT88s or EL34s. Bias adjustment is MANDATORY.

Back to top

Models 4140, 4145 Reverb

V1 is the preamp for the first (main) channel. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the reverb driver and recovery amplifier. A low noise tube is not critical. This position is very hard on the cathode, due to high current. NOS JAN 12AX7s are recommended for this position.

V3 is the amplifier after the tone control and the predriver before the phase inverter. A low noise tube is not needed here. Any 12AX7 of good quality will work, although guitarists picky about their tone may prefer a top-grade NOS version.

V6 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V7 is the preamp for the second channel. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

Power tubes (V4, V5, V8, V9) are 4 x EL34 or 6550. These models were designed for 6550s, and can also accept KT88s or EL34s. Greater reliability will be had with aligned-grid tubes such as 6550s, KT77s or KT88s. Bias adjustment is MANDATORY.

Back to top

The 2000 Series (Not to be mistaken for the JCM2000 Series)

Lead model 2000

V1 is the first preamp for A channel. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the driver and cathode follower for the tone controls in A channel. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V3 is the post-driver and cathode follower for the tone controls. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V4 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V5 is the first preamp for B channel. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V6 is the driver and cathode follower for the tone controls in B channel. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

Power tubes are 6 x 6550. KT88s are also usable. EL34s are not recommended. Bias adjustment is MANDATORY.

Back to top

Bass model 2001

V1 is the first preamp for A channel. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V2 is the driver and cathode follower for the tone controls in A channel. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V3 is the phase inverter. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

V4 is the first preamp for B channel. Recommend a low-noise type with a coiled heater, such as NOS 7025s or ECC83s. Preferred Series 7025 and Sovtek 12AX7LPSs or EHs are suitable. NOS 5751s recommended if lower gain is desired.

V5 is the driver and cathode follower for the tone controls in B channel. A low noise tube is not critical, however the heater-cathode insulation must be good quality. NOS JAN 12AX7s are recommended for this position. We understand that some technicians recommend Ei or Preferred Series 7025 for this duty.

Power tubes are 8 x 6550. KT88s are also usable. EL34s are not recommended. Bias adjustment is MANDATORY.