Explaining Marshall Tubes in early amps

[surfnorthwest]

Preamp

12AX7 (7025, ECC83) - The 12AX7 (US Designation) is a very popular 9 pin twin triode preamp tube. This tube was used nearly exclusively throughout the preamp circuit in the early Marshall amps. The 12AX7 was used in the early gain stage(s), tone control stage and in the phase inverter circuit. The 7025 is a high grade version of the 12AX7. The European Designation for the 12AX7 is ECC83.



Power Amp Tubes

6L6 (KT66/5881) - The 6L6, KT66 and 5881 are from the same family of tubes. The majority of the JTM amplifiers were supplied with power amp tubes of this group. The 6L6 (US Designation) is a very popular power amp tube and is also found in many of the larger Fender amps. As used in the JTM 45 applications, a pair of 6L6GC tubes can deliver around 45 watts or slightly more.



The 5881 (US Designation) is an upgraded (Military version) of the 6L6 tube.



The KT66 (European Designation) falls in the same family as the 6L6. The KT66 was manufactured by GEC of England; however these tubes have not been in production by the original manufacturer for more than a couple of decades. The KT66 is technically known as a Beam Tetrode Tube. Many guitarist and audiophiles consider the original GEC KT66 tubes as the "holy grail" of power amp tubes. Today NOS GEC KT66 tubes command very high prices. Marshall rated the JTM with a pair of KT66s at 30 watts, however a fully cranked Marshall with a pair of GEC KT66s can put out closer to 40 watts. There are several current tube manufactures offering their version of the KT66 tube.



While the 6L6, 5881 and KT66 tubes are from the same family, each type provides its own unique tone. These tubes as used in the JTM amplifiers are known for their warm blues tones and excellent classic rock overdriven tone.



EL34 (KT77) – The EL34 (European Designation) was used in the JTM 50 amplifiers and throughout the JMP amplifiers series. The EL34 has been the primary power amp tube from the late 60s through today for the Marshall amplifiers. One major difference in tone between the JTM and the JMP amplifiers is the use of the EL34 tubes. These tubes provide a very tight distortion and the famous Marshall "crunch". A pair of EL34 tubes is typically rated at 50 watts; however 60 watts or more can be achieved. The EL34 is a popular choice for hard rock and heavy metal applications.



6550 (KT88) - Earlier JMP amplifiers delivered to the US were equipped with EL34 tubes, however later versions to the US were provided with the 6550 (US Designation) tubes. The 6550 provides a loud clean tone and is also popular in Bass amplifiers. When cranked up, the 6550 provides a very tight crunch tone, which is popular with heavy metal players.



EL84 (6BQ5) – The EL84 (European Designation) tube was used in the Marshall 18 and Marshall 20 watt amplifiers as well as the VOX AC15 and AC30. These tubes are very popular today in mid sized amplifiers. The EL84 provides a warm blues break up and very nice overdriven tone, with plenty of harmonics.



Rectifier Tubes

5AR4 (GZ34) – The 5AR4 (US Designation) is a very widely used rectifier tube and is found in the Marshall JTM amps, large wattage Fender amps, VOX amps, as well as many other tube amplifiers.



EZ81 - The EZ81 (European Designation) was used in the Marshall 18 watt amps and can also be found in the VOX AC15 and other amplifiers in the 20 watt range.

[washburn 1]

Hi Surf`
As always, very interesting and descriptive. Just a thought, I have an MA50h into two 1936s ( one vintage ) and am generally fairly happy with it. These things CAN produce quite a good tone if set CORRECTLY and are pretty variable . As with all amps of all breeds/designs they can sound crap if you dont know how to manipulate them! While I appreciate it is NOT QUITE a JVM etc I do wish people would spend time and learn the equipment they are using, owned or not. (sorrrry, didnt mean to rant)

Anyway, your post leads me to ask,

CAN, el34s be removed and 5881s or 6550s inserted at least as an experiment, for tonal differences. ( I was led to believe that 5s and 6s were of the same family but one was a ruggedised version ). I understand that there may be bias differences, but, if the bias pots can cope with the extra adjustment ( and there seems to be a lot of varible bias when doing the `34s ) could this actually work or MUST there be circuit mods done also? could it endanger the amp?

I ask as, I have one of the modern Sound City SC50h heads which uses 5881s and it has a very big open sound much `bigger`(not a bad amp at all!! ) than the MA and I would be interested to see what difference the 5881s might make once transplanted . Could be quite useful for the amp`s tone. Any thoughts would be great . Thanks mate. All the best, Kev.