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Tube cross-referencing: what a tube amp-playing bassist needs to know


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It is established that tube bass amps are in a class of their own. Amps like the Mesa Bass 400+, Ampeg SVT line, and Fender Bassman 300 set the standard in their class. But let's say you buy one of these amps, rock out on it...and a year and half to 2 years later you need to retube. What then? You might know your stuff about tubes, you might not. Generally bassists are not as tube-savvy as guitarists because bassists can get along just fine on solid-state amps, whereas guitarists need tube amps so they don't sound like bees in a tin can. Hence, they need at least baseline tube knowledge. So, my aim with this thread is to help out those who will at some point need new tubes.


The majority of bass amps use a large(for instrument amplification) beam tetrode tube called the 6550. This is because they deliver a lot of power, 100 watts per pair. Examples are Hiwatt DR405, Ampeg SVT-CL, Aguilar DB728, Fender Bassman 300, Orange AD200B, George-Dennis Bassic Tube 200 and Trace Elliot V8. The earliest SVT's used 6146 tubes; they were changed to 6550 after the second year of production.


Some bass amps use the medium-sized 6L6 tetrode. This tube does not dissapate as much power as the 6550, but it has a somewhat different sound. Some people like to thinkof the 6550 as a big 6L6. Examples: Pignose B100V, old Fender Bassman amps like the 50, 100 and 135, Matamp Bass 120(i think), and most notably the Mesa Bass 400+.


There is one bass amp currently in production that uses EL34 tubes; this is the George-Dennis Bassic Tube 120. some old Marshall bass amps like Bass Major, Superbass, etc also used EL34.


One special case is the Hiwatt DR201. since its inception in 1969 it used a tube called the EL36, also known as 6CM5. This was another beam power tetrode whose power output was somewhere between the EL34 and KT88. in 1978, the DR201 switched to KT88, which meant it could theoretically push 300 watts, but since the original design was rated for 200, it is doubtful that designer Dave Reeves uprated it without calling it the DR301.


However, that's only for the power section of the amplifier. We also have the preamp to deal with. The vast majority of guitar and bass amps use the 12AX7 tube, a fairly high-gain preamp tube. For our purposes, we will simply say they all use 12AX7s, even though the old SVT's actually used the 12DW7. Most preamp tube positions in a tube bass amp will use a 12AX7. however, sometimes for the phase inversion tube the manufacturer will use a variant of the 12AX7, usually a 12AU7 or 12AT7. These are lower-gain tubes than 12AX7's, but can be substituted for a 12AX7 if needed, though this is uncommon on bass amps. These lower gain tubes look visually similar to 12AX7's, so you need to look at the label.


Here is a listing of gain ratings for preamp tubes:


12AX7: 100/100


5751: 70/100


12AT7: 60/100


12AY7: 45/100


12AV7: 41/100


12AU7: 19/100


A special note for the Phase Inverter tube: you should specify a "matched" or "balanced" PI tube if available! the reason is that since big-power amplifiers run push-pull, each 'side' of the amplifier actually only runs 1/2 the time, amplifying one half of the Sine wave. the phase inverter tube is the one that splits the signal into those two halves for the first and second banks of tubes to amplify. if the two sides recieve a signal that is not the same on both sides, it will sound kind of unbalanced. it won't kill you or your sound, but then, it won't kill you to spend the extra $3 on a balanced tube either. you will have a much clearer sound. if you can hear some notes sustain, but not others, your probably have an unbalanced phase inverter, and not the dead spots in your bass that you might suspect. replace it with a balanced one, but don't throw out the existing one! you may be able to use it somewhere else, or keep it for a backup.


The names for any of the tubes I just talked about are not absolute, and now we're coming to the main point of this post. A tube sold under one designation in North America, example 6BQ5 will be the same tube in Europe, but the name will be different, i.e. EL84. This is important to know because it really opens up a variety of tubes to choose from. With this knowledge, you can shape your sound in a manner unachievable just by EQing. Below, I will list tubes that are the same, and tubes that are interchangeable. Standard text denotes tubes that are the same, bold shows interchangeable tubes.


Also, you may find this article about tubes an excellent read.


6L6: 7581, 5881, 1622, 5932, WT6, EL37, KT66


6550: KT88, KT90, KT99, KT100


EL34: 6CA7, E34L, KT77


EL84: 6BQ5, 7189, 7320


12AX7: ECC83, E83CC, ECC803, 7025, M8137, 6057, 6681, 7494, 7729, CV4004, 5751(70% gain version of 12AX7), 12AT7(lower gain), 12AU7(much lower gain)


12AU7: ECC82, E82CC, ECC802, 5814, 5963, 6067, 6189, 6680, 7316, 7489, 7730, 12AV7(higher gain)


12AT7: ECC81, E81CC, ECC801, CV4024, 6060, 6201, 6671, 6679, 7492, 7728, A2900, 5751(higher gain), 12AY7(lower gain)


Hope I helped.


Below are some pictures of tubes, they may help you out if you can't determine what kind you have.


12AX7, 12AU7, 12AT7, 5751, 12DW7, et al look like this:






































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