During part 1, we talked about how a tube works, discussed about NOS and New Production tubes gap and, named the few plants that are currently making tubes for guitar and bass amps.
In part 2, we talked about some of the technicals characteristics that make a tube different to other, among with which of those characteristics are more interesting for us, guitarists, when searching a tube and, we talked about what tube resellers do for us.
Pre-amp tubes: the 12AX7 family
Probably, the tube type that more confusion generates in all us is the 12AX7 type.
The type was produced first in USA and therefore, the original name is 12AX7.
Some specific models were designed to optimize some aspect, while not changing anything else under the 12AX7 specification.
By example, the 12AX7A was a tube made to have low microphonics, but it's a 12AX7 all the way.
Currently, there are some versions named 12AX7B that have a tad more gain and lower microphonics than a 12AX7 and, they are good for very demanding high gain front ends.
They were made lower gain versions, with less noise, named 12AX7WA, WB and WC.
But, if you read 12AX7, followed with one or more typos, we have a 12AX7 with some special characteristic but, that perfectly can replace any other 12AX7 tube in your amp.
Then, we have the military version of the 12AX7, named the 5751. Has a better transconductancy, a lower gain factor and, it's made to last in a military environment. Their specifications deviate slightly from the 12AX7 tube but, are good enough to be considered closer to 12AX7 than a 12AT7, by example.
SRV was mounting one 5751 in its V1 position. Usually, it works very well in 12AX7 PI slots, to maintain clearer the power amp and achieve more headroom.
The 7025 was a "lowest microphonics" version of a 12AX7 and, it's more suitable for Hi-Fi applications. Has slightly more gain than a 12AX7 and, can sound a tad harsh (to my ears) but, helps to tame noise and microphonics issues in V1.
Then, Europe was producing a tube equivalent to the 12AX7 and, that was named the ECC83.
ECC83S, ECC803 and ECC803S were the "high quality" "low microphonics" versions of such a tube (basically, made by Telefunken).
Mullard had the military equivalent CV4004.
There were other tubes equivalents (as 12AD7, 12DM7, 12DT7, 12DF7, 7729, 6681 and 7058, by example), but those are more difficult to see in a guitar amp.
A 12AU7 has more or less same tone than a 12AX7 and similar technical characteristics but, gain factor drops to 40 (instead of the 100 of a 12AX7). It's European equivalent is the ECC82.
The 12AT7 has more or less same transconductancy as the 5751 and similar gain but, technical characteristics are very different and, very specially the current drain, which is about 10 times the 12AX7's one. Even that there is people that loves to throw a 12AT7 in the slot of a 12AX7, it makes more sense to use a 5751 instead, if want you wanted is to raise the amp's headroom. Both, 12AT7 and 5751 sound very Hi-Fi in tone positions. The European equivalent is the ECC81 (I think that Mullard made it also with the CV4001 name).
The 12AY7 has a different tone respect of the 12AX7, with more basses and a mushy distortion type, instead of crunchy. It was mounted in the very first Tweeds.
And, those are the most usual valves that you can see in a guitar amp preamp.
Other pre-amp tubes in guitar amps
One early tube that was used was the EF86, not compatible with 12AX7 specifications, and that can be in some retakes of vintage amps, mostly in boutique amps (by example, Dr. Z KT-45). The American equivalent is the 6267.
While the American pre-amp tubes had their equivalents in Europe, very close from a sonically point of view, and even more close respect of specifications, the power tubes produced in one or the other side of the Ocean were completely different.
Overall, the American tubes have more basses, while European tubes have more mids. American tubes have a creamy distortion, while European tubes have a crunchy distortion. This is very inaccurate and, we need to go in details but, you get the overall picture.
The most famous American tubes are the 6V6 and the 6L6GC. The 6L6GC is the powerfuler of both and has more balls. Both have tight basses.
The most famous European tubes are the EL84 and the EL34. The EL84 is mellower than the EL34, more powerful, with high projection in mids. The EL34 is Marshall's sound, while the EL84 is Vox' sound (simplifying things a lot!).
But, there were produced other tubes that combined both characters. The KT66, KT77 and KT88 are some kind of combination of the best characteristics of both trends. Those have a lot of fans, also.
Equivalents to the 6V6 were 6V6G, 6V6GT, 6V6GTY, 5992 and 7408. The first three changed in the design of the bottle or the socket. The 5992 was a very good sounding and sturdy tube. The 7408 was an industrial version of the 6V6GT and, not so interesting for a guitar amp.
Equivalents to the 6L6GC were the 7581 and the European KT66. The 7581A had a higher plate voltage.
Equivalents to the EL84 were the American 6BQ5, the E84L / 7320 (Premium EL84s) and the 7189.
Equivalent to the EL34 is the American 6CA7.
Equivalent to KT88 is the 6550.
Tubes and their position in pre-amp
Usually, all pre-amp tubes are auto-biases so, you can safely swap one with a new one (while respecting equivalences) and test the changes. You don't need to BIAS them.
The most important position, that we will call V1, is the corresponding to the very first tube that amplifies the signal of our guitar. Since this is the beginning of the gain cascade, we need a tube with very low noise and very low microphonics issues. We will need a good output if the amp has a complex front end. Gain will depend on if we can krank the amp or not. If we want lower loudness, we need a gainy tube. If we want more headroom at higher loudness, we need a less gainy tube.
Other tone shaper positions (V2...)
In an amp that has more than one tube responsible for the tone (usually, amps with two or more channels), this positions are also very important tone-wise. Respect of noise and microphonics, those positions aren't so exigent as V1 but, still important enough. Gain and output aren't so critical, maybe, just in the case that each tube drives a separated channel. Usually, to mount a tube of the same brand/model as in V1 works fine but, you can test other combinations and, choose the one that better sounds to you.
Cathode Follower positions
Usually, the tube that drives the Tone Stack and the FX Loop are working in a Cathode Follower mode. They are not directly responsible for the tone and, therefore, tone-wise aren't critical but, they are critical for their specific function (to recover gain after the tone stack or the FX loop) and, a tube with correct output and gain are required. In Cathode Follower positions, tubes with spiral filament often have issues so, it's better to use other kind of tubes here. The types 12AX7WA, 12AX7WB and 12AX7WC work very well there.
Driver / Phase inverter position
This position, even not being a tone-shaper one, is critical for the sense of immediateness of the amp and to select the break up point of the power amp. Is usually the last tube of the pre-amp stage but, it's more related to the power amp than to the pre-amp itself and, it's recommended to swap this tube when you swap the power tubes.
What we want here is a tube with high transconductancy, to get immediate response with speedy riffs. We want a nice output also. Gain depends on what we want to achieve. If we want the power tubes to distort early, at lower volumes, we need a gainy tube. If we want the power tubes to have more headroom, at higher volumes, we need a less gainy tube.
Tubes that can work well here are: a 5751 (for 12AX7 sockets and more headroom desired), Sovtek 12AX7-LPS and, Sovtek 12AX7WC (usually, specifications-correct, with very balanced triodes).