In article , Ken Harris writes:
> I’m getting confused on tubes. Would it be possible
> for someone to list the different types of tubes,
> both preamp and power amp tubes? (OR are there
> too many?)
>
> Creating this confustion, I took my
> old preamps tubes in, they were 12AX7As, and
> they guy gave my 12AX7s, saying there was no
> difference. The power tubes I think were 6L6s
> or something, and he gave me ones with a four
> digit code. They worked fine, but left me
> confused.
>
> Any help in this would be appreciated. Is there
> a FAQ covering this kind of stuff?

Tubes were made in a bewildering array of types, just as transistors were
in the golden age of transistors. And since many of these differed only
trivially, they could be substituted for one another in many cases. As
manufacturers noticed this, they began manufacturing fewer types which
were labeled with different numbers, but which could work at least passably
as the different types. (gosh, how’s that for a fast, broad brush?)

Today, there are very few tube lines in operation, and even before the
US and Euro lines shut down, the number of different tubes contracted
drastically, until today, any difference between say a 12AX7 and a 7025
is academic or accidental. They may well have come from the same tube
line, and just be labelled differently. This is one reason people search
out new-old-stock (NOS) tubes, to actually get a quieter 7025.

The following is a run down of audio tube types. I’ll probably miss a few
and make a mistake or two, but it should give the flavor.

Class – twin triodes

Primary member – 12AX7 and substitutes/equivalents
12AX7 – original high gain twin triode, amp factor 100
12AX7A- “improved” 12AX7, better specs on some things
ECC83 – Euro designation for a 12AX7 compatible twin triode
7025 – originally an industrial number for a lower noise/hum 12AX7
5751 – originally a military special type that is slightly lower gain,
but sturdier, less microphonic

Other members
12AT7, 12AU7, 12AY7, ECC82, many others; lower gain, higher current
twin triodes with a type “9a” pinout. These are used where lower gain
or wider frequency response, or higher current is needed.
6EU7 and clones – high gain twin triode, functionally similar to 12AX7,
different pinout and some parameters.
6DJ8, 6922, and clones; high transconductance twin triode intended for
radio use but used in some hifi audio stuff

Class – power output tubes

6L6 and its other variants – the “standard” beam power tube for Fenders
and many others; a pair can put out 50 watts if they are pushed to the
edge; 6L6 is an old design, but became a standard
5881 – a “military” type like the 6L6, may be directly substituted
EL34/6CA7 – Euro/US versions of a true power pentode roughly similar to
the 6L6 in power output, but not directly pin or circuit compatible
without modifications
6550 – an industrial power pentode, higher voltage and power output than
6L6 types; used in the SVT
7027/7027A – yet another power tube slightly higher ratings than 6L6, not
directly plug or circuit compatible; used in old Ampeg amps.
8417 – late design (60’s) power tube with high transconductance, low Rp,
suitable for high power, easier design of output transformers. never
produced in high quantity because transistors came along.
6V6 – Smaller output tube than 6L6, produces 15-20 watts when pushed. 6L6
can be sub’ed into a 6V6 socket, but needs rebiased to work right.
6V6 into a 6L6 socket kills the 6V6.
6AQ5 – mini-envelope version of 6V6, same characteristics but lower power
and voltage ratings. Used only in old, obscure guitar amps, but sounds
good.
EL84/6BQ5/7189 – mini-envelope pentode, produces 20-30 watts when pushed;
famous for use in quads in the Vox AC-30

Class – miscellneous tubes
7199 – combined pentode/triode in one envelope. Used as an amplifier/phase
splitter in many hifi amps, lots of old Ampeg, Gibson, other guitar amps
6AN8 – different pinout, same discription
6U8 – ditto
6AU6 – high gain pentode used as an audio amp in some older gear

Can anyone help me with making this more accurate and inclusive? I’ll
include it in the tube amp mini-faq.

R.G.

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