From howard–(at)–aiwan.com Tue May 30 22:42:37 CDT 1995
Article: 1481 of alt.guitar.amps
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From: howard–(at)–aiwan.com (Howard Christeller)
Newsgroups: alt.guitar.amps
Subject: Re: Tube questions
Date: 30 May 1995 08:33:36 -0700
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In article <3q8s07$j8--(at)--anix2.panix.com>,
mgarvi–(at)–anix.com (Mark Garvin) wrote:
> In <3q81oi$g6--(at)--ver.mhv.net> Dr.Distortio–(at)–bs.mhv.net (Dr Distortion) writes:
>
> >The “G” stood for glass envelope. B, C and so on indicated upgrades. How
> >come there was never a 6L6GA? I have no idea… I believe the
>
> > We could probably get more definitive answers to these questions on the
> >newsgroup rec.antiques.radio+phono. Those guys are real knowledgeable
> >about ancient electronics lore.
>
> Yeah, it would be nice. Unfortunately such questions are frowned
> upon there. I hate moderated news groups.
r.a.r+p is not moderated. It is a good source, and one that I read,
even though I am not a radio collector. Mark it “read-only” 😉
The tube numbering scheme used by US companies comes from the Radio
Manufacturer’s Association. It consists of a number, a one or two
letter type designation, another number, and suffixes.
The first number indicates, very roughly, the filament voltage. If
there is a center tap, the series voltage is used. For example, a
12AX7 may used with 12.6V or 6.3V connections.
The type designator is a sequentially assigned letter, or pair of
letters. Other than the fact that they started assigning rectifiers
in reverse order, the letter doesn’t mean anything. When they ran
out of single letters, they used two. In some cases, if the first
letter is L, then that means a locking-base type. If the first
letter is S, it may mean a single-ended (using pins on one end only)
version of a tube which has a plate or grid cap.
The second number is the number of useful elements. Elements which
are internally connected count as one. Using the 12AX7 again, there
are two cathodes, two grids, two plates, and a heater. If you
wonder what the six elements of a 6L6 are, they are the cathode,
grid, screen, plate, heater, and shield. The shield is the metal
envelope of the original 6L6. Glass versions of the 6L6 were not
renumbered, in spite of not having the sixth element, because that
would confuse the replacement issue.
Suffixes A, B, C, D, E, and F indicate a newer version which can
replace any previous version. G indicates a large glass bulb, and
GT a small, straight-sided glass bulb (T9). W indicates an
industrial or military version. An industrial version may be more
rugged, have lower noise, or a longer life. The reason for the
industrial version may or may not be something that you care about.
X and Y mean low-loss or medium-loss bases, which affect RF
performance.
The European numbering system has been described on r.a.r+p, and I
can check my files if anyone is interested.
The two, three, and four digit industrial numbers have no meaning,
except perhaps the order of design. Some industrial types are based
on standard types, but with different specs. They are similar, but
may not be suitable replacements.
GE ETRM-15N lists these industrial variations on the 6L6:
6L6 1622
6L6G 5881, 5932
6L6GC 7581, 7581A
The 6L6 family:
TYPE BASE OUTLINE PLATE PLATE SCREEN SCREEN
WATTS VOLTS WATTS VOLTS
6L6 7AC 10-1 19 360 2.5 270 DESIGN CENTER
6L6-G 16-3
6L6-GA 14-3
6L6-GB 12-15
6L6-GC 12-15 30 500 5.0 450 DESIGN MAX
Outline Drawing 10-1 shows a small-wafer octal base with a metal
MT10 envelope. 16-3 shows a medium-shell octal base with a glass
ST16 bulb. 14-3 uses a smaller ST14 bulb. 12-15 uses the
straight-sided T12 bulb. The ST bulbs are the “Coke-bottle” shape.
The RMA Design Center ratings for power line operated equipment
assume a 10% power line variation. In 1957 the RMA switched to the
Design Max ratings and eliminated the power supply tolerance. Thus,
the Design Center ratings are about 10% lower than the Design Max
ratings. The 5881 Design Max ratings of 21W & 400V are equivalent
to the Design Center ratings of 19W & 360V of the 6L6G it is derived
from.
So the 6L6 started as a metal tube, went to a large Coke bottle (G),
a small Coke bottle (GA), and then the straight-sided bulb (GB).
Finally, the ratings were increased (GC). Rugged (W) versions exist.
NOS tubes follow this naming scheme. Some distributors screw up, so
you get bullshit like a Coke bottle labeled 6L6GC, which it can’t be.
—
Howard Christeller howard–(at)–aiwan.com Irvine, California
