Last updated 24 JUL 03
Reviews of Beam Pentodes
Included are:
6L6   -  5881   -   KT-66   -  6V6
Don't mix up the tubes if you came here via one of the forums on the Internet that
linked to this location as there are a number of referrals from there.
Some of the following reviews are coutesy of Watford Valves in the U.K.  My personal reviews are
posted on a quarterly basis on the music forums on Harmony and Music  My
reviews are more technical in nature and a bit more "black and white", so these reviews by Watford
Valves are really nice when you want some "tone" viewpoints.  I have to say, I cannot disagree with
these folks.  There is a link at the end of the reviews to their website.

Technical reviews are done also in this section by Guitar Amplifier Blueprinting

Groove Tubes has a new AMERICAN 6L6 that is made by them.  I saw the components at the NAMM
show, right down to the grids, plates, pins, etc.  It is a gorgeous tube.  I think the price is going to be
$90.00 per duet and $180.00 per quad.  The workmanship and materials are really A+++, and I cannot
wait to get my hands of some of these to test.  Groove Tubes will probably be the only place to get
these initially, but I am not sure of that at this time.  Projected delivery for the first tubes are before
this summer.

Update - as of early March, I was able to procure this tube and it has just completed our testing.  
Please scroll down for test report.
August 2002 new information: The Watford Valves reviews below are a bit on the old side.  
There are comments in them in regard to Chinese tubes.  In the last 4-6 months, there have been
changes in tooling and manufacturing in China.  What I am finding in testing, is that the quality is on
the rise, big time, so keep an eye out on the Chinese tubes.

The latest 6L6C's are holding up to higher voltages, and now have an output that is right up there
with the Russian tubes.

The latest generation 12AX7C with the octagon spacer, is much more consistant than any from the
past, and may be behaps the most versitile newly made 12AX7.  I have detailed reports on the entire
family of new 12AX7's from China, Europe, and Russia if folks are interested.
The 6V6 output tube ... used in some of the finest Blues Amps ever made.  
Unfortunately, there are not too many choices today for newly made tubes.  It
seems that NOS is still the only viable option at this time, but some of the newer
offerings are closing the gap!


The 6V6GT is one of my favourite valves. So many modern amps that use this valve do not have the
tone of those classic tweed deluxes. We set out to find out why.


The amplifier used was a Fender princeton reverb 2 Paul Rivera model.  This was chosen as it is a
good amp to evaluate 6V6 and it also can supply some very crunchy modern sounds as well as
vintage. Guitars used where a 1973 Fender stratocaster, 1980 Yamaha SA 2000S semi acoustic and a
1980 Gibson Les Paul Standard.

These were used as the reference and seem at home with single coils or humbuckers. They
produced a fat controlled bass with a strong mid character. The valve has great balance and notes
can be easily distinguished even under heavy distortion. Every slight variation of tone was
accurately reproduced. When pushed hard the mid range character distorted beautifully with clarity
and refinement. The current stock of valves we have are from the 50's and 60 's which are the most
sought after. This was going to be a hard valve to beat.

These are 1950's French Military Valve that we decided to test at 500 vdc and 500 Screen on our test
rig. 5 hours later this Valve is rock solid where others had died. This is a serious item we thought. In
the Princeton it rocked, the sound stage was big, the distortion fat and punchy. The bass response
was not as deep as the R.C.A and it sounded slightly edgy. When over driven the valve had a raw
organic sound which I fell in love with. This I liked as it is better suited to the grungy side rock. With
The Les Paul The sustain was Full and rich with no harsh edges. Sound staging was big G.E style, it
was if the wall of sound was behind me, not a little princeton. These babies are great little blues
valves and worked well with the semi acoustic Yamaha in both single and Humbucking mode. The
valve did not lose punch or go muddy even when effects where introduced.

Sovtek have some good valves in it's line up but this in not one of them. If you try and use in old
Fender deluxes it will blow up as it cannot handle plate voltage of much above 345 Vdc. Indeed many
died at 325Vdc so be warned. The valve is very harsh sounding with little bass response. When
overdriven it was very muddy. Always a good measure is does your ears hurt after prolonged use
and boy with this valve I was hurting.

Brown base with semi smoked glass looked identical to and possibly made by R.C.A The top end of
this valve really did sing , the more you threw at it the more it wanted. Killer Kossoff like sustain was
achieved with amp really pushed with no harshness. Bass was bigger than the Mazda with full
definition even on fast runs. Balance was superb clear and precise.The valve had nice balance as
was at home with country or jazz .
We can thoroughly recommend this valve.

This is identical to the valves used by Fender in the 70's and would have been the standard valve
fitted to the Princeton when new.  The valve was a lot brighter than the Mazda or Standard and
worked very well with the Fender Strat. The amp sounded thinner due to the sound staging being
not as large. The sustain was not as fat or as long lasting as the Mazda or Standard. The bass was
not as well defined but was rich and warm. A great rock and roll valve that would improve any
current Boogie amp.

G.E 6V6GT Large and nude base
We have two types of G.E one with the pins straight out the bottom like the Sovtek 5881 and one
with a full base. Both valves have the large grey box plate and sound quality I found to be Identical
there both are placed together.  The characteristic G.E sound staging big, fat and proud were
present which I happen to really like. The top end response was noticeably less than the Philips with
mid range twang more pronounced than any other. Balance was forward in your face type with great
punch. Ideally suited to country or steel guitar.

The valves are marked U.S.A and come in the original box. These are for the Neil Young fans.
Fantastic bass response which is possibly due to the bass forwardness of the valve.  When
distorted the bottom notes on the guitar are in full focus. Great for riffing or heavy Zep style
music.The top end is not as detailed but sustain is good and clear. The mid range seemed a little
recessed but the Strat twang could still be produced. This is defiantly for the hard rockers.

This is a very warm sounding valve a little bass light when compared to the Standard or Mazda. The
mid range response of this valve is its real voice. The clarity and Fender twang shone through. The
sustain was warm and rich, again not as forward as the Mazda.  When pushed hard bass seem to
become less defined but the valve still retained a very musical sound.

We have seen recently a lot of dealers trying to make a market in these by saying that these are a
super Hi-Fi valve and great Guitar valve with high prices to match.  If you want a Hi-Fi valve the
Mazda Stamps all over this valve. In guitar amps the Brimar is a traditionally British sounding. The
bass response is lighter than the R.C.A, Mazda and Westinghouse. It has a very smooth laid back
sound with no harshness. It does not have the get up and go of most of the other valves listened to
which for my taste is a shame. This may be ideally suited to the Jazzer. It has a clear midrange which
never gets flustered even on heavy rock. Rock solid stability and a good all round Valve.

Another French Military valve with a lot of the same characteristics of the Mazda. The valve had rich
sustain with good bass. Not as deep as the R.C.A, Standard or Westinghouse but the same as the
Mazda. The Mids where very smooth like the Brimar but did not have the raw edge of the Mazda but
had that Brimar controll and smoothness. The top end response seemed recessed compared to the
Philips or Mazda. However plenty of traditional Fender Twang. Sustain was as you might expect
superb rich and clear. The valve was very well balanced and is a good choice for all applications.


The first rule when fitting 6V6GT is not to the Russian 6p3s valve which an American designer valve
company rebrands as there own 6V6HD. This valve is awful and is very hard to bias . This valve is
also sold by many U.K dealers under the guise of 6L6GC, 6L6GT and 6L6GB. We do not recommend
fitting this as either. These valves were also more gassy than any other and do not like the guitar
amp plate voltages.

The Sovtek valve is very poor in the sound quality department so until a new valve is made and that
will only be if its commercially viable then we can only look to new old stock items.

The R.C.A is still the valve to beat in this market but two valves I feel come very close.

If I had to choose two favourite valves It would be the Mazda 6V6GT & the Standard 6V6GTY.

The Mazda really rocked from Zep riffs to Muddy waters. It had a raw wild feel, up and in your face
which I loved. It makes you want to play the guitar and feel good. It was also very easy to get a great
sound quickly. Some of the Pistols Steve Jones Guitar parts sounded stack like with plenty of

The Standard 6V6GTY did everything that the Mazda but with more control. The harder you pushed
the valve it would simply shout back give me more. Huge bottom end response was clear as a bell.
These are 1958 production and are identical to the R.C.A that we used as a reference. Under heavy
distortion the valve just seemed to get bigger. Sustain was simply remarkable. Bass at this level was
bigger and more defined than the Mazda.

Both of these are exceptional valves The Mazda raw and on the edge The Standard always in
control. Try both sets before they are all gone.

These tubes are all available from many sources or via the net from
General 6V6GT Data
Electro Harmonix 6V6GT test results
To see the worksheet and curves on this tube, with its associated comments, please click here
The 6L6 Family - Tube Reviews

These tubes are availble from a number of sources and these review were done
by the folks at
watford valves

6L6 types are one of the biggest selling valves in American history. These valves are still the favoured choice of
many manufactures. We wanted to see if the current production types could compete with some of the finest
valves ever made.

The amplifiers used was a 70 's Fender Twin reverb fitted with J.B.L's. A 70's Fender Twin reverb fitted with
original Fender blue back speakers. A Mesa/Boogie Mark 4 combo.
Guitars used where a 1973 Fender stratocaster, 1980 Yamaha SA 2000S semi acoustic and a 1980 Gibson Les
Paul Standard.

R.C.A 6L6GC Where used as the reference. The Fender twin fitted with J.B.L where used to establish If untested
valves proved to be Microphonic. All Valves were run on our special test rig at 500vdc 500volt plate and screen to
establish reliability.

The R.C.A's handled ever situation with great authority . From Boogie hard Rock to the steely Fender twang
These where a true reference. The bass was big with perfect balance and definition between top, middle and
bottom. The sonic presentation was full and as the valve when into distortion which was about half way one the
Fenders channel volume control it was sweet and musical. The amp sounded huge which is something I always
look for in a valve. Not a hint of harshness even when saturated . It makes you want to play. ORDER

This valve is offered by many amplifier manufactures and many designer valve companies All I can say is why?
after finding a choice quad, after 10 valves failed a simple burn in test the sound quality was abysmal. The sound
was very harsh and brittle and the valve compressed very early. The bass was loose and fizzy and in the Fender
with J.B.L I found myself thinking that a Transistor amp would sound better. Then the amp went down.   
Cathode/heater insulation was found to be poor and the valves where found to be gassy whichever brand name
was placed on them. They also where fairly Microphonic.

Stocking this valve does not have any commercial sense as they cannot operate at normal guitar amp plate
voltages. So Our advice is don't bother with these.

(as a 2002/2003 note by Myles, Chinese tubes these days have come a long way.  The Chinese plate
materials, cathode coatings, and quality have come a long way.  Current Chinese tubes can now
handle the plate voltages the tubes from Europe handle, are equally consistent or more consistent,
and have 15% more power than most of the non-Chinese offerings in many cases)

Sovtek 5881/5881WXT
This is the industry standard which is fitted as standard by Fender, Marshall and Boogie.
The valve a warm tone but lacks sound stage or harmonic detail of the R.C.A.

When pushed hard tends to go lose control and can sound muddy. The fine detail was blurred compared to other
valves especially with the J.B.L loaded twin. The Sovteks Performed very well on the Microphony test. Only a few
Valves failed the burn in test.

Sovtek 6L6WXT PLUS
The valve differs from the 5881 by having larger plate dimensions and improved grids to allow higher power
handling. The valve performed very well in the microphonic tests and has more top end clarity than The 5881. Mid
range was still muffled when pushed . Bass response was the same as the 5881 soft and sweet. In the Boogie and
Twins we noticed a little more detail and clarity than the 5881. This, however, was not as good as the majority of
other valves in this test in the clarity department.

Sylvania 6L6GC U.S.A
These Sylvania valves which are late 60's early 70's production sounded full and rich. No problems on the high
plate volt tests solid as a rock. No problems with microphony. The bass was nice a tight with that classic warm
sound associated with Sylvania . In the Fender amps they sounded a lot brighter than the Boogie but in no way
where harsh. Mid range not as detailed as the R.C.A. but The valves where very punchy and forward sounding.

Imaging and presentation were far better than the Sovteks Great choice for all applications if you what that Fender

The early type has a more rounded appearance and is slightly smaller than later production items. The sound
stage and imaging is huge. This making the notes fly out of the amp.

Bass was deep rich and plentiful and equally as extended as the R.C.A. The famous strat voice was not lost even
under driven conditions. The top end had a rich bell like clarity which begged you to sustain a note. In the Boogie,
which came with poor quality Chinese 6L6 valves fitting the G.E gave the amp a richer fatter tone ,which I thought
sounded better than the R.C.A's. The sound stage made the Boogie sound like a stack. Improvement in clarity and
mid range sparkle shone out of the J.B.L twin. My conclusion must be that these G.E are one of the greatest 6L6
GC of all time and come with my strongest recommendation.

The valve that gave the Fender bassman it's true Voice the original valve in the original bassman. The bass is
very tight well defined and in your face. The valve is well balanced and has the classic midrange twang. This gave
less power than the Sylvania, G.E or R.C.A but has superb drive and early distortion. The sound stage makes the
amp sound bigger than it really is. A great sounding valve that deserves it classic status.

This American made military specification valve has all the tight bass and midrange clarity and twang of the
Tung-Sol and is some what of an bargain. The valve has a very musical break up with a big soundstage.The
dominant mid range response is uniquely classic Fender with no harshness. The valve a little less defined than
the Tung-Sol and is less aggressive than original Tung-Sol but warmer with a touch less bass. This valve is
superb in Fenders if you require early distortion at club volume levels. It also has all the mid range honk you could
want. We have found these to be an ideal choice in reissue Baseman's, Boogies and Soldano.  The Harma
SB-6L6 is a specially selected version of the Philips.

This is the best current production 6L6. The glass envelope is made from high quality heavy grade glass made
the valve the most unmicrophonic of any we tested. The gold plated grids and special plate alloy gives this valve
improved plate dissipation and greater stability ant high voltages. The valves bass response is deep and carries
more weight than the RCA In Quad 2 the bass is easily equal if not better than the Original G.E.C KT66. When
pushed into distortion the amp sounded full and vibrant. This Valve was only just second to the G.E in sound
staging. The treble response is full smooth and creamy. The sustain was long and pleasing with punch and
definition. One of customers who has an original 1989 Soldarno SLO 100 head took out the Philips/ Sylvania
S.T.R 387 and will only fit the Harma S.T.R 6L6. This for the record, is the best sounding combination that I have
heard. Ultimate rock tone The Harma even tames this amps bright presentation. This valve gets our highest

The Svetlana has a growing list of name professional users. The specification claim design based on the Famous
Philips / Sylvania S.T.R 387. The extra thick mica spacer did make this valve less prone to Microphony. In all amps
the balance was perfect just like the RCA Bass was crisp and clear but not as deep as the Harma. Distortion was
sweet with good punch. When pushed hard the valve never got muddy like the Sovteks and is in our opinion the
second best current production item available behind the Harma.
 (note by Myles - 2002 and later date code
tubes may have quality issues, so be sure to know and trust your vendor)

We tested a number of the famed 387 Valves. In sound terms we could detect no difference between this and the
Sylvania 6L6GC. Indeed this tube sounded brighter than it's earlier brother. Which I felt was over bright in the Twin
fitted with J.B.L's

Same characteristics as the Sylvania and can be recommended.

Many U.K valve dealers sell this valve under all sorts of numbers such as 6L6GT, 6L6GB, 6L6GC and even one
world wide dealer has Even branded it KT66.

This valve does not like even moderate voltages therefore cannot be recommend under any circumstances.

This is a military long life 35 watt plate dissipation version of the S.T.R 387 . The bass was controlled mids clear
and proud with nice balance. This valve is the cleanest sounding of all the 6L6 types. We strongly rate this valve
for the big clean Fender sound as is a superb valve for bassists to fit.

G.E.C KT66
This valve was tried using a reissue Marshall blues breaker combo fitted with vintage 30 speakers. This was a
simply awesome sounding set up. After trying the replica KT66 of CHINESE and G.T variety which did not happen
at all. The G.E.C had it all. Control, deep bass with midrange beef and definition. This had the blues stamped all
over it. The Les Paul was easily the best combination giving the sound balls and grit. Sustain was both clear and
loud with the ability to produce long sustained notes with ease. These are not cheap But then again nor is a
vintage Les Paul. With these valves you defiantly get what you pay for. The best of it type and a true legend.

6L6GC Test Conslusion

With the 6L6 valves the first step is to decide how you want the amp to sound in relation to your style of music.

The Russian 6P3S and the CHINESE 6L6 's are not worth fitting in guitar amps as they sound poor and most of
them just don't work.

The Top two current production 6L6GC are the Harma S.T.R and Svetlana .  The Harma was a better Hi-Fi Valve
and excelled in our original Quad 11.

The Harma had a bigger G.E sound stage presentation and was more vibrant.  The Harma tube easily ranks along
side the U.S.A valves.

The real star of the U.S.A valves was the GE 6L6 early production and not surprisingly the RCA black plate.
The bass was not as deep as the RCA but then would you really notice when your Boogie or Fender is cranked up
in your favourite club. I think not.  What the G.E had which won the day for me was the sound stage. The more you
turned It up the bigger it got. No valve was as big as the G.E. The R.C.A was better balanced but I would gladly
trade that for the forward slightly brighter sound of the G.E. The G.E. is pure Rock and Roll.

The Sylvania 6L6GC was another great valve, warm rich and full bodied. This was a great valve for the Fender
twin. With the Strat in the out of phase position this was heaven. I just wish I had a real vintage strat as this was all

The real revelation was the Harma SB-6L6. This tube is an American made military specification small bottle 6L6
Which has been stabilised at 500Vdc plate and screen then specially selected.  The valve had less power than the
other U.S.A items. They did have the most detail mid range presentation. The great thing is that it broke up earlier
than the other U.S valves. The twins really benefited form these valves. Big fat distortion with taking your head off.
I would recommend these to any club player. In the Mesa the Harma SB provided it with a nice break up with any
amount of pre-amp distortion
The Groove Tubes 6L6GE "NVM" ("New Vintage Manufacture") tube - due out
before this summer
Preliminary first report on the Groove
Tubes 6L6GE tube

3/6/02 - Guitar Amplifier Blueprinting - West Hills CA.

We hear the term NOS (new old stock).  After seeing some
of the initial curve traces of this tube, I had to wonder if a
new "moniker" such as NVM (new vintage manufacture) or
NMOS (newly manufactured old stock) seemed more
appropriate.  When talking to the folks at Groove Tubes,
the term "reissue" was said by one of them, but "reissue"
as in amplifiers, can mean the same circuit, but with
changes.  Items such as printed circuit boards or other
changes are a bit different than the originals.

I looked at this tube very closely.  To my way of thinking, it
is anything but a reissue.  It is an original GE tube made
today.  It is made on machines from decades ago.  The
parts inside are of the same metals, dimensions, and
design as the GE.  The coatings are of the same chemical
content, formulations, and thickness.  Even the glass case
is the same glass formula, thickness, and dimensions.  The
getters are side getters, and the mica spacers are identical
to the originals, made of the same material on the same
machines as the originals.

Well, sometimes a copy of something can be a copy
visually and physically, but in practice it can be a different
matter.  So ? it was off to the vacuum curve tracer and
scopes, meters, and loads.

First, a standard was needed.  What better standard than
the original GE data sheets.  These sheets show what was
expected from an "average" tube.  As we changed
voltages, one thing became more clear as time passed,
this tube (which is still not released) did not have a Groove
Tubes rating number, but probably would have been a #6.  
The curves (as some of the charts show), were just a touch
stronger than the average GE tube.  This was well within
the range of a number of the originals.  These curves are
as linear as anything we have tested.  The usable range of
this tube as various voltages was wider than the Svetlana,
Electro Harmonix, or JJ Tesla tubes.  The wider range is
not a plus or minus, just an indication of a very well
engineered piece of equipment.

Listening tests done earlier showed this tube to be a
winner to the folks tastes that were present.  This tube was
tested in the new Groove Tubes Solo Class A amp, in both
the combo and head version.  It was also tested in Mikey
Wright?s Fender Custom Shop Vibroverb.  Mikey can
probably give the ability to have a listen if you are in the
area.  If you are interested, drop me an email.  I normally
refrain from listening tests as these are so subjective, but I
will say that my old favorite KT-66 tubes will possibly be
replaced by these 6L6GE?s when they are commercially
available in quantity.  For now, this tube is in something of
BETA test, with about 30 or so musicians out there.

This is the first phase of out testing, and I will post more as
I have more to tell you all.  I will say that so far, this is the
best "data wise" 6L6 available.  As far as sound quality,
that is subjective.  If you like the original Fender sound,
than this tube will be work investigation.

Also down the road, Groove Tubes is developing a 6CA7
(EL-34).  The USA 6CA7 had a higher vacuum than their
Europe counterparts, and were of a heavier construction
also.  This may be the ticket for the folks out there that
want a strong EL-34 type tube with a high degree of

Now, I did not see any spare equipment lying around to
make another tube type at Groove Tubes ? but ? maybe if I
can get a lot of people pushing hard enough, we can get
them to make a "NVM" versions of a 6V6 !!!

Availability of this tube is expected by this early summer if
not before.

For a writeup on this tube by
Harmony Central posted on April
click here for a reprint of the
article in the Groove Tubes section
of this website.  This tube has been
reviewed by Legendary Tones, and
was the first time Guitar Player
Magazine did a tube review (Feb
2003 issue).
To see the curves on this tube
click here
The Electro Harmonix 6L6GC tube
Electro Harmonix 6L6GC test

3/13/02 - Guitar Amplifier Blueprinting, West
Hills CA.

For this test we used a very low bias in order to
drive the tube as hard as possible at lower
voltages.  There is also a set of curves on this
tube against an NOS reference standard
where the bias was set to a level to allow
higher plate voltages to be used in further
testing.  The notes below are for the first
phase of testing with the lower bias.

Recently we have tested some of the offerings
from Electro Harmonix.  Their EL-34 well
enoough for the most part.  Their 6V6GT
offering looks to be perhaps the best choice if
you don?t want to pay the extra for NOS tubes,
and did very well in our testing.  Their
12AX7EH will be tested in the upcoming weeks,
but some use so far look very nice.

In the case of their 6L6GC offering, the results
were not quite as encouraging as that of their
other tubes.  Hopefully they will change some
aspect of this tube, and try to align the
performance with the conventional standards
for the original 6L6GC tube.  Perhaps the way
this tube performs is to a target design, and
this may have been their intention all along,
but their advertisement states that it was
modeled after the RCA Blackplate, and in this
comparison it fell short of the mark.

Right from the start, it was noted that this tube
performs below the standard "average" RCA
Blackplate at lower voltages (from 150 volts to
300 volts).  Its output was less, by a fair
margin, than any of the RCA, GE, or Sylvania
reference tubes.  As voltages increased at
higher current requirements, the differences
became smaller, but the curves were not linear
by any observation.

The rise time of this EH tube was slower than
the NOS offerings in all tests.  What this
means, is the tube is slower to react and
slower to amplify to a given voltage or current.

Above 350 volts, where most guitar amplifiers
operate, the differences between this tube and
the NOS tube grew even wider.  Its output
dropped even more than at voltages that were
less in earlier tests.  Its rise time became
increasingly longer, and the tube became less
linear in all aspects.  We stopped recording the
results at over 375 volts, but continued testing
to 525 volts.  At over 375 volts, the
characteristics of this tube were so far off from
the reference standards, that is seemed
non-realistic to look at this tube anymore as a
6L6GC tube.  Perhaps this will be a nice
feature for some folks, having a tube that
sounds different than the conventional 6L6GC.

In closing, I think if you had an older Fender
style amp or an amp from the 1950?s that had
plate voltages below 330 volts or so, and you
did not want to spend extra for NOS offerings,
then these tubes might be a choice.

Our sample was limited to 22 tubes from 8
vendors.  In all cases we asked for a rating that
was the same as a Groove Tubes #5 rating,
and told the suppliers the tubes were to be
used in a Fender Bandmaster, a Fender Blues
DeVille, a Rivera Quiana, and a Carr Rambler

In a few weeks, we will retest this tube, but next
time as for the tubes to be supplied at a higher
rating.  The fact that 8 vendors were spread all
over the place when asking for a mid rating
was more of a surprise than in the past.  
Perhaps they are just getting their own scale or
system set up for this tube.  If you use
something like a "maxi-matcher", you may want
to test these at both the 325v and the 400v
setting, and play a bit with the bias selection
too, when you are trying to match these for
customers.  Be sure to also look at
transconductance, and this varied a bit too.  If
you are looking for a matched pair, be sure to
look at current and transconductance if you do
not have access to other equipment that takes
additional factors into consideration.
To see the curves on this tube that go
with the review to the left
click here
To see the reference comparison with
an NOS lab reference tube
click here
Ruby Tubes / Magic Parts 6L6GCMSTR Tube Test

06 December 2002 Guitar Amplifier Blueprinting -  Myles S. Rose

These tubes were noted to have printed information of the boxes of:

PC:  35 in all cases
TC:  2950 in all cases
Nr.: 195, 92, 80, and 78

These tubes appear to be of the same construction as the current GT 6L6C, with a visible exception,  the plate
assembly.  On the Ruby version, there are holes in the plates allowing the internal beam forming plates to be
seen.  In the GT version, there are no holes.  The radiator is of different construction.  The cathode appears to
be slightly different.  The plates may also be of a different material, as may the cathode coating.

The Ruby version is slightly lower powered tube than the GT 6L6C for average samples (GT using #5 and #7 as
a reference).

The output at a static reference test setting for the Ruby version was 67.3 ? 69.3 milliamps, with
transconductance in triode mode between 6260 and 6330.  The current GT version of the 6L6C in a #5 rating
was a bit higher at 71.3 milliamps.  The spec for a ?standard? 6L6 tube at reference voltages is 72 milliamps, so
each of these versions are a bit down on output, so these  would be very nice for folks with amps that they
wanted to play nicely at slightly lower levels with more dynamic touch and feel.

The static match on the Ruby Tubes was off by about 3 milliamps, but the dynamic match was right on the
money.  These folks are doing matching in a much more upscale way than other tube vendors out there such as
The Tube Store and the "mom and pop" shops using simple static devices such as the Maxi Matcher.  Static
matching is something like saying my VW Jetta set to idle at 800rpm is the same car as a BMW M5 set to idle at
800rpm.  Ruby Tubes does an excellent matching job, appearing to be dynamic, rather than simply statically
matched, and it shows on the curve matches.

This tube performed very well over voltage ranges from 225 plate volts to 490 plate volts.  It?s change in
character when tested in pentode and also in triode modes, was very slight.  This is an indication that the grid
windings / screen assembly, are done with a high degree of consistency.

Comparing to other tubes and known reference samples:

The curves of the Ruby was a bit slower than those of the 6L6B / 5881WXT Sovtek.  This is a good aspect, as the
Sovtek tube is not very linear, and is more harsh at plate voltages anywhere about 390.  The Ruby would
probably be a nicer sounding tube for rock (60?s) and much nicer for blues or jazz styles than the Sovtek.

The Ruby has curves faster than the current JJ 6L6S.  We are looking at about 15 milliseconds here, at middle
output ranges, but it is enough that it may indicate more pure cathode coatings over the JJ tubes as an example.  
This may be an indication of overall quality, which on this tube is very high.  This is also an indication that the
plate material may also be more consistent than the materials used in some Eastern Europe tubes.  The faster
curves and rise time are not too fast, so this tube may show even more articulation than the JJ tube at lower
volume levels.  Possibly the Ruby would be better suited to playing smaller venues with a Fender type 50 ? 100
watt amp.  This tube would be more versitile than a JJ 6L6S, or should I say, less agressive.  It has a nice big and
full character.

The curves and performance were better in all areas over the current 2002 date code production Svetlana 6L6
tubes.  This may be due to the 2002 inconsistency of the Svets also.

The curves and performance were better in all cases than the current Electro Harmonix 6L6 offering.  In every
aspect, this Ruby offering performs better than the EH tube from a pure math and spec standpoint.  Every player
preferred this tube in addition to the technical testing aspects.  In a Black Face Fender Bassman, a BF Fender
Super Reverb, and a Marshall that had 5881's in it, all players preferred this tube over the EH tube.  The EH tube
sounded thin next to this tube.  This Ruby 6L6 may have had less current output than the EH in the same circuit
by a few milliamps, but biased to the same idle current, had a much bigger sound, a wider sound stage image by
about 8-10 degrees, and more harmonic content and response.

Compared to an NOS 6L6GE, the Ruby was a bit faster in rise time (and hence a bit less smooth).  This was only
observed at middle and higher plate voltages, and when driving the tube from 60% and higher of it?s possible
output.    At plate voltages above 375 volts, the Ruby had a faster rise time and less linear curves than the GE.  
The Ruby prefers voltages of less than 450, but does not have any problem with higher voltages than 450, it is
just a ?happier? tube at less than 450.

The bottom line is, the Ruby is a very good tube.  It has nicer curves and response than any of the current
Russian offerings or the JJ offerings.  This is from a strict ?data? sense, the sound in various amps and user
personal preference is the final authority on any tube test.  I will be listen testing over the weekend on a set of

The current Groove Tubes 6L6GE cannot be compared to this tube as the GE is a much more expensive tube,
and it perhaps the most linear tube and has the most harmonic content transfer characteristics of any current
6L6.  The GT and NOS 6L6?s trace identically again in this current test.

I think the ?Chinese? 6L6?s have come a very long way in the last year.  They are well suited to be in the world of
Rock and Roll, standing right there with the older standards such as the Sovteks, Svets, and JJ?s ? and in many
cases, walking past them and setting new standards for quality and consistency.   

I will be trying a duet of these in a Marshall JTM-45, a duet in a Fender Pro Reverb, and then also in the GT Solo
45 that seems to get the most out of any power tube as there is less preamp coloration than with most other
amps.  I will update this when the listening tests are completed.  I will also have two fairly well respected jazz
players take two of the Ruby duets for a ?test drive?.

Myles S. Rose