| Preamp tube information |
Preamp Tubes; most critical, least expensive, most overlooked
tubes in your amp.
The tonal signature of your sound and interchangeable without adjustment or the need of an amp
tech.
Unlike power/output tubes, which are routinely matched when they are sold (in different ways,
some much better than others), preamp tubes are tested at best to: (a) make sure they work ? (b)
they are not microphonic. In testing, we have found that some suppliers don?t seem to test their
preamp tubes at all, as we have found one side of the triode that is dead at times. Since most
warranty preamp tubes for up to 6 months and longer, they possibly figure that is cheaper to just
send them out as they get them in, and if there is a problem, it is cheaper to just give the
customer another tube. This is of little comfort to somebody that either has to make another trip
to their music store, or worse, box up the bad tube and ship it back to the supplier, and then wait
for its replacement. This is one reason to consider a proven supplier when you buy preamp tubes.
Today's amplifiers, whether modern high gain types or boutique amplifiers, have one thing in
common; the preamp tube in the first gain stage (usually V1 and / or V2) sets the tone and initial
gain structure of the amplifier.
Amp design -
Today?s modern amps get just about all of their characteristics in the preamp section. How the
gain stages are set up, how the EQ is set up, gain structure, and tone stacks, all are the main
aspect of the sound character of the amplifier.
Amps such as Mesa Boogie, Fender, Marshall, Bogner, Peavey, and others, all use the same
Sovtek, Svetlana, JJ/Tesla, Electro Harmonix, and other power tubes from the same factories. In
spite of the same output sections, and in many cases the same range of B+ voltages on the plates
of the output tubes, these amps sound different. This is all because of different designs, primarily
in the front end, or initial gain section of the amplifier.
Inconsistencies -
Today?s newly made preamp tubes are very inconsistent compared to the tubes of the 1940?s to
1960?s. There is little need in the medical sector or the military sector for tubes. They are
primarily used today in audio applications. For the high end audiophiles, their needs are more
easily met, as their tubes are not subjected to the same stresses as those on a guitar amplifier,
they use less of them, and they last much longer. There are high end audio suppliers that will
match tubes and hand select them, at much higher costs (check out a Western Electric 300B
matched pair for example). They pop their tubes in, and ten years later, all is still just fine.
Tubes for the guitar and bass player for use in the preamp section, are a different story. The
tubes today are very inconsistent. You contact your local tube supplier, plunk down your money,
and the roulette wheel is now set into motion.
To show the inconsistency, we went through a batch of over 100 tubes that were from the Electro
Harmonix 12AX7EH, ECC83, 7025, Sovtek 12AX7WA, LP, LPS, Chinese 12AX7C (old tooling and new
tooling), and a few others.
Basically, the standard 12AX7 spec that applies to 12AX7 / ECC83 / 7025 tubes, has a reference of
1.2 mA at 250 volts with a -2 volt bias.
Some people like to use those little references that say if you want less gain than a 12AX7, use a
12AT7, as it has only 70% of the gain of a 12AX7 etc. These little tips are cute, but with the wide
range of inconsistency out there, they are not all that useful, as it is still a matter of chance. The
12AT7 has a different current capacity than a 12AX7, so if you are just looking for less gain, then
you may, or may not get it with just a different 12AX7, even from the same brand, same date code,
and same batch ? just by swapping tubes around already in your amplifier. With today?s
inconsistent offerings, the old tables of gain cannot be used with much accuracy.
In the tubes we went through, keeping in mind our 1.2 mA / 1600 transconductance industry
standard spec, we found our samples ranged from 0.7 mA to 1.6 mA. When you take into account,
that the amplification factor of a 12AX7 is 100, there is a dramatic difference in these tubes.
Looking at a 1.6 mA tube, we see a factor of increase over the standard of 33%. This is a LARGE
number. A 1.0 tube versus a 1.2 tube will turn the gain you loved in your 5150 into something less
than what you used to know what you liked there. You sit dumbfounded. How can this be? I just
put in new tubes, the same as what I had before?
You want even MORE GAIN from your Triple Rectifier or Bogner ? look at those first gain stage
preamp tubes, and get some tube vendor to measure them for you. If you have a 1.1 in there, and
put in a 1.3, you will hear the difference in gain IMMEDIATELY. This is not a subtle change that
only the "experts" can hear. Leave the settings on the guitar and amp the same, swap the tube,
and listen again.
When we see a transconductance of 1200 versus the 1600, the way the tube reacts is different
too, in this case, its rise time is about 25% slower. This might be just the ticket for a blues player,
looking for some nice initial compression on the pick attack, but it may not be the sound for a
metal or speed player.
Transconductance in the testing, ranged from 1060 to 1790. 1600 is the industry standard.
There is one other aspect of preamp tubes. Unlike power tubes, where one tube is one tube ? a
preamp tube is two tubes in one bottle. There is an A side and a B side. The are independent
units sharing only the heater. In a Fender blackface amp as an example, input 1 uses one side of
the tube, and input 2 (lower gain input) the other side. In the NORMAL channel, V1 is used, and in
VIBRATO CHANNEL V2 is your first gain stage. Since these are two different circuits, it is not as
much of a concern if the A and B side of the tube are close to the same performance, or
"matched". BUT, anytime we use that tube in the phase inverter position or driver position of the
amp (which is the driver for the power tubes), then having the two sides matched is important.
This matching subject has been covered before, so I won?t elaborate on this again here.
New versus Vintage amp needs -
NOS tubes are sought after by folks that have original amps like Fender Tweeds and the like. If
you want the original sound, feel, and character of these amps, then NOS is about the only way to
go. Getting NOS tubes for your amp to be correct is much easier in some ways than getting
decent new tubes for an amp. There are folks that deal with NOS tubes. Some of them are on my
website, but I will say here, that I can recommend KCA and Tube World very highly. Please DO NOT
confuse Tube World with TheTubeStore, they are NOT even close. Either of these folks have the
tubes and the equipment that will let you know that when you purchase a Mullard or RCA 12AX7 or
whatever, it?s characteristics will be noted for your information. If you are in Europe, check with
Watford Valves, as they also have NOS offerings. Eurotubes in the USA, a JJ importer, also may
have some NOS offerings.
When it comes to new tubes for you modern or class amp, or new boutique amp, there is the
problem. Preamp tube suppliers guarantee the tubes to work, and not be microphonic. That is
about all they can do. Going though tubes that retail for less than $20 in most cases, one at a time
to measure them, is beyond reason economically. Watford Valves will do this for folks. Other
folks offer these services, and in most cases, they are an additional cost. In my mind, the money
is well spent. When you want a nice high gain tube for your Rivera or Demeter, putting in a tube
that is 30% down from spec., is not the ticket! At that point, what can you do with that tube? Take
it back? Why? It works. The store or vendor never stated it would do anything more than "work".
Perhaps they will exchange it, and now you start the process over? And over. And over.
Recently I was working with a fellow named Tom Dunn. He plays a 5150 II. We performed a
blueprinting session of his amp, and it was found after going through maybe 2 dozen of his own
tube stock, that he had picked out his tubes by ear, and placed them in the most advantageous
position in his amp. You can do this by ear, if you have the ears of this guy, and also the time (he
did this over many months), and the tube stock.
Conclusion -
Your first gain stage in your amp is its soul, sound, and character. We talked here about gain, and
a little about rise time, which is a subject in itself. We did not get much into "sound", such as the
articulation and definition that comes from NOS tubes like the Mullards and Telefunkens. If you
have an older amp with a more moderate gain structure, and want it to sound closer to magic, than
this is the way to go. In a modern amp, a lot of the articulation from the output section is not the
target of these designs. Today's designs look for two or three or more stages of gain, channel
switching which we did not have on the older amps of yesterday, and flexibility. The only flexibility
we had when I was the age of most of you, was a high gain input and low gain input or tuning the
reverb on or off.
All I can suggest, is try to find a tube vendor that can supply you with the tubes you need with
some degree of classification. This way, if you have a 1.3/1670 tube in there now, and you want to
tone it down a bit, then maybe go for a 1.1 ? it will make a difference. If you want tonal changes in
color, rather than gain and compression, then you want to go with a little stash of tubes,
depending on your use for the day or evening. Most of my clients keep the following:
JAN 12AX7A - Most often general use.
12AX7C - Chinese 12AX7 - take off some amp edge or brightness
12AX7EH - Electro Harmonix - general use
ECC83 - for the Marshall sort of sound
7025 - for the Fender 60?s and 70?s sound
5751 - for blues and less aggressive attack (and perhaps less gain as compared to an in spec
12AX7)
Marshall trick if you use pedals: When you use most pedals, the input of your amp is loaded in a
different way than without the use of these. This also add capacitance, and rolls of some of your
high end. If you want to get back the "edge" with your Marshall, try a 7025 in V1. This will usually
make a Tele or Strat sound too bright without pedals, but may be just the ticket to fix the problem
when pedals are used. On which 7025's to use ... this tube has a long plate structure that is a bit
prone to microphonics in some cases. My clients use the Groove Tubes version of this tube as
they seem to be screened for microphonics to the point that they work more often than many of
the other versions available, even from the same manufacturer.
tubes in your amp.
The tonal signature of your sound and interchangeable without adjustment or the need of an amp
tech.
Unlike power/output tubes, which are routinely matched when they are sold (in different ways,
some much better than others), preamp tubes are tested at best to: (a) make sure they work ? (b)
they are not microphonic. In testing, we have found that some suppliers don?t seem to test their
preamp tubes at all, as we have found one side of the triode that is dead at times. Since most
warranty preamp tubes for up to 6 months and longer, they possibly figure that is cheaper to just
send them out as they get them in, and if there is a problem, it is cheaper to just give the
customer another tube. This is of little comfort to somebody that either has to make another trip
to their music store, or worse, box up the bad tube and ship it back to the supplier, and then wait
for its replacement. This is one reason to consider a proven supplier when you buy preamp tubes.
Today's amplifiers, whether modern high gain types or boutique amplifiers, have one thing in
common; the preamp tube in the first gain stage (usually V1 and / or V2) sets the tone and initial
gain structure of the amplifier.
Amp design -
Today?s modern amps get just about all of their characteristics in the preamp section. How the
gain stages are set up, how the EQ is set up, gain structure, and tone stacks, all are the main
aspect of the sound character of the amplifier.
Amps such as Mesa Boogie, Fender, Marshall, Bogner, Peavey, and others, all use the same
Sovtek, Svetlana, JJ/Tesla, Electro Harmonix, and other power tubes from the same factories. In
spite of the same output sections, and in many cases the same range of B+ voltages on the plates
of the output tubes, these amps sound different. This is all because of different designs, primarily
in the front end, or initial gain section of the amplifier.
Inconsistencies -
Today?s newly made preamp tubes are very inconsistent compared to the tubes of the 1940?s to
1960?s. There is little need in the medical sector or the military sector for tubes. They are
primarily used today in audio applications. For the high end audiophiles, their needs are more
easily met, as their tubes are not subjected to the same stresses as those on a guitar amplifier,
they use less of them, and they last much longer. There are high end audio suppliers that will
match tubes and hand select them, at much higher costs (check out a Western Electric 300B
matched pair for example). They pop their tubes in, and ten years later, all is still just fine.
Tubes for the guitar and bass player for use in the preamp section, are a different story. The
tubes today are very inconsistent. You contact your local tube supplier, plunk down your money,
and the roulette wheel is now set into motion.
To show the inconsistency, we went through a batch of over 100 tubes that were from the Electro
Harmonix 12AX7EH, ECC83, 7025, Sovtek 12AX7WA, LP, LPS, Chinese 12AX7C (old tooling and new
tooling), and a few others.
Basically, the standard 12AX7 spec that applies to 12AX7 / ECC83 / 7025 tubes, has a reference of
1.2 mA at 250 volts with a -2 volt bias.
Some people like to use those little references that say if you want less gain than a 12AX7, use a
12AT7, as it has only 70% of the gain of a 12AX7 etc. These little tips are cute, but with the wide
range of inconsistency out there, they are not all that useful, as it is still a matter of chance. The
12AT7 has a different current capacity than a 12AX7, so if you are just looking for less gain, then
you may, or may not get it with just a different 12AX7, even from the same brand, same date code,
and same batch ? just by swapping tubes around already in your amplifier. With today?s
inconsistent offerings, the old tables of gain cannot be used with much accuracy.
In the tubes we went through, keeping in mind our 1.2 mA / 1600 transconductance industry
standard spec, we found our samples ranged from 0.7 mA to 1.6 mA. When you take into account,
that the amplification factor of a 12AX7 is 100, there is a dramatic difference in these tubes.
Looking at a 1.6 mA tube, we see a factor of increase over the standard of 33%. This is a LARGE
number. A 1.0 tube versus a 1.2 tube will turn the gain you loved in your 5150 into something less
than what you used to know what you liked there. You sit dumbfounded. How can this be? I just
put in new tubes, the same as what I had before?
You want even MORE GAIN from your Triple Rectifier or Bogner ? look at those first gain stage
preamp tubes, and get some tube vendor to measure them for you. If you have a 1.1 in there, and
put in a 1.3, you will hear the difference in gain IMMEDIATELY. This is not a subtle change that
only the "experts" can hear. Leave the settings on the guitar and amp the same, swap the tube,
and listen again.
When we see a transconductance of 1200 versus the 1600, the way the tube reacts is different
too, in this case, its rise time is about 25% slower. This might be just the ticket for a blues player,
looking for some nice initial compression on the pick attack, but it may not be the sound for a
metal or speed player.
Transconductance in the testing, ranged from 1060 to 1790. 1600 is the industry standard.
There is one other aspect of preamp tubes. Unlike power tubes, where one tube is one tube ? a
preamp tube is two tubes in one bottle. There is an A side and a B side. The are independent
units sharing only the heater. In a Fender blackface amp as an example, input 1 uses one side of
the tube, and input 2 (lower gain input) the other side. In the NORMAL channel, V1 is used, and in
VIBRATO CHANNEL V2 is your first gain stage. Since these are two different circuits, it is not as
much of a concern if the A and B side of the tube are close to the same performance, or
"matched". BUT, anytime we use that tube in the phase inverter position or driver position of the
amp (which is the driver for the power tubes), then having the two sides matched is important.
This matching subject has been covered before, so I won?t elaborate on this again here.
New versus Vintage amp needs -
NOS tubes are sought after by folks that have original amps like Fender Tweeds and the like. If
you want the original sound, feel, and character of these amps, then NOS is about the only way to
go. Getting NOS tubes for your amp to be correct is much easier in some ways than getting
decent new tubes for an amp. There are folks that deal with NOS tubes. Some of them are on my
website, but I will say here, that I can recommend KCA and Tube World very highly. Please DO NOT
confuse Tube World with TheTubeStore, they are NOT even close. Either of these folks have the
tubes and the equipment that will let you know that when you purchase a Mullard or RCA 12AX7 or
whatever, it?s characteristics will be noted for your information. If you are in Europe, check with
Watford Valves, as they also have NOS offerings. Eurotubes in the USA, a JJ importer, also may
have some NOS offerings.
When it comes to new tubes for you modern or class amp, or new boutique amp, there is the
problem. Preamp tube suppliers guarantee the tubes to work, and not be microphonic. That is
about all they can do. Going though tubes that retail for less than $20 in most cases, one at a time
to measure them, is beyond reason economically. Watford Valves will do this for folks. Other
folks offer these services, and in most cases, they are an additional cost. In my mind, the money
is well spent. When you want a nice high gain tube for your Rivera or Demeter, putting in a tube
that is 30% down from spec., is not the ticket! At that point, what can you do with that tube? Take
it back? Why? It works. The store or vendor never stated it would do anything more than "work".
Perhaps they will exchange it, and now you start the process over? And over. And over.
Recently I was working with a fellow named Tom Dunn. He plays a 5150 II. We performed a
blueprinting session of his amp, and it was found after going through maybe 2 dozen of his own
tube stock, that he had picked out his tubes by ear, and placed them in the most advantageous
position in his amp. You can do this by ear, if you have the ears of this guy, and also the time (he
did this over many months), and the tube stock.
Conclusion -
Your first gain stage in your amp is its soul, sound, and character. We talked here about gain, and
a little about rise time, which is a subject in itself. We did not get much into "sound", such as the
articulation and definition that comes from NOS tubes like the Mullards and Telefunkens. If you
have an older amp with a more moderate gain structure, and want it to sound closer to magic, than
this is the way to go. In a modern amp, a lot of the articulation from the output section is not the
target of these designs. Today's designs look for two or three or more stages of gain, channel
switching which we did not have on the older amps of yesterday, and flexibility. The only flexibility
we had when I was the age of most of you, was a high gain input and low gain input or tuning the
reverb on or off.
All I can suggest, is try to find a tube vendor that can supply you with the tubes you need with
some degree of classification. This way, if you have a 1.3/1670 tube in there now, and you want to
tone it down a bit, then maybe go for a 1.1 ? it will make a difference. If you want tonal changes in
color, rather than gain and compression, then you want to go with a little stash of tubes,
depending on your use for the day or evening. Most of my clients keep the following:
JAN 12AX7A - Most often general use.
12AX7C - Chinese 12AX7 - take off some amp edge or brightness
12AX7EH - Electro Harmonix - general use
ECC83 - for the Marshall sort of sound
7025 - for the Fender 60?s and 70?s sound
5751 - for blues and less aggressive attack (and perhaps less gain as compared to an in spec
12AX7)
Marshall trick if you use pedals: When you use most pedals, the input of your amp is loaded in a
different way than without the use of these. This also add capacitance, and rolls of some of your
high end. If you want to get back the "edge" with your Marshall, try a 7025 in V1. This will usually
make a Tele or Strat sound too bright without pedals, but may be just the ticket to fix the problem
when pedals are used. On which 7025's to use ... this tube has a long plate structure that is a bit
prone to microphonics in some cases. My clients use the Groove Tubes version of this tube as
they seem to be screened for microphonics to the point that they work more often than many of
the other versions available, even from the same manufacturer.
Technical article on the importance of a matched output section and matched phase
inverter / driver tube
MATCH YOUR OUTPUT SECTION!
Why do two Marshalls or two Fenders, of the same model, and even year, sound different?
For one thing, there was a +/- 20% or more variance in components used, but the more common
reason is below.
I originally wrote this with a subject of dead spots in your sound or tone that people felt were due
to dead spots in their guitar (or bass) neck.
In the last few months while blueprinting amplifiers, I have had to explain over and over about a
mis-matched output section and its impact to many.
I thought I'd write a little here ....
In any class A/B amplifier, because of the NFB (negitive feedback loop ... usually labeled as the
PRESENCE control), any disparity between the upper part of the sine wave (produced by half of
the output tubes), and the lower part of the sine wave (produced by the other half of the output
tubes), is cancelled out by the NFB circuit by design. This is the reason some notes "sing" when
your amp is pushed in the output section (rather than pushing the input in a master volume amp),
and other notes do not have the same magic.
The reason some of the great blues players have that tone, is that their amps are taken care of
people that know how to adjust or deal with some of the issues that cause this lack of luster.
Since no tubes are even close to identical, this cancellation is always going on. The object is to
limit this as much as possible.
The most common way people match an output section, is to use good quality matched tubes. The
industrial spec for a match can be as high as +/- 20%. A good match by a lot of tube vendors is +/-
10%. I believe that even the untrained ear can hear the difference when a output section
matched within +/- 5% is used. In the amps I set up for the folks that retain me, my spec is less
than 2.5%.
The most overlooked and misunderstood part of the output section is the 12AX7/ECC83 (Marshall
style) or 12AT7 (Fender style in vintage cases) Phase Inverter tube. This is the tube that drives
the output tubes. A lot of folks that specialize in making amps sound great don't understand this,
but fix this accidentally. They tend to use very good tubes, such as JAN spec 5751's etc., where
the match is closer, and closer matched tubes in the output section. They also use tubes that
sound good in the first gain stage positions, rather than the common Sovtek WA tubes which
most manufacturers use (because they are sturdy, not as expensive, and ship well without
developing microphonics).
When I scope an amp in the lower frequency region, the vast majority of the time, the upper and
lower parts of the sinewave are not even close to equal. This is more disparent than just a
slightly mismatched set of output tubes. At this point, I install a matched phase inverter / driver.
The problem with phase inverters, is finding a matched tube. You have to remember that a 12AX7
/ 12AT7 etc., is NOT a single function tube as an output tube. It is TWO tubes (two triodes), sharing
a single bottle.
VERY FEW TUBE COMPANIES MATCH THE A AND B SIDES OF PREAMP TUBES. They warrant the
tubes to work, and warrant them not to be microphonic, but do not say they are matched. This is
not any bad commentary on tube suppliers .... to do this matching is time consuming and requires
specialized equipment. If you can find somebody that has a Tektronix tube curve tracer, and bring
them a bunch of tubes, maybe you will be lucky and find a match. There are a few tube vendors
on my feeble little website that do offer matched preamp tubes, mostly those dealing in high end
audio applications.
Matched phase inverters and output tubes are one of the reasons some amps "sing" and others
are pedestrian compared to their brothers and sisters.
If you seem to have a lot of dead spots, try a new phase inverter tube. This is usually the preamp
tube that is the closest to your output tubes. It is a trial and error process, but you may get lucky.
By the way ... THIS IS EXTREMELY CRITICAL WITH DROPPED TUNINGS, 7 STRING GUITARS WHEN IN
THE LOWER RANGES, AND EVEN MORE CRITICAL WITH BASS GUITARS WHEN USING TUBE
AMPLIFICATION.
inverter / driver tube
MATCH YOUR OUTPUT SECTION!
Why do two Marshalls or two Fenders, of the same model, and even year, sound different?
For one thing, there was a +/- 20% or more variance in components used, but the more common
reason is below.
I originally wrote this with a subject of dead spots in your sound or tone that people felt were due
to dead spots in their guitar (or bass) neck.
In the last few months while blueprinting amplifiers, I have had to explain over and over about a
mis-matched output section and its impact to many.
I thought I'd write a little here ....
In any class A/B amplifier, because of the NFB (negitive feedback loop ... usually labeled as the
PRESENCE control), any disparity between the upper part of the sine wave (produced by half of
the output tubes), and the lower part of the sine wave (produced by the other half of the output
tubes), is cancelled out by the NFB circuit by design. This is the reason some notes "sing" when
your amp is pushed in the output section (rather than pushing the input in a master volume amp),
and other notes do not have the same magic.
The reason some of the great blues players have that tone, is that their amps are taken care of
people that know how to adjust or deal with some of the issues that cause this lack of luster.
Since no tubes are even close to identical, this cancellation is always going on. The object is to
limit this as much as possible.
The most common way people match an output section, is to use good quality matched tubes. The
industrial spec for a match can be as high as +/- 20%. A good match by a lot of tube vendors is +/-
10%. I believe that even the untrained ear can hear the difference when a output section
matched within +/- 5% is used. In the amps I set up for the folks that retain me, my spec is less
than 2.5%.
The most overlooked and misunderstood part of the output section is the 12AX7/ECC83 (Marshall
style) or 12AT7 (Fender style in vintage cases) Phase Inverter tube. This is the tube that drives
the output tubes. A lot of folks that specialize in making amps sound great don't understand this,
but fix this accidentally. They tend to use very good tubes, such as JAN spec 5751's etc., where
the match is closer, and closer matched tubes in the output section. They also use tubes that
sound good in the first gain stage positions, rather than the common Sovtek WA tubes which
most manufacturers use (because they are sturdy, not as expensive, and ship well without
developing microphonics).
When I scope an amp in the lower frequency region, the vast majority of the time, the upper and
lower parts of the sinewave are not even close to equal. This is more disparent than just a
slightly mismatched set of output tubes. At this point, I install a matched phase inverter / driver.
The problem with phase inverters, is finding a matched tube. You have to remember that a 12AX7
/ 12AT7 etc., is NOT a single function tube as an output tube. It is TWO tubes (two triodes), sharing
a single bottle.
VERY FEW TUBE COMPANIES MATCH THE A AND B SIDES OF PREAMP TUBES. They warrant the
tubes to work, and warrant them not to be microphonic, but do not say they are matched. This is
not any bad commentary on tube suppliers .... to do this matching is time consuming and requires
specialized equipment. If you can find somebody that has a Tektronix tube curve tracer, and bring
them a bunch of tubes, maybe you will be lucky and find a match. There are a few tube vendors
on my feeble little website that do offer matched preamp tubes, mostly those dealing in high end
audio applications.
Matched phase inverters and output tubes are one of the reasons some amps "sing" and others
are pedestrian compared to their brothers and sisters.
If you seem to have a lot of dead spots, try a new phase inverter tube. This is usually the preamp
tube that is the closest to your output tubes. It is a trial and error process, but you may get lucky.
By the way ... THIS IS EXTREMELY CRITICAL WITH DROPPED TUNINGS, 7 STRING GUITARS WHEN IN
THE LOWER RANGES, AND EVEN MORE CRITICAL WITH BASS GUITARS WHEN USING TUBE
AMPLIFICATION.
Preamp Tubes ? Gain / Output and Matching
I have written other papers on each of these subjects, and questions and comments come up where
additional explanation has been helpful for a lot of folks.
The first basic point to remember here, is that unlike a power tube such as a 6L6, EL-84, EL-34, 6550,
and others, a preamp tube is a ?dual triode? in most cases. This includes tubes such as a 12AX7,
ECC83, 7025, 12AT7 and others.
In regard to matching of two sides of a preamp, many folks feel that in a balanced circuit which in a
way, "sums" the two sides of the tube, that balancing is not necessary. The high end audio industry
recognizes the need for a balanced phase inverter or drivers, unlike some in the musical instrument
industry.
This is necessary (the balance of two sides of a triode). I will use an example to try to illustrate.
Taking a twin engine airplane, let?s say we have a typical light twin with two 300 HP engines designed
to cruise a 250 knots. We have each engine at 1800 rpm. One engine is at 24" of manifold pressure,
while the other is at 22" of MP. The airplane structure itself, is the "balance", much like the balanced
circuit that some amp builders feel will negate the need for a balanced phase inverter. The plane may
fly just fine, but we need to add a bit of rudder trim, and out fuel use will be higher, and overall
performance and balance of the airplane (or amp) will suffer.
Now in the "art" of balancing a dual triode, there are many folks that will perform "matching" for a few
dollars of additional cost. In almost all cases, this ?match? is a gain match, or a current match, or a
transconductance match. Due to very high inconsistencies in today's preamp tubes, matching for any
of these factors, is an improvement over an unknown tube. This sort of matching at least makes sure
that we do not have a 200 HP engine on one side of our airplane and a 400 HP engine on the other
side. These numbers seem like wide examples? Not in the least. A typical new preamp tube these
days at a given bias and voltage, in the 12AX7 family, will run from ½ a milliamp to three times that.
They typical average is about 70% of what is expected as standard spec by the way.
Now, in our airplane example, our horsepower may be way off with two different HP engines in the
stock amp, and with the easiest form of balance as above, at least our HP is the same, which is of help.
But, we still have the factor that one of the engines is running at a different RPM or Manifold
Pressure. This is the ?time? component that is missed in most matching. You can balance the circuit
all you want, from an amp designers standpoint, to compensate for voltages or current, but you
cannot balance the time component, all one can do is average in a manner.
A true balanced dual triode, is just like two output tubes. We want their characteristics balanced in
many aspects. This is why we do not plug in one EL-34 and one 6L6, even though this would work, and
make some sound come out of the amp. In fact, in this example, these two totally different tubes
would be closer in characteristics than the typical new dual triode of today.
What is required here is to select for output, and TIME. This can only be done on very sophisticated
equipment such as a vacuum tube curve tracer. The two curves, as all voltages of operation, and with
a signal applied to the tube, are compared and sought to be as close to identical as can be achieved.
This is very costly as it is very time consuming. At times, only 1 in 50 tubes will make the grade.
Remember, it is THIS little tube that dives your final output stage.
The folks in the high end audio industry know the difference in balanced or matched inverters, and
many amp makers do also. There are still a lot of ?amp? folks out there that want to fight the points
here, mostly because they have not taken the time component into their thinking. If they have tried
?matched? phase inverters from simple sources with simple current match methods, this may be one
reason that there was not as much difference as they had expected.
The Special Applications Group (SAG) at GT was formed specifically to address issues such as vacuum
tube development, testing of factory samples and production, and unique products. The SAG-AX7-MPI
and SAG-AT7-MPI are just two of their products for the audio industry.
Part 2 - Gain Versus Output in preamp tubes.
Think of your days back in science class, where you built or saw one of those big ball devices, that
created a half a million volts, made a great light show, and you could touch it, and have the classes
hair stand on end! Gain in a guitar amp is much the same. You can have a 20 watt amp with high gain,
and shred all day long, just as ?gainy? as a 100 watt monster. It is the wattage, or output, that differ.
Today?s 12AX7?s as an example, all have about the same gain, which in this case should be about 100.
Most today fall below this, in the range of maybe the mid 80?s, with some samples going up to maybe
110. The big difference in tubes though is current output. A typical 12AX7 is expected to put out 1.2
milliamps at a given test voltage. Today?s tubes in general, put out as little as ½ of that in 80% of the
cases. A tube at 0.8 milliamps has a full 30% less output than what is expected. This is like a 50 watt
amp putting out 30 or so watts. Some tubes are better than others from various manufacturers. Some
examples of this are tubes like the widely used, and perhaps the most popular are the Sovtek
12AX7WA. This is a sturdy tube, generally free of microphonics, with acceptable gain. Part of the
reason they are quiet, is they tend to be lower in gain than many other 12AX7?s, but are also much
lower on output current. It is much the same as with power tubes; where in the same amp, one duet
of output tubes will put out 50 watts, while another set only 45 watts. Thus, the Sovtek 12AX7WA, is
quiet, due to lack of output and gain in many amp circuits. These tubes are sturdy and inexpensive,
and help a lot of amps make it through the warranty period. On the other side of the coin are the JJ
ECC83 tube. This is a part of the 12AX7 family, but different construction, plate materials, cathode
coatings, and other factors give this tube a bit more gain than most others. This can be 100-120 Mu,
rather than the 85 or so of a 12AX7WA Sovtek. The big difference in the ECC83 in general though, is
it?s current output, as times over 1.5 milliamps, or in some cases, three times MORE than a
Sovtek12AX7WA/
Like I have said in the past, preamp tubes are a crapshoot. You buy your tubes and take your
chances. Some folks like Groove Tubes, screen and test for microphonics, noise, AND low output.
The ones that do not pass all tests are rejected, in some cases, over 50% of the factory run. There is
still a range of specs even in these more rigidly tested tubes, but the spread is much tighter. In any
of these cases, the end user still does not know what the tube is actually doing. The SAG area at GT
takes tubes, and runs them through another process, where all the specs are recorded and traces are
performed. These traces show rise time, and other factors. Like the SAG-MPIs, there are other ?kits?
such as the MHG (Marshall High Gain) kits, which can be used in any 12AX7 based amp, not just
Marshalls. I guess I should have called this a High Power kit, which would have been more accurate
actually. There are Fender High Gain Kits, and Fender Soft Touch Kits, and the SAG generally tailors
preamp tubes for specific uses and tastes.
In any case, don?t confuse gain and output. They are very different qualities in a tube.
Myles S. Rose
Guitar Amplifier Blueprinting
2003
I have written other papers on each of these subjects, and questions and comments come up where
additional explanation has been helpful for a lot of folks.
The first basic point to remember here, is that unlike a power tube such as a 6L6, EL-84, EL-34, 6550,
and others, a preamp tube is a ?dual triode? in most cases. This includes tubes such as a 12AX7,
ECC83, 7025, 12AT7 and others.
In regard to matching of two sides of a preamp, many folks feel that in a balanced circuit which in a
way, "sums" the two sides of the tube, that balancing is not necessary. The high end audio industry
recognizes the need for a balanced phase inverter or drivers, unlike some in the musical instrument
industry.
This is necessary (the balance of two sides of a triode). I will use an example to try to illustrate.
Taking a twin engine airplane, let?s say we have a typical light twin with two 300 HP engines designed
to cruise a 250 knots. We have each engine at 1800 rpm. One engine is at 24" of manifold pressure,
while the other is at 22" of MP. The airplane structure itself, is the "balance", much like the balanced
circuit that some amp builders feel will negate the need for a balanced phase inverter. The plane may
fly just fine, but we need to add a bit of rudder trim, and out fuel use will be higher, and overall
performance and balance of the airplane (or amp) will suffer.
Now in the "art" of balancing a dual triode, there are many folks that will perform "matching" for a few
dollars of additional cost. In almost all cases, this ?match? is a gain match, or a current match, or a
transconductance match. Due to very high inconsistencies in today's preamp tubes, matching for any
of these factors, is an improvement over an unknown tube. This sort of matching at least makes sure
that we do not have a 200 HP engine on one side of our airplane and a 400 HP engine on the other
side. These numbers seem like wide examples? Not in the least. A typical new preamp tube these
days at a given bias and voltage, in the 12AX7 family, will run from ½ a milliamp to three times that.
They typical average is about 70% of what is expected as standard spec by the way.
Now, in our airplane example, our horsepower may be way off with two different HP engines in the
stock amp, and with the easiest form of balance as above, at least our HP is the same, which is of help.
But, we still have the factor that one of the engines is running at a different RPM or Manifold
Pressure. This is the ?time? component that is missed in most matching. You can balance the circuit
all you want, from an amp designers standpoint, to compensate for voltages or current, but you
cannot balance the time component, all one can do is average in a manner.
A true balanced dual triode, is just like two output tubes. We want their characteristics balanced in
many aspects. This is why we do not plug in one EL-34 and one 6L6, even though this would work, and
make some sound come out of the amp. In fact, in this example, these two totally different tubes
would be closer in characteristics than the typical new dual triode of today.
What is required here is to select for output, and TIME. This can only be done on very sophisticated
equipment such as a vacuum tube curve tracer. The two curves, as all voltages of operation, and with
a signal applied to the tube, are compared and sought to be as close to identical as can be achieved.
This is very costly as it is very time consuming. At times, only 1 in 50 tubes will make the grade.
Remember, it is THIS little tube that dives your final output stage.
The folks in the high end audio industry know the difference in balanced or matched inverters, and
many amp makers do also. There are still a lot of ?amp? folks out there that want to fight the points
here, mostly because they have not taken the time component into their thinking. If they have tried
?matched? phase inverters from simple sources with simple current match methods, this may be one
reason that there was not as much difference as they had expected.
The Special Applications Group (SAG) at GT was formed specifically to address issues such as vacuum
tube development, testing of factory samples and production, and unique products. The SAG-AX7-MPI
and SAG-AT7-MPI are just two of their products for the audio industry.
Part 2 - Gain Versus Output in preamp tubes.
Think of your days back in science class, where you built or saw one of those big ball devices, that
created a half a million volts, made a great light show, and you could touch it, and have the classes
hair stand on end! Gain in a guitar amp is much the same. You can have a 20 watt amp with high gain,
and shred all day long, just as ?gainy? as a 100 watt monster. It is the wattage, or output, that differ.
Today?s 12AX7?s as an example, all have about the same gain, which in this case should be about 100.
Most today fall below this, in the range of maybe the mid 80?s, with some samples going up to maybe
110. The big difference in tubes though is current output. A typical 12AX7 is expected to put out 1.2
milliamps at a given test voltage. Today?s tubes in general, put out as little as ½ of that in 80% of the
cases. A tube at 0.8 milliamps has a full 30% less output than what is expected. This is like a 50 watt
amp putting out 30 or so watts. Some tubes are better than others from various manufacturers. Some
examples of this are tubes like the widely used, and perhaps the most popular are the Sovtek
12AX7WA. This is a sturdy tube, generally free of microphonics, with acceptable gain. Part of the
reason they are quiet, is they tend to be lower in gain than many other 12AX7?s, but are also much
lower on output current. It is much the same as with power tubes; where in the same amp, one duet
of output tubes will put out 50 watts, while another set only 45 watts. Thus, the Sovtek 12AX7WA, is
quiet, due to lack of output and gain in many amp circuits. These tubes are sturdy and inexpensive,
and help a lot of amps make it through the warranty period. On the other side of the coin are the JJ
ECC83 tube. This is a part of the 12AX7 family, but different construction, plate materials, cathode
coatings, and other factors give this tube a bit more gain than most others. This can be 100-120 Mu,
rather than the 85 or so of a 12AX7WA Sovtek. The big difference in the ECC83 in general though, is
it?s current output, as times over 1.5 milliamps, or in some cases, three times MORE than a
Sovtek12AX7WA/
Like I have said in the past, preamp tubes are a crapshoot. You buy your tubes and take your
chances. Some folks like Groove Tubes, screen and test for microphonics, noise, AND low output.
The ones that do not pass all tests are rejected, in some cases, over 50% of the factory run. There is
still a range of specs even in these more rigidly tested tubes, but the spread is much tighter. In any
of these cases, the end user still does not know what the tube is actually doing. The SAG area at GT
takes tubes, and runs them through another process, where all the specs are recorded and traces are
performed. These traces show rise time, and other factors. Like the SAG-MPIs, there are other ?kits?
such as the MHG (Marshall High Gain) kits, which can be used in any 12AX7 based amp, not just
Marshalls. I guess I should have called this a High Power kit, which would have been more accurate
actually. There are Fender High Gain Kits, and Fender Soft Touch Kits, and the SAG generally tailors
preamp tubes for specific uses and tastes.
In any case, don?t confuse gain and output. They are very different qualities in a tube.
Myles S. Rose
Guitar Amplifier Blueprinting
2003
The Groove Tubes SAG tubes may be ordered directly off the Groove Tubes
website by cllcking here. You may specify specific ratings which may or may not be
available. You may direct inquires, or receive assistance in selection or building a
specific kit by emailing me directly at techsupport@groovetubes.com
website by cllcking here. You may specify specific ratings which may or may not be
available. You may direct inquires, or receive assistance in selection or building a
specific kit by emailing me directly at techsupport@groovetubes.com
| Expected preamp tube characteristics Tube mA TC Mu (gain) 12AX7 ECC83 7025 1.2 1600 100 12AT7 ECC81 10.0 5500 60.5 12AY7 3.0 1750 44 12AU7 ECC82 10.6 2200 17-20 mA is current in milliamps. This is standard 1957 industry spec at standard test reference voltage and bias settings. |
| Click Here to return to the main page |