Lake Placid Green
Tele-Meister
It gets confusing when trying to figure out what I want to put in my amp. It's not just a 12ax7, there are like 8 different kinds.
Can anyone tell me the differences in these tubes: 12AX7, 12AX7A/WA/WB/WC, ECC83, 7025
- Thread starter Lake Placid Green
- Start date
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danlad
Tele-Meister
Not a lot.
Historically they might be special versions (low noise, low microphonics, heater warm up times etc) or a result of different regional naming (ECC83 is European, 12AX7 is US) but basically the same functionality.
Probably modern manufacturing just picks different names to give an illusion of choice ;-)
Historically they might be special versions (low noise, low microphonics, heater warm up times etc) or a result of different regional naming (ECC83 is European, 12AX7 is US) but basically the same functionality.
Probably modern manufacturing just picks different names to give an illusion of choice ;-)
theprofessor
Poster Extraordinaire
In terms of interchangeability, what danlad said:
12AX7 US = ECC83 European. Between 12AX7 and 12AX7A, for example, one is designed to be used only in parallel circuits (12AX7) whereas the other can be used in circuits wired in parallel or in series (12AX7A). As for the 7025 versus the 12AX7, the 7025 is a ruggedized military grade low noise version of a 12AX7. I'll leave the other 12AX7 suffixes for others. It seems that the suffixes mean less with new production tubes than they did with vintage tubes. ׂׂׂׂ**edit: as Brent Jessee notes in the information I included below, and as other posters have stated below, the 7025 is not so much milspec as it is hi fi and low noise. It is the W suffixes that indicate milspec.**
All that said, there are some sonic differences between all these types, especially in vintage tubes. But the main point is that in certain circuits, they're all interchangeable.
The following is from Brent Jessee's page, www.audiotubes.com/12ax7.htm:
"The main differences with this family of tubes are as follows:
12AX7: the original version of this tube. This can only be used in parallel filament circuits. This is not a big deal as virtually all audio equipment is of a parallel filament design. These often have large rectangular plates with several horizontal ribs. The older versions have blackplates, which are often preferred by audiophiles.
12AX7A: This version can be used in series or parallel filament circuits. These usually date to the 1960s and have greyplates. Vintage versions of these are about the most sought after tubes of any type today. Often RCA and GE made these for electronic organ manufacturers, and have the organ brand name on the label. These are usually specially selected tubes, and are a great buy---when available! Sometimes, 12AX7A tubes made for the US Military are labeled 12AX7WA, and I have seen WB and WC versions. The W is the military type code, A,B, and C are progressively later productions. These are nice military spec tubes. DO NOT confuse these with current production Russian or Chinese crap with the suffix WX, WB, or WC! These are not military tubes and are not NOS tubes at all!
ECC83: This is the European version of the 12AX7, and is identical to it. Most tubes with ECC83 listed on the label are European, and the ones in demand are Telefunken, Amperex, Mullard, and Siemens. More difficult to find but worth seeking are German made Valvo and the early Holland made Philips Miniwatt and Philips "SQ". Telefunken tubes have a diamond shape molded into the glass on the bottom center of the tube. The most desirable of the Telefunken ECC83 tubes is the "smoothplate" or "flatplate" versions. Like the name implies, the dull grey plates are perfectly smooth. It does not matter if some of these on the market are labeled for Fisher or Dynaco, as Telefunken made these tubes for many OEM applications. They are the EXACT same tube regardless of the label. Watch for the rare Telefunken smoothplates with a cloth label on the glass marked "selected tube". Often these have red painted tips, and watch for blue painted tips, and laboratory grade smoothplates made for Leeds & Northrup as well. These were all screened for critical use in medical equipment, with the "selected tube" or L&N versions being the best. Grass Instruments also had a yellow tip version which was even lower noise, but they are rare and difficult to find these days. All of the red, yellow, and blue tips are incredibly quiet and very three dimensional in sound. Don't miss out on the Amperex version of the Selected Tube with the rare "Electronics for Medicine" label tubes that were made for hospital equipment, and are the same exacting quality as the Telefunken medical tubes, but have the sweet Amperex sound. Amperex and Mullard tubes have a pair of alpha-numeric date codes printed in dark grey near the bottom of the tube. The rarest of these are the BugleBoy cartoon tube label for Amperex (especially if it also has the treble clef music symbol next to the Bugleboy image), and the older Mullard logo that looks like a shield, especially with the letters "BVA" below it. Watch for the Mullard "10M" series of ECC83 tubes in the distinctive royal purple and gold boxes. These sweet tubes were factory screened for a 10,000 hour heater life, matching internal triodes, and low noise, rather like the Telefunken ECC803S tubes. The 10M Master Series has gold plated pins, and the 10M Concert Series has standard pins. Demand and scarcity has driven the price of these babies to record heights lately. The 1950s versions of Mullard and Amperex (and some rare Siemens) tubes often had long narrow plates, with D or square getters, and are the scarcest and finest sounding of these two brands. The 6L13 is the rarest of the longplates, made for ultra low noise. They are nearly impossible to find today. BEWARE: the Amperex Bugleboy and Telefunken smoothplates are being faked! The fake Amperex may have the words "Bugleboy" and the cartoon tube on the box, and the label will not wash or wipe off. The fake Telefunken may have gold pins (real Telefunken ECC83 tubes had only standard pins) and a perfect, baked enamel label that won't wipe off. Our tubes are carefully screened for fakes when we obtain them, and are guaranteed genuine.
7025: This is the low noise, "industrial" version of the 12AX7. This has a spiral wound filament which reduces hum and noise. These are usually the best choice for demanding hi-fi applications, although the premium brands and blackplate types also have very low noise.
ECC803S, E83CC, E803CC, 6L13: This is a premium European version of the ECC83. The "S" indicates Special, and these tubes were screened for low noise and matching internal triode elements. These are rare in the USA. Telefunken and Siemens are the brands most often spotted, although several of the Mullard factories made these for various labels. Most of the Telefunken and Siemens tubes had gold plated pins. BEWARE: I have seen smoothplate Telefunken ECC83 tubes on auction sites with aftermarket gold plated pins, being passed off as NOS ECC803S tubes!! The ECC803S is a frame grid tube, and it's inside construction is different from a smoothplate. The ECC83 smoothplate or ribbed plate Telefunken tubes NEVER had gold pins! Be careful, and get a solid return policy like ours whenever you buy costly tubes like these. Sometimes the E83CC also carried the 12AX7WA military type label, even though it is a European made tube. The build quality and performance of these tubes are similar to the Mullard 10M type listed above, although the sonics will differ between German, Dutch, and British tubes. The 6L13 is rarely seen, and then usually only in a Mullard made longplate, but these are well worth the high price they command. They were made for other brands as well but all have the 6L13 on the label."
12AX7 US = ECC83 European. Between 12AX7 and 12AX7A, for example, one is designed to be used only in parallel circuits (12AX7) whereas the other can be used in circuits wired in parallel or in series (12AX7A). As for the 7025 versus the 12AX7, the 7025 is a ruggedized military grade low noise version of a 12AX7. I'll leave the other 12AX7 suffixes for others. It seems that the suffixes mean less with new production tubes than they did with vintage tubes. ׂׂׂׂ**edit: as Brent Jessee notes in the information I included below, and as other posters have stated below, the 7025 is not so much milspec as it is hi fi and low noise. It is the W suffixes that indicate milspec.**
All that said, there are some sonic differences between all these types, especially in vintage tubes. But the main point is that in certain circuits, they're all interchangeable.
The following is from Brent Jessee's page, www.audiotubes.com/12ax7.htm:
"The main differences with this family of tubes are as follows:
12AX7: the original version of this tube. This can only be used in parallel filament circuits. This is not a big deal as virtually all audio equipment is of a parallel filament design. These often have large rectangular plates with several horizontal ribs. The older versions have blackplates, which are often preferred by audiophiles.
12AX7A: This version can be used in series or parallel filament circuits. These usually date to the 1960s and have greyplates. Vintage versions of these are about the most sought after tubes of any type today. Often RCA and GE made these for electronic organ manufacturers, and have the organ brand name on the label. These are usually specially selected tubes, and are a great buy---when available! Sometimes, 12AX7A tubes made for the US Military are labeled 12AX7WA, and I have seen WB and WC versions. The W is the military type code, A,B, and C are progressively later productions. These are nice military spec tubes. DO NOT confuse these with current production Russian or Chinese crap with the suffix WX, WB, or WC! These are not military tubes and are not NOS tubes at all!
ECC83: This is the European version of the 12AX7, and is identical to it. Most tubes with ECC83 listed on the label are European, and the ones in demand are Telefunken, Amperex, Mullard, and Siemens. More difficult to find but worth seeking are German made Valvo and the early Holland made Philips Miniwatt and Philips "SQ". Telefunken tubes have a diamond shape molded into the glass on the bottom center of the tube. The most desirable of the Telefunken ECC83 tubes is the "smoothplate" or "flatplate" versions. Like the name implies, the dull grey plates are perfectly smooth. It does not matter if some of these on the market are labeled for Fisher or Dynaco, as Telefunken made these tubes for many OEM applications. They are the EXACT same tube regardless of the label. Watch for the rare Telefunken smoothplates with a cloth label on the glass marked "selected tube". Often these have red painted tips, and watch for blue painted tips, and laboratory grade smoothplates made for Leeds & Northrup as well. These were all screened for critical use in medical equipment, with the "selected tube" or L&N versions being the best. Grass Instruments also had a yellow tip version which was even lower noise, but they are rare and difficult to find these days. All of the red, yellow, and blue tips are incredibly quiet and very three dimensional in sound. Don't miss out on the Amperex version of the Selected Tube with the rare "Electronics for Medicine" label tubes that were made for hospital equipment, and are the same exacting quality as the Telefunken medical tubes, but have the sweet Amperex sound. Amperex and Mullard tubes have a pair of alpha-numeric date codes printed in dark grey near the bottom of the tube. The rarest of these are the BugleBoy cartoon tube label for Amperex (especially if it also has the treble clef music symbol next to the Bugleboy image), and the older Mullard logo that looks like a shield, especially with the letters "BVA" below it. Watch for the Mullard "10M" series of ECC83 tubes in the distinctive royal purple and gold boxes. These sweet tubes were factory screened for a 10,000 hour heater life, matching internal triodes, and low noise, rather like the Telefunken ECC803S tubes. The 10M Master Series has gold plated pins, and the 10M Concert Series has standard pins. Demand and scarcity has driven the price of these babies to record heights lately. The 1950s versions of Mullard and Amperex (and some rare Siemens) tubes often had long narrow plates, with D or square getters, and are the scarcest and finest sounding of these two brands. The 6L13 is the rarest of the longplates, made for ultra low noise. They are nearly impossible to find today. BEWARE: the Amperex Bugleboy and Telefunken smoothplates are being faked! The fake Amperex may have the words "Bugleboy" and the cartoon tube on the box, and the label will not wash or wipe off. The fake Telefunken may have gold pins (real Telefunken ECC83 tubes had only standard pins) and a perfect, baked enamel label that won't wipe off. Our tubes are carefully screened for fakes when we obtain them, and are guaranteed genuine.
7025: This is the low noise, "industrial" version of the 12AX7. This has a spiral wound filament which reduces hum and noise. These are usually the best choice for demanding hi-fi applications, although the premium brands and blackplate types also have very low noise.
ECC803S, E83CC, E803CC, 6L13: This is a premium European version of the ECC83. The "S" indicates Special, and these tubes were screened for low noise and matching internal triode elements. These are rare in the USA. Telefunken and Siemens are the brands most often spotted, although several of the Mullard factories made these for various labels. Most of the Telefunken and Siemens tubes had gold plated pins. BEWARE: I have seen smoothplate Telefunken ECC83 tubes on auction sites with aftermarket gold plated pins, being passed off as NOS ECC803S tubes!! The ECC803S is a frame grid tube, and it's inside construction is different from a smoothplate. The ECC83 smoothplate or ribbed plate Telefunken tubes NEVER had gold pins! Be careful, and get a solid return policy like ours whenever you buy costly tubes like these. Sometimes the E83CC also carried the 12AX7WA military type label, even though it is a European made tube. The build quality and performance of these tubes are similar to the Mullard 10M type listed above, although the sonics will differ between German, Dutch, and British tubes. The 6L13 is rarely seen, and then usually only in a Mullard made longplate, but these are well worth the high price they command. They were made for other brands as well but all have the 6L13 on the label."
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7025 is the hifi low noise hum version of the 12ax7
http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/7025.PDF
12ax7wa is the milspec version
https://frank.pocnet.net/sheets/127/1/12AX7WA.pdf
here is a data sheet for the 12ax7a:
https://frank.pocnet.net/sheets/127/1/12AX7A.pdf
the wb version seems to be a sovtek invention I can't find a data sheet for it
they say it higher gain, easier to distort, has a low end bump but then "they" say a lot of things
http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/7025.PDF
12ax7wa is the milspec version
https://frank.pocnet.net/sheets/127/1/12AX7WA.pdf
here is a data sheet for the 12ax7a:
https://frank.pocnet.net/sheets/127/1/12AX7A.pdf
the wb version seems to be a sovtek invention I can't find a data sheet for it
they say it higher gain, easier to distort, has a low end bump but then "they" say a lot of things
Lake Placid Green
Tele-Meister
Awesome. Thank you
Lake Placid Green
Tele-Meister
Thank you. This is exactly what I was looking for. Much appreciated
Viejo
Tele-Afflicted
The NOS tubes are well covered above. For new production anything manufactured by New Sensor are apparently dipped in alphabet soup and named for whatever letters stick.
thickwood
TDPRI Member
You only need to put them in wring down v1- on v2 and roll the tubes around , v1 closest too the input instrument jack has the most effect on the overall eq and tone.
I can not find anything but a sovtek long plate 12ax7 in the phase inverter too get the right sound .
Breakup is natural not smeared into mush with that tube !
For a thicker bottom end a 23 buck GE 5751 in v1 gets it rocking.
mgreene
Tele-Afflicted
One thing to remember - the meaningful difference in 12AX7 types is the way they sound.
Different preamp tubes of the exact same type, manufacturer, and even the same date code can sound different - and have different levels of noise and microphony.
It is always a surprise when a super-duper vintage tube sounds "blah" in a certain amp where a modern cheapo tube sounds good or great.
I had a Silver anniversary Marshall way back when that only sounded good with cheap chinese 12AX7s - Mullards, Telefunken, Amperex et al all sounded terrible in this particular amp.
Recently, I got a Silverface PR that sounds really good with the JJ 12AX7s that came in it. No vintage tubes, so far, sound better in this amp.
Mike
Different preamp tubes of the exact same type, manufacturer, and even the same date code can sound different - and have different levels of noise and microphony.
It is always a surprise when a super-duper vintage tube sounds "blah" in a certain amp where a modern cheapo tube sounds good or great.
I had a Silver anniversary Marshall way back when that only sounded good with cheap chinese 12AX7s - Mullards, Telefunken, Amperex et al all sounded terrible in this particular amp.
Recently, I got a Silverface PR that sounds really good with the JJ 12AX7s that came in it. No vintage tubes, so far, sound better in this amp.
Mike
You can use any of them. Electrically they are basically the same with any equipt made after 1960's, some just have extra features to reduce noise. There is as much sound variation between different brand tubes of same type as there are between the different types themselves. Since about 1980's all that labeling turned into marketing primarily too...the original specs necy to designate a certain type are no longer adhered to. What you want is a good quiet tube of any of those types that sounds good.
Karl Pilkington
Tele-Meister
Don't forget the 12AD7. Non-microphonic and with a 3 millivolt maximum RMS hum level at plate.
As stated, tonally they vary. I am not a fan of the 12AX7 overall, they tend to sound a little harsh when pushed. In some amps however, they sound great. They are ubiquitous so they get used for many amps. Across my several amps I use 5751, 12AY7, 12AT7, 7025 for some variation. Substitutes are great for the amps used at home but, when you play out a bit, you want something that you can replace easily thus the 12AX7s popularity i.e. they can be found everywhere.
Penguinchit
TDPRI Member
Here's a good description (from Billm) of what each tubes does in a Blues Junior:
12AX7-compatible tubes
Many other twin-triode tubes are pin-compatible with the 12AX7. Not all are suited for the job. The 12AX7 is designed for high gain–lots of amplification. That’s why it’s used in so many guitar amps. It’s easy to generate distortion.
The 5751 has virtually identical electronic specifications as the 12AX7, but it only has 80 percent of the gain. If your Blues Junior breaks up too readily and you need a bit more control over clean headroom, the 5751 is a good choice for V1. Many 5751s sound warmer or rounder than equivalent 12AX7s, so if that’s a tone you seek, by all means give them a try.
The 12AY7 is also very similar to the 12AX7, but has less than half the gain. It’s the preamp tube that Fender used in classic amplifiers such as the Tweed Deluxe, many Champs, and the revered 57 Twin. Does half the gain mean half the volume? No. A Blues Junior with a 12AY7 in V1 will produce just as loud a clean tone as one with a 12AX7, but at 7 on the volume knob, while the 12AX7 will produce maximum clean at about 4. After that, the 12AX7 starts producing serious amounts of distortion. With the 12AY7, the onset is much more gradual.
The 5751 or 12AY7 don’t give you any more clean headroom. They only delay the onset of distortion and limit the total amount of distortion.
12AX7: Clean range ends at 4
5751: Clean range ends at 5
12AY7: Clean range ends at 7
So the effect of these lower-gain tubes is to expand the clean headroom across more of the volume control’s range.
These measurements were made with the Fender-specified 1KHz input signal at 50mV, roughly equivalent to a fairly hot guitar being run wide open tone/vol. When the THD (total harmonic distortion) goes over 5 percent, you start hearing the distortion.
The 12AT7 is the wrong tube to use in a preamplifier stage, even though it’s pin-compatible. It has 60 percent of the gain of a 12AX7, but it’s designed with heavier plates and lower internal resistance to push large amounts of current. The lower internal resistance affects the frequency response of the preamp stage, dulling the highs. Some people like that, but it’s better to use the right tube and turn down the treble control!
The 12AU7 has even lower internal resistance than the 12AT7 and even less gain (20 percent of a 12AX7). So it makes for a very dark and quiet amp. The 12AU7 is designed to deliver buckets of current, not a beautiful preamplified signal.
So stick to the 12AX7, 5751, and 12AY7 for your preamp.
Putting a lower-gain tube into V2 is pretty much the same thing as turning down your master volume. So keep a 12AX7 in there and don’t worry about it.
Both the 12AY7 and 12AT7 can be used in the phase inverter (V3) position if you want less drive to your output tubes. The 12AT7 typically has a somewhat darker tone than the 12AY7, but the effect is subtle. Both tubes are darker than the stock 12AX7.
Harp (harmonica) players may get a kick out of using a 12DW7, also known as an ECC832, in V3. This is a hybrid tube that has the gain of a 12AX7 on one side and the gain of a 12AU7 on the other. It drives the output tubes unequally, which causes distortion pretty much all the time. This can be good for harp, because you generally want to drive the amp into distortion to get your blues tone, but don’t want to play loud enough to cause feedback into the microphone.
Some Experiments with Different 12AX7 Brands
I was testing a customer’s Blues Junior that I’d just finished modding and thought it sounded a little dull. It had the usual tone stack and coupling mods, plus the cathode follower, TO20 output transformer, and 6V6 conversion.
I was using the customer’s 12AX7s, so I switched to my bench tubes, which are fairly low-mileage Sovtek 12AX7WCs. The amp sounded better.
With good tubes in V2 and V3, I decided to mess around a little.
Now I’m not a tube-rolling kind of guy. As I said earlier, tubes are like tires: they’re round and they wear out. And most of the differences you can hear are about the same as one notch on one of the tone controls (usually the treble).
So why bother?
Because I had all these different, new 12AX7s on hand. (Don’t ask me how these things happen.) I adjusted the amp to just below the verge of breakup on the volume, set the master for just below ear ringing, set the tone controls for a nice Fender-y scoop. No reverb. There are subtle differences; here are my observations:
Sovtek 12AX7WC The workingman’s tube. Reliable, long-lasting, “standard” tone. There’s a reason why Fender stuffs these into just about everything, but the tone is very straightforward, little harmonic complexity.
Sovtek 12AX7WA The same overall tone as the WC, but slightly more gain, closer to breakup on full barre chords. A bit more complexity. Maybe a little more brightness. Maybe.
Tung-Sol 12AX7 Conventional wisdom says that longer-plate tubes have better lows. The Tung-Sol reissue disproves that. This is a noticeably bright tube with a lot of shiiinng!: shimmery high harmonics, high-end complexity. A little quieter than the Sovteks. I tried another copy of the tube and it sounded identical. The low filament-to-cathode voltage rating means that you should not use these in a cathode follower circuit.
Electro Harmonix 12AX7EH The EH has noticeably more bottom end than the Tung-Sol, more than the Sovteks, too. The top end is smooth and sweet. The plate stamping is identical to the Tung-Sol (they’re from the same company), but the mica and getter are different, and who knows about the grid and cathode structure. One thing I can say about the cathode is that these tubes tend to go hummy in circuits that have un-bypassed cathodes, like the second preamp stage of Blues Juniors. I would not use these in a cathode follower either, but they sure sound pretty. Like the Sovtek WA, this one wants to crunch early on. I don’t use them anymore because of the hum tendency.
JJ ECC83S Bright, aggressive, jangly. Just the thing to wake up a dull-sounding amp. On the other hand, my alarm clock is bright, aggressive, jangly. You can get these selected for high or low gain from some vendors, but the couple that I had on hand were on the quiet side. Overall, I think they’re a little harsh-sounding.
JJ ECC803S Some of the hi-fi guys think this is the second coming of the fabled Mullard and Telefunken tubes of yore. I couldn’t wait to get it out of the amp. Dull, dull, dull. Did I mention dull? Or should I add lifeless? Or that hackneyed phrase about a blanket over the amp? However, I did another modded amp recently, with 6L6s and the TP24/TO26 power and output transformer. Normally, this is a very bright setup. The customer had supplied me with three ECC803s, and the combination of bright output and mild preamp was just perfect. A tech friend in the UK says that the hi-fi guys like the 803 because it’s so neutral.
Tube Amp Doctor 7025WA Antique Electronics was touting these on their page, so I bought a couple with my last order. They’re supposed to be premium, selected tubes. And, yes, that seems to be the case. They sound great, like the Sovtek WA after it’s been ported and polished and running on aviation gas. Firm lows, nice complexity in the highs, balanced from bottom to top. This is a triple-mica tube with short plates, so it should be somewhat more rugged in a combo, less likely to go microphonic.
Sovtek 12AX7 LPS The “long plate spiral” has been around for years and it’s often overlooked. Don’t ask me why; it has the best bass of the bunch, sweet clear highs, and is maybe the loudest tube of the lot. But it also has a bit of headroom. The supposed downside of long-plate tubes is that they get beat up in combo amps and fail quickly. But you can always put a couple of damping rings on it. I have one of these in V1 of my old green board Blues Junior and my jazzer and acoustic-y customers gravitate to it like a comet falling into the sun.
I wish I had a Mullard reissue for comparison, but the last time I played with tubes, I thought it sounded a lot like the EH, warmer and rounder than the Tung-Sol, pretty close to the Sovtek LPS.
Remember, these observations are the result of obsessive, close listening. Each of these tubes is very workable, only a tone knob tweak away from perfection. But you can, if you want, use the tubes’ tone in V1 to jump-start the signal chain and get it headed in the right direction for your playing. And yeah, when you’re up and playing, nobody in the audience would hear a difference with any of these tubes.
12AX7-INCOMPATIBLE Tubes
At first glance, the ECC88, also known as the 6DJ8, looks like a good 12AX7 substitute. It’s not.
The ECC88’s pinout for the plates, grids, and cathodes is the same as a 12AX7. And it has this cool internal shield to separate the two halves. But in circuits intended for the 12AX7 family, the shield will be connected to one side of the filament line. Worse, the ECC88 has a maximum plate voltage of 130V (one datasheet cites only 90V). Your Blues Junior will probably toast it. Someone who tried it wrote to me–the tube evidently shorted out internally, which took out one or both cathode caps and destroyed the Fat circuit. He’s still assessing the damage.
The ECC88/6DJ8 is on the left. Note the shield attached to pin 9. The 12AX7 family uses the pinout on the right. The 12AX7 has a 12 volt filament, but 4 and 5 are usually connected together and the other side of the filament line is connected to pin 9, so the tube can run on 6.3V. When you plug an ECC88 into a 6.3 volt socket wired for a 12AX7, the tube won’t light because 4 and 5 are both connected to the same point. Pin 9, the hum shield, doesn’t connect to anything else. The guy who tried it said he heard a notiecable volume drop when he installed the ECC88; I’m surprised he heard anything at all. Maybe the grid fused to the plate and it just became a path to the next stage.
Some folks in the hifi community evidently rewire their amps so they can replace their 12AX7s with ECC88s. It has a reputation for quietness in the phono circuit. In guitar amps, it’s an “oh no” circuit.
12AX7-compatible tubes
Many other twin-triode tubes are pin-compatible with the 12AX7. Not all are suited for the job. The 12AX7 is designed for high gain–lots of amplification. That’s why it’s used in so many guitar amps. It’s easy to generate distortion.
The 5751 has virtually identical electronic specifications as the 12AX7, but it only has 80 percent of the gain. If your Blues Junior breaks up too readily and you need a bit more control over clean headroom, the 5751 is a good choice for V1. Many 5751s sound warmer or rounder than equivalent 12AX7s, so if that’s a tone you seek, by all means give them a try.
The 12AY7 is also very similar to the 12AX7, but has less than half the gain. It’s the preamp tube that Fender used in classic amplifiers such as the Tweed Deluxe, many Champs, and the revered 57 Twin. Does half the gain mean half the volume? No. A Blues Junior with a 12AY7 in V1 will produce just as loud a clean tone as one with a 12AX7, but at 7 on the volume knob, while the 12AX7 will produce maximum clean at about 4. After that, the 12AX7 starts producing serious amounts of distortion. With the 12AY7, the onset is much more gradual.
The 5751 or 12AY7 don’t give you any more clean headroom. They only delay the onset of distortion and limit the total amount of distortion.
12AX7: Clean range ends at 4
5751: Clean range ends at 5
12AY7: Clean range ends at 7
So the effect of these lower-gain tubes is to expand the clean headroom across more of the volume control’s range.
These measurements were made with the Fender-specified 1KHz input signal at 50mV, roughly equivalent to a fairly hot guitar being run wide open tone/vol. When the THD (total harmonic distortion) goes over 5 percent, you start hearing the distortion.
The 12AT7 is the wrong tube to use in a preamplifier stage, even though it’s pin-compatible. It has 60 percent of the gain of a 12AX7, but it’s designed with heavier plates and lower internal resistance to push large amounts of current. The lower internal resistance affects the frequency response of the preamp stage, dulling the highs. Some people like that, but it’s better to use the right tube and turn down the treble control!
The 12AU7 has even lower internal resistance than the 12AT7 and even less gain (20 percent of a 12AX7). So it makes for a very dark and quiet amp. The 12AU7 is designed to deliver buckets of current, not a beautiful preamplified signal.
So stick to the 12AX7, 5751, and 12AY7 for your preamp.
Putting a lower-gain tube into V2 is pretty much the same thing as turning down your master volume. So keep a 12AX7 in there and don’t worry about it.
Both the 12AY7 and 12AT7 can be used in the phase inverter (V3) position if you want less drive to your output tubes. The 12AT7 typically has a somewhat darker tone than the 12AY7, but the effect is subtle. Both tubes are darker than the stock 12AX7.
Harp (harmonica) players may get a kick out of using a 12DW7, also known as an ECC832, in V3. This is a hybrid tube that has the gain of a 12AX7 on one side and the gain of a 12AU7 on the other. It drives the output tubes unequally, which causes distortion pretty much all the time. This can be good for harp, because you generally want to drive the amp into distortion to get your blues tone, but don’t want to play loud enough to cause feedback into the microphone.
Some Experiments with Different 12AX7 Brands
I was testing a customer’s Blues Junior that I’d just finished modding and thought it sounded a little dull. It had the usual tone stack and coupling mods, plus the cathode follower, TO20 output transformer, and 6V6 conversion.
I was using the customer’s 12AX7s, so I switched to my bench tubes, which are fairly low-mileage Sovtek 12AX7WCs. The amp sounded better.
With good tubes in V2 and V3, I decided to mess around a little.
Now I’m not a tube-rolling kind of guy. As I said earlier, tubes are like tires: they’re round and they wear out. And most of the differences you can hear are about the same as one notch on one of the tone controls (usually the treble).
So why bother?
Because I had all these different, new 12AX7s on hand. (Don’t ask me how these things happen.) I adjusted the amp to just below the verge of breakup on the volume, set the master for just below ear ringing, set the tone controls for a nice Fender-y scoop. No reverb. There are subtle differences; here are my observations:
Sovtek 12AX7WC The workingman’s tube. Reliable, long-lasting, “standard” tone. There’s a reason why Fender stuffs these into just about everything, but the tone is very straightforward, little harmonic complexity.
Sovtek 12AX7WA The same overall tone as the WC, but slightly more gain, closer to breakup on full barre chords. A bit more complexity. Maybe a little more brightness. Maybe.
Tung-Sol 12AX7 Conventional wisdom says that longer-plate tubes have better lows. The Tung-Sol reissue disproves that. This is a noticeably bright tube with a lot of shiiinng!: shimmery high harmonics, high-end complexity. A little quieter than the Sovteks. I tried another copy of the tube and it sounded identical. The low filament-to-cathode voltage rating means that you should not use these in a cathode follower circuit.
Electro Harmonix 12AX7EH The EH has noticeably more bottom end than the Tung-Sol, more than the Sovteks, too. The top end is smooth and sweet. The plate stamping is identical to the Tung-Sol (they’re from the same company), but the mica and getter are different, and who knows about the grid and cathode structure. One thing I can say about the cathode is that these tubes tend to go hummy in circuits that have un-bypassed cathodes, like the second preamp stage of Blues Juniors. I would not use these in a cathode follower either, but they sure sound pretty. Like the Sovtek WA, this one wants to crunch early on. I don’t use them anymore because of the hum tendency.
JJ ECC83S Bright, aggressive, jangly. Just the thing to wake up a dull-sounding amp. On the other hand, my alarm clock is bright, aggressive, jangly. You can get these selected for high or low gain from some vendors, but the couple that I had on hand were on the quiet side. Overall, I think they’re a little harsh-sounding.
JJ ECC803S Some of the hi-fi guys think this is the second coming of the fabled Mullard and Telefunken tubes of yore. I couldn’t wait to get it out of the amp. Dull, dull, dull. Did I mention dull? Or should I add lifeless? Or that hackneyed phrase about a blanket over the amp? However, I did another modded amp recently, with 6L6s and the TP24/TO26 power and output transformer. Normally, this is a very bright setup. The customer had supplied me with three ECC803s, and the combination of bright output and mild preamp was just perfect. A tech friend in the UK says that the hi-fi guys like the 803 because it’s so neutral.
Tube Amp Doctor 7025WA Antique Electronics was touting these on their page, so I bought a couple with my last order. They’re supposed to be premium, selected tubes. And, yes, that seems to be the case. They sound great, like the Sovtek WA after it’s been ported and polished and running on aviation gas. Firm lows, nice complexity in the highs, balanced from bottom to top. This is a triple-mica tube with short plates, so it should be somewhat more rugged in a combo, less likely to go microphonic.
Sovtek 12AX7 LPS The “long plate spiral” has been around for years and it’s often overlooked. Don’t ask me why; it has the best bass of the bunch, sweet clear highs, and is maybe the loudest tube of the lot. But it also has a bit of headroom. The supposed downside of long-plate tubes is that they get beat up in combo amps and fail quickly. But you can always put a couple of damping rings on it. I have one of these in V1 of my old green board Blues Junior and my jazzer and acoustic-y customers gravitate to it like a comet falling into the sun.
I wish I had a Mullard reissue for comparison, but the last time I played with tubes, I thought it sounded a lot like the EH, warmer and rounder than the Tung-Sol, pretty close to the Sovtek LPS.
Remember, these observations are the result of obsessive, close listening. Each of these tubes is very workable, only a tone knob tweak away from perfection. But you can, if you want, use the tubes’ tone in V1 to jump-start the signal chain and get it headed in the right direction for your playing. And yeah, when you’re up and playing, nobody in the audience would hear a difference with any of these tubes.
12AX7-INCOMPATIBLE Tubes
At first glance, the ECC88, also known as the 6DJ8, looks like a good 12AX7 substitute. It’s not.
The ECC88’s pinout for the plates, grids, and cathodes is the same as a 12AX7. And it has this cool internal shield to separate the two halves. But in circuits intended for the 12AX7 family, the shield will be connected to one side of the filament line. Worse, the ECC88 has a maximum plate voltage of 130V (one datasheet cites only 90V). Your Blues Junior will probably toast it. Someone who tried it wrote to me–the tube evidently shorted out internally, which took out one or both cathode caps and destroyed the Fat circuit. He’s still assessing the damage.
The ECC88/6DJ8 is on the left. Note the shield attached to pin 9. The 12AX7 family uses the pinout on the right. The 12AX7 has a 12 volt filament, but 4 and 5 are usually connected together and the other side of the filament line is connected to pin 9, so the tube can run on 6.3V. When you plug an ECC88 into a 6.3 volt socket wired for a 12AX7, the tube won’t light because 4 and 5 are both connected to the same point. Pin 9, the hum shield, doesn’t connect to anything else. The guy who tried it said he heard a notiecable volume drop when he installed the ECC88; I’m surprised he heard anything at all. Maybe the grid fused to the plate and it just became a path to the next stage.
Some folks in the hifi community evidently rewire their amps so they can replace their 12AX7s with ECC88s. It has a reputation for quietness in the phono circuit. In guitar amps, it’s an “oh no” circuit.
Thanks tatittle for reviving this thread. This is a fantastic thread and timely for me as I start to experiment with these types of tubes. Thanks for posting and thanks for all of the knowledgeable input. This is where TDPRI really shines.
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To some degree, the design differences between versions of the 12ax7/ ecc83 applies to way back during the constant changing development of the tube.
By now for example, there is generally no design or construction difference between a 12ax7 and a 7025, instead the quietest 12ax7s are set aside and labeled 7025, as opposed to manufacturers make two different tubes.
Most of todays suffix' pertains to which manufacturer and which brand, bearing in mind that there are few manufacturers but many brands selling the same tube with different names.
Likewise with current production Mullards and Tung Sols etc; Chinese and Russian factories or US EU distributors bought the names and print them on Sovtek and Shugang tubes etc.
There is no Mullard in the UK and no Tung Sol in the US.
But we all knew that, right?
Maybe one day there will only be Pignose and Gorilla making guitar amps, but they will put Fender and Marshall and Mesa logos on them and we will discuss the new Marshall on forums, skipping the fact that it's really a pignose wearing a mask.
By now for example, there is generally no design or construction difference between a 12ax7 and a 7025, instead the quietest 12ax7s are set aside and labeled 7025, as opposed to manufacturers make two different tubes.
Most of todays suffix' pertains to which manufacturer and which brand, bearing in mind that there are few manufacturers but many brands selling the same tube with different names.
Likewise with current production Mullards and Tung Sols etc; Chinese and Russian factories or US EU distributors bought the names and print them on Sovtek and Shugang tubes etc.
There is no Mullard in the UK and no Tung Sol in the US.
But we all knew that, right?
Maybe one day there will only be Pignose and Gorilla making guitar amps, but they will put Fender and Marshall and Mesa logos on them and we will discuss the new Marshall on forums, skipping the fact that it's really a pignose wearing a mask.
FWIW next time you can simply use Google and type in "12AX7 tube comparisons" and similar terms. You'll find exactly what others copied-and pasted. Similar results are available for just about every group of common tubes - and other products.
This chart is very useful. TAD 7025WA are my go to basic, quiet, neutral tone tubes in modern production stuff.
From this article
http://www.guitarsite.com/news/features/12ax7-comparison-of-current-made-tubes/
From this article
http://www.guitarsite.com/news/features/12ax7-comparison-of-current-made-tubes/
