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Vintage threads from the first ten years

Re: Mook's Taste Tests: 12AX7s

5/6/1999 11:52 AM
jason	Re: Mook's Taste Tests: 12AX7s Welcome to ampage! Yes it is cool to post URL's for sites here. Ned does post here from time to time (he does have an informative site doesn't he). jason

5/6/1999 12:05 PM
Reid Kneeland	I'll join the Ned Admiration Chorus as well. Usually a buyer has to make some tradeoff between price and service, but I've gotten both the lowest prices I've seen and excellent service from Triode. Reid

5/8/1999 6:51 PM
MKB	One more member of the Ned Admiration Chorus pipes in...... The last couple times I placed an order with Ned, he gave me brief but important lessons on tubedom. One time I was modifying an ST70 and needed some 6DJ8's, he asked about the circuit and gave me a great explanation on the differences between beam power tubes and power pentodes (6L6 vs. EL34). He recommended a brand of tubes, gave the reasons why, and of course they worked great! These are the guys that we need to give much business to for they truly deserve it!

5/8/1999 6:51 PM

MKB

One more member of the Ned Admiration Chorus pipes in......

The last couple times I placed an order with Ned, he gave me brief but important lessons on tubedom. One time I was modifying an ST70 and needed some 6DJ8's, he asked about the circuit and gave me a great explanation on the differences between beam power tubes and power pentodes (6L6 vs. EL34). He recommended a brand of tubes, gave the reasons why, and of course they worked great! These are the guys that we need to give much business to for they truly deserve it!

5/5/1999 9:38 AM
Bruce	Hey Mook, Have you ever tried these tube tests with a few of your known "good ears" guitar player buddies and their tube amps/guitars? Sometimes I'm very surprised by what my customers say about the tone of certain tubes in certain amps and it makes me scratch my head in bewilderment at times as amp tone is so personal and subjective. Bad tubes sound bad to just about everyone but good tubes and great preamp tubes are not quite so obvious to most and they really sound different in different hands and gear. I like the fact your including what all your test gear is during these little shoot outs. Since so many of your preamp tubes leaned into a honky sound or middy sound I wonder if this is also an altered tonal response from the guitar, speaker or amp too. Maybe the tubes that did not sound honky were scooped in the 1Khz region or somewhere else in frequncey bandwidth that complimented the slightly skewed tested equipment. What do you think? Bruce Mission Amps

5/5/1999 9:38 AM

Bruce

Hey Mook,
Have you ever tried these tube tests with a few of your known "good ears" guitar player buddies and their tube amps/guitars?
Sometimes I'm very surprised by what my customers say about the tone of certain tubes in certain amps and it makes me scratch my head in bewilderment at times as amp tone is so personal and subjective.
Bad tubes sound bad to just about everyone but good tubes and great preamp tubes are not quite so obvious to most and they really sound different in different hands and gear.
I like the fact your including what all your test gear is during these little shoot outs.
Since so many of your preamp tubes leaned into a honky sound or middy sound I wonder if this is also an altered tonal response from the guitar, speaker or amp too.
Maybe the tubes that did not sound honky were scooped in the 1Khz region or somewhere else in frequncey bandwidth that complimented the slightly skewed tested equipment.
What do you think?

Bruce
Mission Amps

5/5/1999 6:45 PM
Steve A.	Bruce: I'm glad that you brought up those points! I have about 10 different tube amps and preamps, and some preamp tubes sound really great in one amp but like total crud in another. While I like the EI 12AX7's in most of my amps, there are a few amps that just don't work well with them. Some of the amp makers really "tweak" their preamp designs to work well with the preamp tubes they are using; I haven't noticed as much of that tweaking with the output sections. Thanks! Steve Ahola

5/5/1999 6:45 PM

Steve A.

Bruce:

I'm glad that you brought up those points! I have about 10 different tube amps and preamps, and some preamp tubes sound really great in one amp but like total crud in another. While I like the EI 12AX7's in most of my amps, there are a few amps that just don't work well with them. Some of the amp makers really "tweak" their preamp designs to work well with the preamp tubes they are using; I haven't noticed as much of that tweaking with the output sections.

Thanks!

Steve Ahola

5/5/1999 9:27 PM
Joe Zuzze	Mook, It's always a pleasure reading your taste tests. I was surprised about hearing a 5751 used as a PI tube, I thought these were almost exclusively 1st stage. Yep, the Ei's are pretty nice, at least I know I'm on the waiting list for the golden ear awards. Joe

5/6/1999 11:02 AM
MBSetzer	5751 is a balanced tube, nostalgia sets in I have had a number of different 5751's in my surplus box for years. They were fairly common in precision industrial amplifiers. These are some pretty old and used tubes but I always seem to like the sound in any socket where a 12AX7 is intended. In the GE book the ordinary consumer 12AX7 is described as a High-Mu Twin Triode, based on its design. It is the highest Mu of tubes in the family that fit sockets wired according to digram 9A. The available signal gain of a preamp triode is based on its Mu value, higher Mu triodes will give more ultimate signal gain than lower Mu ones. Of course 12AX7 was intended for radio, TV, audio, guitar, accordion, bass, etc. as well as most other types of vintage electronics where a transistor would be used today. So many applications have no contribution whatsoever to producing sound, it is considered a general purpose tube. This was one of the most common and useful tubes which is why they are still being made in more than one plant, I would assume to exceed the same specification as GE lists: 12AX7 shows a heater spec of 0.3 amps at 6.3 volts, maximum plate watts of 1.2W per section, and a Mu of 100. For reference the 7025 is listed as a High-Mu Twin Triode, a special 12AX7 for use in audio; heater 0.3A, Mu 100, but maximum plate watts only 1.0W per section. This is difficult to interpret without real inside information, but if a manufacturer made all their high Mu 9A tubes to exceed 1.2W capability & 100 Mu, and if they all passed the additional audio specs such as noise & microphonics, then they would all qualify as 7025. Other manufacturers which ordinarily made 12AX7's that did not meet audio specs would likely have been able to make 7025's when necessary, and they might have been completely different tubes internally, or by manufacturing procedure. The way the 7025 spec is based on the 12AX7 spec, it would also be possible to select ordinary 12AX7's individually or by batch that meet the 7025 requirements. IIRC when the SF Fenders were first coming out, I had to dig into this to help musicians who saw 7025 on the tube chart & thought CBS had put in some weird tube that they couldn't get at the local drugstore or TV shop. It was confusing then because by that time the major US tubemakers' 12AX7's were probably about 99 percent exceeding the 7025 spec. The general consensus among oldtimers then was that it was hype, and whether a tube was marked 12AX7, 7025, or both didn't make a difference any more. Anyway, the 5751 is listed as a High-Mu Twin Triode too, a special 12AX7 for industrial or military use as a voltage amplifier, with balanced sections, lower ratings, and higher heater current. It has maximum plate watts of 0.7W per section, Mu of 70, and heater current 0.35A at 6.3V. The balanced sections would come in handy for some types of phase splitter where it could almost seem essential. I would review schematics technically to find out if it theoretically would help a particular amp, or scope it on the bench trying various tubes to be sure. Too late for me since I've already put so much effort into circuitry to make unbalanced sections give similar performance. Too much information so far? Intermission Now might as well go over the top . . . I did find an NOS 5751 just now brand new in the box, and decided to test against 3 more, including a brand new NOS 12AX7WA from the same company & decade. Using the Hickock 539C tube tester the conditions for 12AX7, 7025, & 5751 are identical, and the minimum transconductance (Gm)in micromhos (uMho) is 875 per triode to be considered a serviceable tube, this is interesting, here goes: 12AX7WA NOS JAN Philips 1987 week 26 triode #1 2700 uMho triode #2 2400 uMho 5751 NOS JAN Philips 1983 week 26 triode #1 2700 uMho triode #2 2750 uMho 7025 Used Fender Special Design 1977 week 35 the above info is printed in orange lettering, the 7025 slightly overlaps where the glass is permanently etched 12AX7A U.S.A. with a pattern of dots below that. triode #1 2400 uMho triode #2 2550 uMho 5751 Used RCA 1959 week 52 triode #1 2750 uMho triode #2 2750 uMho Since Mu is directly proportional to Gm, and test conditions were identical, it appears that the Mu specification of 70 for the 5751 is probably a minimum value that must be exceeded to qualify as an on spec tube. Most likely ordinary 12AX7's were officialy acceptable as long as the sections were balanced within the 5751 spec, since they would all have Mu of 70 or greater. Looks like in reality there is no actual intended difference in Mu of the triodes, just a more lenient spec for the 5751, kind of a tradeoff for the requirement for balanced sections. Looking at the tubes is a different story. The Philips 12AX7WA has the longest plates, but not anything untypical, it has a halo getter. The Philips 5751 does look extremely similar, the plates are the same shape, just a little shorter, they are the size that is the most common for US tubes. Its halo getter is not open but contains a disc like some overseas tubes. The Fender 7025 (12AX7A) looks very much like the Philips 5751, with the same size & shape plates but they are notched toward one side of the tube, the halo is open, but there is an additional piece of mica pushed down upon the regular upper spacer, looks like this additional piece is acting as a spring to keep the filaments from moving, rumor is that by 1977 Fender was working closely with Sylvania to make sure the tubes they used were optimized for performance upside-down, as they are installed in Fender amps. That is supposed to be the origin of the STR Sylvania 6L6GC's, initials mean Special Test Requirement. Wouldn't be surprised if this had an influence on their 12AX7/7025 also. Philips is supposedly the one who took over the Sylvania tube making operations for the final decade, so that may explain why these 3 tubes look so much alike. The old RCA 5751 is noticably different even though the plates are the same size & shape as the Philips 5751 & Fender 7025, they are black not grey like the other three tubes. The RCA halo getter is squarish not round, and there is a nonperforated disc of additional mica separating the getter ring from the upper spacer, isolating it from the other tube electrodes. Nothing like the extra mica in the Fender 7025. Looks like this wafer of mica is metallized from the getter flash to a certain extent like the inside of the glass envelope at the top. All except the NOS 5751 have been tried in a Fender Champ and sound excellent, I didn't listen to them back-to-back, but I seem to remember liking the old RCA 5751 best for tone while I was comparing it to some other used 12AX7's. Everybody's preferences are different, but I like the Champ circuitry a lot for tube evaluation since it only has one each preamp tube & power tube, you can really hear the difference between tubes, and of course matching is not applicable, just have to use a 2 x 12 cabinet with 8 ohm speakers wired parallel to substitute for the built-in 8 inch 4 ohm weinie speaker. Mook, your efforts are extremely valuable, I wish you were in Houston, it would be nice to get together and swap tubes, amps, and guitars around in even more combinations, and maybe take a few readings from amps on the bench. Last week I had a visitor who brought some old RCA NOS Radiotron replacements that were discarded by the Shell Oil Refinery here. Just happened to be two black plate 6L6GC's in there, you know, they really are nice! Yesterday another oldtimer said he will make me a good deal on his collection of surplus from the last decade or two, hope he comes through, most of his was from Shell & Exxon, he says a few hundred tubes, hope there's some guitar types. I'm getting better at using oddballs though, still trying to make the best of what I already have before going out and buying more electronic junk. So much tonnage, so little time . . . MBS P.S. If there is any interest I could post some close-up photos of these tubes, let me know.

5/6/1999 11:02 AM

MBSetzer

5751 is a balanced tube, nostalgia sets in

I have had a number of different 5751's in my surplus box for years. They were fairly
common in precision industrial amplifiers. These are some pretty old and used tubes but I always seem to like the sound in any socket where a 12AX7 is intended.
In the GE book the ordinary consumer 12AX7 is described as a High-Mu Twin Triode, based on its design. It is the highest Mu of tubes in the family that fit sockets wired according to digram 9A. The available signal gain of a preamp triode is based on its Mu value, higher Mu triodes will give more ultimate signal gain than lower Mu ones. Of course 12AX7 was intended for radio, TV, audio, guitar, accordion, bass, etc. as well as most other types of vintage electronics where a transistor would be used today. So many applications have no contribution whatsoever to producing sound, it is considered a general purpose tube. This was one of the most common and useful tubes which is why they are still being made in more than one plant, I would assume to exceed the same specification as GE lists: 12AX7 shows a heater spec of 0.3 amps at 6.3 volts, maximum plate watts of 1.2W per section, and a Mu of 100.
For reference the 7025 is listed as a High-Mu Twin Triode, a special 12AX7 for use in audio; heater 0.3A, Mu 100, but maximum plate watts only 1.0W per section. This is difficult to interpret without real inside information, but if a manufacturer made all their high Mu 9A tubes to exceed 1.2W capability & 100 Mu, and if they all passed the additional audio specs such as noise & microphonics, then they would all qualify as 7025. Other manufacturers which ordinarily made 12AX7's that did not meet audio specs would likely have been able to make 7025's when necessary, and they might have been completely different tubes internally, or by manufacturing procedure. The way the 7025 spec is based on the 12AX7 spec, it would also be possible to select ordinary 12AX7's individually or by batch that meet the 7025 requirements. IIRC when the SF Fenders were first coming out, I had to dig into this to help musicians who saw 7025 on the tube chart & thought CBS had put in some weird tube that they couldn't get at the local drugstore or TV shop. It was confusing then because by that time the major US tubemakers' 12AX7's were probably about 99 percent exceeding the 7025 spec. The general consensus among oldtimers then was that it was hype, and whether a tube was marked 12AX7, 7025, or both didn't make a difference any more.
Anyway, the 5751 is listed as a High-Mu Twin Triode too, a special 12AX7 for industrial or military use as a voltage amplifier, with balanced sections, lower ratings, and higher heater current. It has maximum plate watts of 0.7W per section, Mu of 70, and heater current 0.35A at 6.3V. The balanced sections would come in handy for some types of phase splitter where it could almost seem essential. I would review schematics technically to find out if it theoretically would help a particular amp, or scope it on the bench trying various tubes to be sure. Too late for me since I've already put so much effort into circuitry to make unbalanced sections give similar performance.

Too much information so far?

**Intermission**

Now might as well go over the top . . .

I did find an NOS 5751 just now brand new in the box, and decided to test against 3 more, including a brand new NOS 12AX7WA from the same company & decade. Using the Hickock 539C tube tester the conditions for 12AX7, 7025, & 5751 are identical, and the minimum transconductance (Gm)in micromhos (uMho) is 875 per triode to be considered a *serviceable* tube, this is interesting, here goes:

12AX7WA NOS JAN Philips 1987 week 26
triode #1 2700 uMho
triode #2 2400 uMho

5751 NOS JAN Philips 1983 week 26
triode #1 2700 uMho
triode #2 2750 uMho

7025 Used Fender *Special Design* 1977 week 35
the above info is printed in orange lettering, the *7025* slightly overlaps where the glass is permanently etched *12AX7A U.S.A.* with a pattern of dots below that.
triode #1 2400 uMho
triode #2 2550 uMho

5751 Used RCA 1959 week 52
triode #1 2750 uMho
triode #2 2750 uMho

Since Mu is directly proportional to Gm, and test conditions were identical, it appears that the Mu specification of 70 for the 5751 is probably a minimum value that must be exceeded to qualify as an *on spec* tube. Most
likely ordinary 12AX7's were officialy acceptable as long as the sections were balanced within the 5751 spec, since they would all have Mu of 70 or greater. Looks like in reality there is no actual intended difference in Mu of the triodes, just a more lenient spec for the 5751, kind of a tradeoff for the requirement for balanced sections.
Looking at the tubes is a different story. The Philips 12AX7WA has the longest plates, but not anything untypical, it
has a halo getter. The Philips
5751 does look extremely similar, the plates are the same shape, just a little shorter, they are the size that is the most common for US tubes. Its halo getter is not open but
contains a disc like some overseas tubes.
The Fender 7025 (12AX7A) looks very much like the Philips 5751, with the same size & shape plates but they are notched toward one side of the tube, the halo is open, but there is an additional piece of mica pushed down upon the regular upper spacer, looks like this additional piece is acting as a spring to keep the filaments from moving, rumor is that by 1977 Fender was working closely with Sylvania to make sure the tubes they used were optimized for performance upside-down, as they are installed in Fender amps. That is supposed to be the origin of the STR Sylvania 6L6GC's, initials mean *Special Test Requirement*. Wouldn't be surprised if this had an influence on their 12AX7/7025 also. Philips is supposedly the one who took over the Sylvania tube making operations for the final decade, so that may explain why these 3 tubes look so much alike.
The old RCA 5751 is noticably different even though the plates are the same size & shape as the Philips 5751 & Fender 7025, they are black not grey like the other three tubes. The RCA halo getter is squarish not round, and there is a nonperforated disc of additional mica separating the getter ring from the upper spacer, isolating it from the other tube electrodes.
Nothing like the extra mica in the Fender 7025.
Looks like this wafer of mica is metallized from the getter flash to a certain extent like the inside of the glass envelope at the top.
All except the NOS 5751 have been tried in a Fender Champ and sound excellent, I didn't listen to them back-to-back, but I seem to remember liking the old RCA 5751 best for tone while I was comparing it to some other used 12AX7's. Everybody's preferences are different, but I like the Champ circuitry a lot for tube evaluation since it only has one each preamp tube & power tube, you can really hear the difference between tubes, and of course matching is not applicable, just have to use a 2 x 12 cabinet with 8 ohm speakers wired parallel to substitute for the built-in 8 inch 4 ohm weinie speaker.
Mook, your efforts are extremely valuable, I wish you were in Houston, it would be nice to get together and swap tubes, amps, and guitars around in even more combinations, and maybe take a few readings from amps on the bench. Last week I had a visitor who brought some old RCA NOS Radiotron
replacements that were discarded by the
Shell Oil Refinery here.
Just happened to be two black plate 6L6GC's in there, you know, they really are nice! Yesterday another oldtimer said he will make me a good deal on his collection of surplus from the last decade or two, hope he comes through, most of his was from Shell & Exxon, he says a few hundred tubes, hope there's some guitar types. I'm getting better at using oddballs though, still trying to make the best of what I already have before going out and buying more electronic junk. So much tonnage, so little time . . .
MBS
P.S. If there is any interest I could post some close-up photos of these tubes, let me know.

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