PM Guitar Tubes-A Test

Discussion in 'The Squawk Box' started by Cygnus X1, Oct 8, 2010.

  1. Cygnus X1

    Cygnus X1 Senior Member

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    Jul 18, 2009
    Got a few PM tubes to test, try out, and give some opinions on.
    I use all kinds of tubes, from NOS, used vintage, new, newer, and
    whatever is kicking around in the parts drawer.

    My tube tester is a Hickok 6000A, a portable used by techs in the field
    "back in the day".
    It was calibrated before I bought it, and I trust it.
    I use a set of tubes to test just for calibration only and it is running
    within spec.


    I have not played the tubes yet, here is my test results and opinion thus far:

    Tube Test:
    Hickok Model 6000A Dynamic Mutual Conductance tester
    First tested expected sample Fender branded Chinese 12AX7A
    for calibration sample.
    Normal Mutual Conductance: Normal is 1250.
    Sample Tested at 1400/1425.
    Life test good, stays in the positive 1/3 range, -4 mark drop.
    NOTE: Life test is subjective, I measure by the amount of drop
    under load. Hickok manual indicates if it stays in the green, it is OK.

    PM 12AX7A #1: Strong silk screen.
    Pins were bent, straightened to fit in the tester socket.
    Marked “Made in China”.
    Filament Continuity: OK
    Triode #1: Tests Good
    1350 Mut Cond.
    Life test –3 mark drop (Good)
    Triode #2:
    Filament: OK
    Test: Good
    1325 Mut. Cond.
    Life test –5 mark drop (good).
    Conclusion: Tube tests “moderate”, stronger than most premium NOS,
    softer than most Chinese.

    PM 12AX7A #2: Weak silk screen.
    Pins also bent, all over to one side. Straightened.
    Filament: OK
    Triode #1: Tests Good (Low)
    1075 Mut. Conductance (Low)
    Life test –3 mark drop (Good)
    Triode #2:
    Filament: OK
    Test: Good (Low)
    1100 Mut. Cond. (Low)
    Life Test: -3.5 drop (Good)

    Conclusion: Significant difference between the two 12AX7 tubes.
    Although they should last a long time there should be a noticeable
    difference in gain characteristics.
    Both were well balanced from one triode to the other.
    Either tube could be used as a phase inverter.
    Not sure why the pins were bent, the boxes look good and the
    tubes were not bottomed out in the boxes.

    Base looks cheap, sharp corners on the top of the base.
    Has an appearance of plastic rather than bakelite.
    Silk screened, no other markings other than the silk screen
    on the tube glass.
    No indication of manufacturer or country of origin on the
    tube or on the box.
    Shipped in a duet.
    6L6GC #1:
    Filament: OK
    Test: Good
    Expected Mut Cond: 5000 Tests 5250 (Good)
    Life Test: -2.0 (Good)
    6L6GC #2:
    Filament: OK
    Test: Good
    Mut Cond: 4950 (Weak)
    Life Test: -.5 (Excellent)

    Conclusion: 6L6 is near the top of the range of the tester.
    Although it tests accurately I depend on actual amplifier conditions
    to detect any mismatch. Mutual conductance is always borderline
    at this range.
    I can tell more from current draw on the line test, which was within 5%,
    which usually indicates a good match.
    Ed B, GPDUSA and rocknhorse1 like this.
  2. Phildog

    Phildog Senior Member

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    Apr 11, 2010
    Kool reading! Thanks Cyg. I've been looking for some good reading on this subject. Everything on the web seems so subjective though.

    That tester reminds me of the dashboard of an old jet. Kool!!!

    Do/Can you compare different 12AX7's or 6V6's and EL84"s. It would be interesting to see their respective performances under load.
  3. Splattle101

    Splattle101 V.I.P. Member

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    May 30, 2008
    Interesting. Thanks for doing this and posting the results. It's nice to see some solid information instead of the usual opinions. :hmm:

    And thanks to GPD Tubes for sending the valves to be tested, too. That shows confidence in their product at the very least. :thumb:
  4. rocknhorse1

    rocknhorse1 Senior Member

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    Feb 22, 2010
    Thanks for this Cyg!! Nice job. :thumb:

    GPDUSA Member

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    Sep 23, 2010
    Great Tests Cygnus, it's obvious you have a lot of knowledge in this area. Can't wait to read the 'ear test' results!

  6. Cygnus X1

    Cygnus X1 Senior Member

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    Jul 18, 2009
    Playing/listening tests:
    First the guitar...1981 or 82 Phantom A5.
    Some info here:
    The Hamer Prototype

    Andy Summers used this guitar for the Police "Synchronicity" album.
    It has a good LP sound with the bridge HB and a ringing clear sound
    with either bridge or neck single coil.
    Also has a good "Rosewood FB" Tele sound.
    Other players that favor this guitar are Steve Stevens (Billy Idol)
    and Glenn Tipton (Judas Priest) who went with different trem systems for it.
    Mine is just after the Prototype:


    Shown without that goofy looking scratchplate.


    The amp is the Cygnus-7 combo.
    2x12AX7, single ended 6L6GB or GC for the power tube.
    Good, touch sensitive amp when dialed in just on the
    verge of breakup.

    First I installed both the PM 12AX7's to isolate the power tube tests.
    Tube #1: Groove Tubes 6L6GTR grade#7.
    Quiet...but kind of cold. Breakup is fizzy a broken speaker.
    The speaker is a Celestion G12M70, no risk of it's a tube
    making that sound.

    Tube#2: 1960's GE 6L6GC:
    Cleaner, more headroom. Adjusted volume for more breakup.
    Midrange more pronounced, it started to "bloom" easily
    fretting on the D and G strings.
    Harmonics noticeably more pronounced.

    Tube #3: PM 6L6GC
    Volume went down from the GE.
    Adjusted again for a bit of breakup.
    Distortion wasn't bad like the Groove Tubes 6L6 was,
    but it sounded muffled.
    Upper midrange was pronounced, but no bloom
    unless I really wheeled up the volume...and that is a
    result of the speaker reacting to the guitar.
    Another thing...the tube was noticeably hotter when
    I gave it ten minutes to cool like the others.

    Tube#4: Conn 6L6GB (RCA)
    Volume output lower (It's a GB rather than a GC).
    Clear, very high headroom...the most of them all.
    Did not bloom as easily as the GE did.
    Nice breakup at twice the volume setting on the amp.
    But the animals look scared and my wife came out and gave me a funny look...
    (No wonder they used this for an organ tube)!

    Tube #5: the other PM 6L6GC
    Sounded much clearer than the first one.
    Not as good as the GE or RCA, but far better than
    the Groove Tubes.
    It didn't get as hot as the first PM.

    So ranking:
    1: Conn/RCA cleanest
    2: GE best tone, best breakup, and nice bloom at low volume.
    Some people would consider that to be microphonic in high gain situations though.
    3: PM #2: Like a cross between GE and RCA, nice sounding tube
    4: PM #1: Muddy, but not as bad as
    5: Groove Tubes. I already know why they get a bad rep, this confirms it.

    12AX7 test in the next post.
  7. Cygnus X1

    Cygnus X1 Senior Member

    Likes Received:
    Jul 18, 2009
    12AX7A testing:
    Switched guitars to a Jackson solidbody DKMGT “shredder”.
    Dual humbucker, Jackson J90C in the bridge, another
    older Jackson pup neck position.
    The guitar has a nice clear high midrange tone typical of a good
    It really emphasizes differences between the preamp tubes.


    Testing is same amp with MV set to clean, preamp drives set
    to varying levels as will be noted.
    The preamp setup of the Cygnus-7 is:
    V1: Gain stages 1 and 2
    V2: Gain stage 3 and cathode follower TMB tone circuit.
    Then Master Volume stage is last.
    Power tube is the above noted GE 6L6GC.

    First test is V1, first two gain stages only.
    V2 is fitted with a neutral Phillips JAN 12AX7WA.

    Tube #1: Sovtek 12AX7WC
    Clean: Muddy...breaks up too easily.
    Distortion (Gains at “10”): I expect more,
    especially from a Sovtek.
    Not a pleasing distortion, too much buzz sound.

    Tube #2: PM #1, 12AX7A (Strong silk screen)
    Clean: Much clearer, like the difference between a
    bad pickup and a good one.
    Not quite “bell like”, but far better than the Sovtek.
    Distortion: Clear...harmonics came through quite well.
    Drive is moderate.

    Tube #3: JJ ECC83 (12AX7A)
    Clean: Like the was hard to get a good clean.
    But the tone is very good, would sound great in a Champ, Tweed Deluxe
    or many other Fender circuits where a player wants smooth breakup.
    Distortion: Very crunchy, in a good way.
    But not a “clear distortion” like the PM #1

    Tube #4: Tung-Sol Russian reissue
    Fat tube, just barely fits in the socket retainer.
    Clean: Just a bit muddy. Similar to the Sovtek
    but not as bad. Breakup is...meh.
    Distorted: Reveals what the “issue” with the clean was.
    A dark sounding tube, I resisted the temptation to dime up
    the treb to 10 (I kept the tone controls steady throughout the tests).
    But then I did, and it did not clear up, it just added some high end
    This tube would be great for a SLO guy running everything on “12”!

    Tube #5: Chinese Fender 12AX7A
    Clean: Clean but boring. No breakup early.
    Tried to run the gains up, real tone there.
    Distorted: Real vanilla sounding as well.
    OK, but nothing to cross the street for.

    Tube #6: PM #2 12AX7A (Soft silk screen)
    Clean: Very much like the first PM.
    Not real clean like the Fender.
    I know without plugging one in, that a Phillips JAN
    would be the clearest with barely any breakup.
    Distortion: Again, like the first...not a ton of gain like some others but
    delivered the harmonics. I dialed them up briefly and banged on the amp to check for microphonics...not a problem there.

    I’ll call it a day there.
    Not sure testing all into V2 would tell me much that I
    didn’t find out in V1.
    The differences were very noticeable, and the PM’s held
    up very well.
    I admit I am partial to high gain (as long as they aren’t microphonic)
    12AX7’s of Chinese manufacture.
    Was also surprised by the fact my “new favorite” Tung-Sol sounded
    dead compared to most of the others.

    It takes a lot of time and a clear mind to fairly roll tubes, but it is fun!

    I’d recommend the PM’s for the players who like a clear breakup in
    their preamp tubes. Probably not for the Twin player who wants the
    best cleans...I still stay with old RCA, Sylvania, Mullards, etc...the
    nosebleed territory of the hifi world when dealing with a stodgy

    I need to try them in my 5E3, I think they would be a good match there.

    Appreciate the opportunity to test them!
  8. Ed B

    Ed B Senior Member

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    May 6, 2008
    Great post, thanks!
  9. Clyde Billt

    Clyde Billt Senior Member

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    Apr 22, 2010
    Excellent post geezer!
    Thanks for all the time and effort. Much appreciated.

    I was also happy with my Tung-Sol 12AX7 re-issues. Now I'll have to try some others.
    Yeah, thanks for that as well
  10. GPDUSA

    GPDUSA Member

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    Sep 23, 2010
    Thanks again for all the tests Cygnus, great job!
  11. Cygnus X1

    Cygnus X1 Senior Member

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    Jul 18, 2009
    PM 12AX7 High Gain (HG) Tube Test:
    Tube #1:
    Pins were OK.
    Marked “Made in China”.
    Blue silk screen looks thick but not well applied (crooked).
    Filament Continuity: OK
    Triode #1: Tests Good
    1650 Mut Cond. (1250 expected, other PM tube tested 1350)
    Life test –3.5 mark drop (Good)
    Triode #2:
    Filament: OK
    Test: Good
    2100 Mut Cond. (1250 expected, other PM tube tested 1325)
    Life test –1.5 mark drop (good).
    Conclusion: Tube tests VERY strong, a bit unbalanced from one triode to the other.

    Tube #2:
    Pins were OK.
    Marked “Made in China”.
    Blue silk screen looks thick but not well applied (crooked).
    Filament Continuity: OK
    Triode #1: Tests Good
    2250 Mut Cond. (1250 expected, other PM tube tested 1350)
    Life test –2.0 mark drop (Good)
    Triode #2:
    Filament: OK
    Test: Good
    2175 Mut Cond. (1250 expected, other PM tube tested 1325)
    Life test –1.5 mark drop (good).
    Conclusion: Tube tests VERY strong, better balanced from one triode to the other.

    Listening/playing test:
    Warmed up the amp with normal PM 12AX7 in V1 and Phillips JAN 12AX7WA in V2 as before.
    Guitar is Jackson DKMGT and Cygnus-7 with a GE 6L6GC as before.
    Played the amp setup as described on clean, a little dirt, loud, soft,
    then gains on full.
    I really do like the PM 12AX7’s.
    Not noisy or microphonic, has that “bell like” tone many use
    to describe “holy grail” tubes like Mullard and Amperex.

    PM 12AX7HG:
    Played it first with the gains dimed and the master volume the same.
    Not much more drive, tone was about the same.
    Turned it to clean and applied some different tones, neck pickup, mixed.
    It has a fuller tone than the original PM 12AX7.
    Where this tube really did well was when I turned the volumes to
    mild breakup.
    Full tone, nice bloom, just as I described concerning the best NOS.
    This tube would be great in a Champ, Deluxe, and reminds me of
    “that tone” of the more well known boutique amps. (Ahem, Trainwreck).
    Great touch sensitivity.

    Went back to gains dimed to see how it would compare to the
    #2, a tube that test edmuch higher (the highest I have ever seen)
    for conducatance.
    Went with bridge-only “lead” for a direct comparison.
    Noticeably more gain, but not the fizziness (bees in a bottle)
    that I sometimes get from “borderline” high gain...near microphonic
    I wheeled up the overall master volume to check microphonics.
    Good results, no significant feedback beyond what I would expect
    from normal reaction of amp to speakers.
    Volume was in the range that cats from 3 blocks away would dial 9-1-1 to
    see “What’s up with THAT?”
    Dialing down the gains to normal territory of Marshall non-master volume
    yielded the same pleasing results I heard from the others.
    Clean is OK, not anything I would throw a vintage RCA over the bridge for.
    Not bad, not great.
    A Metallica fan would be pleased at the pronounced midrange.

    My only negatives at this point are this:
    The first PM 12AX7's had bent pins.
    I wouldn't expect every user to have a pin straightener to fix that, and it
    gives an impression of lack of attention to quality.
    Second, the print/silk screen on the tubes looks very cheap, as if they are rebranded knock-offs.
    The tube construction itself is fine, the testing is good, and the sound is better than I expected.

    I'd suggest to PM and GPD that paying attention to those details would go a long way towards working on credibility.

    I appreciate the opportunity to be able to try these out, and have checked, and double checked to be sure my ears were telling me what I thought I was hearing.


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