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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.
PM6L6GC:
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.
Playing/listening tests:
First the guitar...1981 or 82 Phantom A5.
Some info here:
The Hamer Prototype[/URL]
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 sounding...like a broken speaker.
The speaker is a Celestion G12M70, no risk of breakup...so 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
pulled.
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.
). The GTs are supposedly relabeled Shunguang tubes, and are meant to compliment the amp well. However, I wanted some more opinions, so does anyone have any comments on this selection?
