In regards to these tubes, the added 2 metal structures in them, are they some kind of hum sheild as they do not seem to be set up to reduce Microphonics?
The other day I tested a whole stack of new Mesa ( China made ) 12AT7s and not one of them tested above 55%!
Pure crap, how the hell do they get away with reboxing these?
China ax7s
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Stevem
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China ax7s
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: China ax7s
Makes those expensive NOS I use look damn cheap. .No issues and last forever.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
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Turbojunkie
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Re: China ax7s
Hi Stevem,
Your thread subject says Chinese ax7's, but then in the post it says at7's...which type are you referring to?
Your thread subject says Chinese ax7's, but then in the post it says at7's...which type are you referring to?
Re: China ax7s
What parameter does your tube tester test? 55% of what? Spec transconductance, emissions, gain? What tester are you using?
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Stevem
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N
Both the China made AX7 and the AT7 are constructed with the same internal parts I was questioning.
In regards to the output of the Mesa / China made AT7s I tested , that was done on my Hickok 600a which does Transconductance tests.
And the internal construction of many AT7s , not just the China made ones is a question in itself!
A AT7 is a big wattage tube and some have the plate size I woukd expect to see in resgardes to the wattage rating , while others like these China ones and even Tele made AT7s have far smaller plate structures then I would concider a tube to have if it was going to last !
In regards to the output of the Mesa / China made AT7s I tested , that was done on my Hickok 600a which does Transconductance tests.
And the internal construction of many AT7s , not just the China made ones is a question in itself!
A AT7 is a big wattage tube and some have the plate size I woukd expect to see in resgardes to the wattage rating , while others like these China ones and even Tele made AT7s have far smaller plate structures then I would concider a tube to have if it was going to last !
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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martin manning
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Re: China ax7s
But Matt's question remains: What are you reporting as 55% for these 12AT7's you tested? Transconductance or plate current?
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Stevem
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M
Sorry, your right!
A 12AT7 would test 4000gm on my tester if it was 100%.
A 12AT7 would test 4000gm on my tester if it was 100%.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: China ax7s
When is the last time your Hickok was calibrated? Industrial testers measure gm in incredibly goofy ways. The signal to the grid is derived from unfiltered rectified wall voltage, plus another AC component and isn't measured, just assumed to be correct. If the line voltage is off, or the rectifier tube acting up, meter readings will be wrong. My 539C was that way. As to the plate structure of the 12AT7s, those old GEs with the wing-shaped plates look anything but robust, but always worked great.
That said, I always cynically wondered if the Chinese (maybe the Russians, too) didn't just build a batch of generic 12A?7 tubes, test them, and call the higher gm ones 12AT7s.
That said, I always cynically wondered if the Chinese (maybe the Russians, too) didn't just build a batch of generic 12A?7 tubes, test them, and call the higher gm ones 12AT7s.
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Stevem
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N
I check my testers cal twice a year and re-cal as needed.
I have gone as far as installing a voltmeter on the octal sockets Heater pins as a have noticed that when testing output tubes it pulls down the applyed heater voltage and the meter lets me reset it back up to 6.3 during the test.
In regards to these 600 series testers I have also found that in regards to the on board meters line voltage setting and testing out 9 pin preamp tubes that setting the line voltage to the meters center mark will have the tester out putting some 13.2 volts to these tubes , not the 12.6 they should be fed!
I have gone as far as installing a voltmeter on the octal sockets Heater pins as a have noticed that when testing output tubes it pulls down the applyed heater voltage and the meter lets me reset it back up to 6.3 during the test.
In regards to these 600 series testers I have also found that in regards to the on board meters line voltage setting and testing out 9 pin preamp tubes that setting the line voltage to the meters center mark will have the tester out putting some 13.2 volts to these tubes , not the 12.6 they should be fed!
Re: China ax7s
Maybe the 13.2 sags down to 12.6 under load?
Tube junkie that aspires to become a tri-state bidirectional buss driver.
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Leo_Gnardo
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Re: China ax7s
I noticed those "h" shaped supports in Chinese 12AX7's a long time ago. They don't seem to be functional as hum shields. OTOH they allow anybody to identify the Shuggie pre tubes at a glance.
I made a gain checker circuit, simply a copy of the typical Fender first preamp stage, to test "real world" gain for 12AX7's. Most Shuggies measure in the 60's, occasionally over 70. No better and no worse than any other brand whether old or new. Sonically, there's a "jingle" or "ping" in high frequencies revealed by listening though frequency sweep testing shows nothing special going on up there. Some even whistle, a deal breaker for guitar amps - those go on the reject pile.
For those who like some "zing" in their guitar amps, Shug's work fine and I've seen them working with good long lifetimes. 10 or 20 years is not unusual, and on occasion I find one that's been going since the early 90's or late 80s still working fine. Can't say what today's currently made Shugs will do in the future, only time will tell.
I made a gain checker circuit, simply a copy of the typical Fender first preamp stage, to test "real world" gain for 12AX7's. Most Shuggies measure in the 60's, occasionally over 70. No better and no worse than any other brand whether old or new. Sonically, there's a "jingle" or "ping" in high frequencies revealed by listening though frequency sweep testing shows nothing special going on up there. Some even whistle, a deal breaker for guitar amps - those go on the reject pile.
For those who like some "zing" in their guitar amps, Shug's work fine and I've seen them working with good long lifetimes. 10 or 20 years is not unusual, and on occasion I find one that's been going since the early 90's or late 80s still working fine. Can't say what today's currently made Shugs will do in the future, only time will tell.
down technical blind alleys . . .
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Stevem
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O
No, a test of a 9 pin preamp tube does not load the tester enough to pull it down to 12.6.
Testing a octal pre will or a 12bh7 pull it down a bit though!
Testing a octal pre will or a 12bh7 pull it down a bit though!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Stevem
- Posts: 4570
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
M
Leo, I just looked at those real close and I would now assume that the are inverted in the tube to keep the mica supports pulled together in the center section inbetween the plates of the A and B section of the tube.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: China ax7s
The 12AT7 is unusual for the 12A_7 family in that the manufacturing tolerances for the tube as-designed appear to be incredibly wide. Check out the "Design Minimums and Maximums" under the Electrical Tests section in this Raytheon data sheet:
https://frank.pocnet.net/sheets/138/1/12AT7WA.pdf
Acceptable plate current is anywhere from 7 to 14 mA! Spec GM is anywhere from 4500 to 6500 umhos.
https://frank.pocnet.net/sheets/138/1/12AT7WA.pdf
Acceptable plate current is anywhere from 7 to 14 mA! Spec GM is anywhere from 4500 to 6500 umhos.
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Stevem
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L
Thanks for posting the info Matt!
The difference is still not the big.
5500 GM you 6500 is only 15%, the change in plate voltage though of 80 is a big jump!
So I go back to one of my questions on these tubes, how can a small plate structure rated for 1 watt in a AX7 be called upon to handle 2.5 watts if it where to run as a near flat out AT7?
The difference is still not the big.
5500 GM you 6500 is only 15%, the change in plate voltage though of 80 is a big jump!
So I go back to one of my questions on these tubes, how can a small plate structure rated for 1 watt in a AX7 be called upon to handle 2.5 watts if it where to run as a near flat out AT7?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!