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Post by 405x5 on Apr 30, 2020 16:52:23 GMT -5
I may have found the problem tube. Attached is a picture of the Mullard/VM 12AX7A where the upper plates have separated and several pieces of mica are floating about the tube.
No spare to pop in for a test run...I’m presuming..... |
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Post by 405x5 on Apr 30, 2020 16:58:45 GMT -5
Suddenly, I’m reminded of the last time I brought tubes in to check them at a local Shop that had a tester....I wasn’t old enough to drive a car!
This year, I celebrated my 65th.
(With my temper, my wife would tell you I’m still not old enough.)
Bill
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Post by SteveH on Apr 30, 2020 17:06:34 GMT -5
Suddenly, I’m reminded of the last time I brought tubes in to check them at a local Shop that had a tester....I wasn’t old enough to drive a car! This year, I celebrated my 65th. (With my temper, my wife would tell you I’m still not old enough.) Bill As a kid, I remember going to the local drug store with my Dad, he would have a handful of tubes to plug into their tester and that drug store always had a replacement. My Dad always fixed it! |
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Post by 405x5 on Apr 30, 2020 17:21:47 GMT -5
Suddenly, I’m reminded of the last time I brought tubes in to check them at a local Shop that had a tester....I wasn’t old enough to drive a car! This year, I celebrated my 65th. (With my temper, my wife would tell you I’m still not old enough.) Bill As a kid, I remember going to the local drug store with my Dad, he would have a handful of tubes to plug into their tester and that drug store always had a replacement. My Dad always fixed it! Yep! Your local pharmacy was one of a number of places they parked those machines |
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Post by SteveH on Apr 30, 2020 17:28:48 GMT -5
As a kid, I remember going to the local drug store with my Dad, he would have a handful of tubes to plug into their tester and that drug store always had a replacement. My Dad always fixed it! Yep! Your local pharmacy was one of a number of places they parked those machines The TV was sick!!!!!! |
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Post by DavidR on Apr 30, 2020 17:42:52 GMT -5
I remember in the early 70s going to Radio Shack to test tubes for a shortwave radio my grandfather gave to me. It was a MUCH different type of experience on a Saturday back then.
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Post by DavidR on Apr 30, 2020 18:46:29 GMT -5
I may have found the problem tube. Attached is a picture of the Mullard/VM 12AX7A where the upper plates have separated and several pieces of mica are floating about the tube.
No spare to pop in for a test run...I’m presuming..... I do have some old GE 12AX7 and one GE 7025 from early 80's and a pair of cheap Chinese no name brand 12AX7 but I'm going on the suggestion of the designer/builder and getting (3) low noise versions of the same tube. I will put those in and see if I still have a hum issue. If I don't then I will roll one of the (3) NOS tubes at a time into it's intended position and see what I get then. |
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KeithL
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Post by KeithL on Apr 30, 2020 23:58:04 GMT -5
To be candid I doubt that stray bits of mica or even cracked mica would cause a tube to be susceptible to hum.... The inner parts of the tube are simply pressed together so the mica could have simply cracked a little bit while the tube was assembled.
The purpose of the mica is simply to mechanically hold the other parts together... It is also an insulator... but in the sense that it can hold things but won't allow current to pass through it... and not that it physically blocks current from going somewhere... Therefore I would not assume that cracked mica, or even bits of mica laying in the bottom, would cause any problem at all. I also doubt that it would necessarily show up on any functional test of the tube.
(But it could be a symptom that the tube was treated very roughly at some point in its life - which could also have damaged it in other ways.)
However, depending on the exact circuit used, if the tube is weak, and has out-of-spec low gain or high leakage, that could result in more hum.
(How much things like gain and leakage affect performance really depend on the exact design of the circuit in which it is being used.)
However, in fact, the simplest way to test that guess is to substitute another tube, and see if the problem goes away. Note, though, that a 12AX7 is a standard part... so, unless they're not working properly, all tubes with that number should perform VERY similarly, with VERY minor differences. (You'll note that the test parameters on a tube tester for "a 12AX7" specify the number... and make no mention of the brand.)
I should also point out something here....
Back when tubes were actually current technology saying "a 12AX7" was like saying "a 60 watt light bulb". There is some minor difference in performance between brands but, in order to fit the designation, they must all work within a narrow range of characteristics. While some premium brands claimed better performance, and may have cost a few cents more, they were all considered to be interchangeable.... And the very idea that a certain "premium tube" sounded "significantly better" would have been considered a joke. (Back then, a regular RCA 12AX7 cost a buck, and an fancy imported Telefunken 12AX7 might have cost a buck and a half.)
This is still true today. Different brands of the same number may have slightly different electrical characteristics... And that may result in their sounding slightly different in different equipment... But the idea that there are "super premium ones" that "perform significant better" is an expensive myth...
If you want to experiment with different tubes...
Go on eBay and buy a dozen cheap 12AX7's for twenty or thirty bucks... You'll have a lot more fun, and there's just as much chance that one of them will sound wonderful with your particular device, as that any expensive one will. Sadly, many vintage tubes are starting to fail from age, and many modern tubes simply don't work very well because many of them are just plain poorly constructed. (Some of the modern Russian ones are pretty good, and some are terrible, but at least you won't blow a week's pay on something mediocre... because they don't cost that much.)
I may have found the problem tube. Attached is a picture of the Mullard/VM 12AX7A where the upper plates have separated and several pieces of mica are floating about the tube. |
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KeithL
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Post by KeithL on May 1, 2020 0:13:38 GMT -5
Although high leakage or low gain might lead to problems they would be unlikely to have anything to do with mica (unless pieces fell inside the plate). However, as I mentioned in another post, that might suggest that the tube was handled roughly at some point.
And, IF your preamp uses AC to light the filaments, AND the insulation between the filament and cathode is cracked, or the filament itself has shifted, THAT could cause hum. (But that's not something a normal tube tester can even test.)
The best way to diagnose this is to try another tube... if the problem goes away, then there was something unusual wrong with that tube, and it's time for the trash can... And, if the problem doesn't go away, then it's something else...
(One of the benefits of tubes is that it's easy to try a different one if you suspect a problem; you should always have a few spares around for exactly that reason.)
You're getting precariously near a very expensive rabbit hole... with nary a rabbit, or a pocket watch, in sight... Unless there's something wrong with it a tube it will not HUM and a "low noise select tube" will not hum less. A noisy tube might hiss... or it might crackle... and a tube might be "selected" for this... but tubes are not "selected for low hum".
And a shielded tube could pick up hum if its internal shield had a cracked connection... but 12AX7's aren't shielded.
It is remotely possible that a faulty tube might pick up excessive hum from its filament... if the filaments are AC powered... and the cathode insulation is failing.
And some very sophisticated balanced circuits may be quieter if you supply them with matched tubes. But, by and large, buying more expensive tubes is not a good way to try to reduce hum. Hum is almost always a circuit issue.... or a wiring or layout issue... and not a bad tube issue.... and certainly not a "tube quality" issue... Wouldn't that show up as grid leakage on a tube tester? My tube tester does emission strength, shorts and grid leakage. They all showed NOS emission value and no shorts and no grid leakage. Tomorrow I'll put the tube amp back with the XSP-1 and see what happens.
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Post by DavidR on May 1, 2020 8:41:43 GMT -5
@keithl, my thoughts on the tube were that if the insulating spacer plates are dislodged and not seated properly and the mica broke in to pieces then the assembly will not be firmly fixed in place and would result in noise and microphonics. Yes, I am trying some inexpensive new production tubes that will be tested for low noise. The seller of the NOS tubes will allow me to return any that I find unacceptable.
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KeithL
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Post by KeithL on May 1, 2020 9:32:45 GMT -5
Your thought makes sense.... Microphonics, however, refers to the tube being sensitive to mechanical vibration or movement...
For example, if you tap on the tube with a pencil, you hear boinging noises, or crackly crunchies, from the speakers...
While it's possible that the transformer is imparting vibration to the chassis, which is then being picked up by a faulty tube, this would be pretty unusual. You can test this simply by tapping on the chassis... if you don't hear odd noises when you do so then the tube is not especially sensitive to microphonics.
(Note that virtually all tubes will respond... at least a tiny bit... if you tap directly on the tube... at the gain levels used in a phono preamp...) (Also note that, since mica is stiff, and the vibration would be coming into the tube from outside, while the mica will prevent parts from moving, it actually transmits vibration as well.) In general, when a tube is "selected for low noise", they are testing for hiss and "shot noise".... snapping crackling noises. The hiss is the actual noise of "electrons raining down on the plate" - but the exact amount and characteristic of that noise is varied by MANY factors. For example, the size, shape, and composition, of the particles used to coat the cathode makes a difference... And the composition and pressure of the gas filling the tube (there's no such thing as a perfect vacuum)...
(Note that, much as it doesn't matter what grade of steel is used in the screws that hold your fence together, these details shouldn't matter in a properly designed audio circuit.)
In general, hum is something that is picked up by the tube, from outside... And how well shielded a tube is from it depends on things like the shape of the plate... and whether it includes an actual internal shield of some sort. I've NEVER heard of anyone subjecting individual tubes to an outside electromagnetic noise source and testing their immunity to it in a standard "audio tube screening test"... I also haven't heard of anyone actually testing individual tubes for microphonics in years (although certain type numbers are specified as "being better or worse in that regard".)
They generally fire up the tube, with no input source, or a grounded control grid, and measure the amount of hiss and crackle it makes on a meter or equivalent. (Yes, back when tubes were used military devices, the tubes the military used in missile guidance systems were tested for that.) (And, yes, MIL-SPEC tubes, generally carrying the "JAN" designation, are supposed to be more resistant to microphonics in general.)
The usual solution, if this is believed to be an issue, would be to mount the entire phono preamp board on rubber shock absorbers, which was commonly done in the old days.
(The screws holding the board went through rubber grommets inserted into holes in the board; the tube sockets themselves can also be mounted on rubber.) (Metal tube shields, which are little grounded covers that fit over each tube, were also often used in the old days. If you want magnetic shielding they should be steel... not aluminum.)
@keithl, my thoughts on the tube were that if the insulating spacer plates are dislodged and not seated properly and the mica broke in to pieces then the assembly will not be firmly fixed in place and would result in noise and microphonics. Yes, I am trying some inexpensive new production tubes that will be tested for low noise. The seller of the NOS tubes will allow me to return any that I find unacceptable. |
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Post by audiobill on May 2, 2020 4:56:40 GMT -5
David, following this, I think your questions may be better answered over at the Dynaco forum, where the guys have a lot of relevant actual experience with tubes generally and your issue particularly.
Bill
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