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#1
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microphonics
flipper wrote: Not a problem, per see. It's just that I never had occasion to observe it for myself till this little guitar amp I built for the nephew. Did a thanksgiving day field test at the family get together. To keep costs down I'm using 12AT7s in the premp and first thing I discovered on the test bench was being able to hear tapping in the chassis in the speaker. Microphonics. As luck would have it I just happened to have a low microphonic 6201 and, sure enough, no tapping sound with that one. Nice to see theory play out in practice. However, the source I normally use for tubes subs 12AT7WAs for 6201s. Hmm. So I pulled out all my 12AT7s and a couple were WAs. One was quiet (GE) but the Tungsol was microphonic as hell. And the 'plain' Sylvania 12AT7 was as quiet as the GE WA. One thing I learned, I can tell which will and which won't by simply looking for the extra mica spacer. But the question is, how can I order a 'quiet' one when 12AT7WAs don't necessarily have it but even some 'plain' 12AT7s do? Does anyone know if any of the 'current' production 12AT7s are low microphonic? While the few I've got were distinguished by the extra mica spacer when I look at photos at tube depot the Ei 12AT7 Elite Gold claims low microphonics but doesn't have the extra spacer while the 'cheap' Sino does (according to the pictures, the only one that does). but make no microphonics claim. It was kind of fun trying different dual triodes of the same basing. While the microphonic 12AT7s were 'obvious' (if you tap the chassis), they might be acceptable because it's not enough to oscillate but the microphonic feedback screech with a 5965 ('computer' triode) was stunning. Microphonics is a crapshoot. The brand, year of manufacture, etc., means *squat*. If you're going to put a 12AT7 in a gain stage, you'll have to test 'em yourself - in the circuit you're using them in. Either that, or you'll need to get them from someone who does. While the triple mica types will tend to be less microphonic than some, the triple mica is no guarantee. I've tested plenty of 'em that were microphonic. And, as you found out, vaunted badges from days gone by are no guarantee either. As to the claim of low microphonics for the Ei 12AT7, it's bull**** - if you go through 100 of 'em, you *might* get five or six that will be OK. "Ei" (pronounced "AY!") is actually a Yugoslavian word which means "shrieks like a train whistle." Regardless of how fancy the box is or what the pins may be plated with, you are *not* going to get anything special in the microphonics department if it says "Ei" on the glass. As for other currently manufactured 12AT7s, give the EH a miss, too. I recently had a special request for low-microphonics 12AT7s from Randall Aiken, and I went through the 80 or so EH 12AT7s I had on hand in the process of selecting some from every brand I had; not a single one passed. (In fairness to the EH, they work great as phase inverters and reverb drivers.) I don't have much experience with the JJ, as I don't get much call for 'em. Besides, NOS JAN 12AT7WC are cheap and ubiquitous - no real need for me to stock lots of current ones. BTW, the Chinese 12AT7s from the Beijing factory are great tubes, if you can find them. (Production ceased in the early 90s, as I recall.) No experience with the current Chinese 12AT7 from the Shuguang plant, but if it's anything like their 12AX7, it'll be fairly ho-hum. Lord Valve VISIT MY WEBSITE: http://www.nebsnow.com/LordValve I specialize in top quality HAND SELECTED NOS and current-production vacuum tubes for guitar and bass amps. Good prices, fast service. Authorized dealer for QSC amps, Sovtek/Electro-Harmonix, Behringer, Hammond-Suzuki, Leslie, Rolls, Weber VST, etc.! - Partial Client List - * Derek Trucks (Allman Brothers Band/Derek Trucks Band) * * Meatloaf * Catherine Wheel * Yo La Tengo * Let's Go Bowling * * Kofi Burbridge (DTB) * Tod Smallie (DTB) * Susan Tedeschi * * Roy Pritts (former head, Audio Engineering Society) * * Rob Hyckys (guitarist for Commander Cody) * Waky Amps * * Jamie McLean (guitarist for the Dirty Dozen Brass Band) * * John Pierce (bassist for Huey Lewis and the News) * * Rob Eaton (guitarist for Dark Star Orchestra) * * Detroit Frank DuMont (guitarist, Frank DuMont and the Drivin' Wheels) * * Coco Montoya * Clint Black * Bill McKay * Mojo Watson * Dick Dale * * Fleetwood Mac * Tyrin Benoit * Eugene Fodor * Dale Bruning * * Komet Amplification * Dr. Z * Maven Peal * Blockhead Amps * * Jim Kelley * Balls Amplification * Roccaforte Amplifiers * * Gerhart Amplification * Aiken Amplification * Germino Amplification * * Lots More * NBS Electronics, 230 South Broadway, Denver, CO 80209-1510 Phone orders/tech support after 1:30 PM Denver time at 303-778-1156 - Our 25th Year - VISA - MASTERCARD - PAYPAL - DISCOVER "It ain't braggin', if ya can do it." - Dizzy Dean |
#2
Posted to rec.audio.tubes
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microphonics
Lord Valve wrote: flipper wrote: Not a problem, per see. It's just that I never had occasion to observe it for myself till this little guitar amp I built for the nephew. Did a thanksgiving day field test at the family get together. To keep costs down I'm using 12AT7s in the premp and first thing I discovered on the test bench was being able to hear tapping in the chassis in the speaker. Microphonics. As luck would have it I just happened to have a low microphonic 6201 and, sure enough, no tapping sound with that one. Nice to see theory play out in practice. However, the source I normally use for tubes subs 12AT7WAs for 6201s. Hmm. So I pulled out all my 12AT7s and a couple were WAs. One was quiet (GE) but the Tungsol was microphonic as hell. And the 'plain' Sylvania 12AT7 was as quiet as the GE WA. One thing I learned, I can tell which will and which won't by simply looking for the extra mica spacer. But the question is, how can I order a 'quiet' one when 12AT7WAs don't necessarily have it but even some 'plain' 12AT7s do? Does anyone know if any of the 'current' production 12AT7s are low microphonic? While the few I've got were distinguished by the extra mica spacer when I look at photos at tube depot the Ei 12AT7 Elite Gold claims low microphonics but doesn't have the extra spacer while the 'cheap' Sino does (according to the pictures, the only one that does). but make no microphonics claim. It was kind of fun trying different dual triodes of the same basing. While the microphonic 12AT7s were 'obvious' (if you tap the chassis), they might be acceptable because it's not enough to oscillate but the microphonic feedback screech with a 5965 ('computer' triode) was stunning. Microphonics is a crapshoot. The brand, year of manufacture, etc., means *squat*. If you're going to put a 12AT7 in a gain stage, you'll have to test 'em yourself - in the circuit you're using them in. Either that, or you'll need to get them from someone who does. While the triple mica types will tend to be less microphonic than some, the triple mica is no guarantee. I've tested plenty of 'em that were microphonic. And, as you found out, vaunted badges from days gone by are no guarantee either. As to the claim of low microphonics for the Ei 12AT7, it's bull**** - if you go through 100 of 'em, you *might* get five or six that will be OK. "Ei" (pronounced "AY!") is actually a Yugoslavian word which means "shrieks like a train whistle." Regardless of how fancy the box is or what the pins may be plated with, you are *not* going to get anything special in the microphonics department if it says "Ei" on the glass. As for other currently manufactured 12AT7s, give the EH a miss, too. I recently had a special request for low-microphonics 12AT7s from Randall Aiken, and I went through the 80 or so EH 12AT7s I had on hand in the process of selecting some from every brand I had; not a single one passed. (In fairness to the EH, they work great as phase inverters and reverb drivers.) I don't have much experience with the JJ, as I don't get much call for 'em. Besides, NOS JAN 12AT7WC are cheap and ubiquitous - no real need for me to stock lots of current ones. BTW, the Chinese 12AT7s from the Beijing factory are great tubes, if you can find them. (Production ceased in the early 90s, as I recall.) No experience with the current Chinese 12AT7 from the Shuguang plant, but if it's anything like their 12AX7, it'll be fairly ho-hum. Lord Valve Well, you make interesting comments about 12AT7, one of the most despised twin triodes of all time with regard to audio by the cognescenti knowa-ferkin-lotski. I don't find so few that they generally could not be used in an phono stage and I am not so sure one would have to test large number before you got some good ones. But your'e right about many old famous brands of tubes testing like ****. Just because one pays top dollar for supposed NOS brands, it don't mean they have not been used a bit, maybe 1,000 hrs, and then sold as NOS, again...... preamp tubes can easily last 10,000 hrs. Some last 5 minutes, but they are the minority.... Just never assume what the quality is until you have tested it. I normally test a twin triode in common cathode test amp with a 40k load and fully bypassed cathode and at about average Ia which is usually less than the data figure, and more like the real world figure. The input grids are grounded via an RCA shorting plug to 0V. The anode output is taken to a low noise amp with 20kHz of bandwidth and at least gain = 1,000. So let's say we have a 1/2 12AT7 giving a gain of about 32 into 40k and let's assume there is an equivalent amount of GRID input noise = 2uV. So there will be 64uV of noise at the AT7 anode. This noise tends to swamp all the other noise mechanisms of the tube if its OK. 64uV x 1,000 = 64,000 uV of noise at the output of the gain amp = 64mV. 64mV is easy to see on a CRO, and it can be watched for awhile to see if there is a lot of sputtery bursts of noise, and this signal can also be amped up further and listened to on a speaker. A constant hiss is what should be heard and without hum or any sputters. After you have listened to a dozen tubes you will know what the good and crook ones sound like. If the tube is knocked lightly with a pencil it should not make a loud long ring sound, but just be a dull thud sound, and definately there should be no acoustic feedback from a nearby speaker. Anyway, I find that plenty of the 12AT7 I have accumulated have less than 1uV of input noise and are not too microphonic. When testing tubes this way you soon find out about tubes that are gassy, have low emission, become mechanically loose inside, have too much hum. The noise will be over 5uv and if its 10uV I chuck em in the bin. nearly all the common twin triodes will measure between 0.6uV, very good, to about 3uV, beginning to be crook, and some which have very low noise are microphonic, but I put black textra pen ID with N = noisy, M = microphonic, NM = noisy + microphonic, and the noise figure for the worst of the two triodes within the twin triode. The ones that measure well just have their noise figure writ on them. One can also test the gain for the tunes in the test circuit I have but without the following gain amp. If one hasn't got the type number for the tube under test then measuring gain with Ia kept constant for 2 different loads, say 20k and 40k will give two gain figures. Since A = µ x RL / ( RL + Ra ) and Ra and µ don't vary, the the µ and Ra can be worked out because one has two equations. This all is difficult to test with a tube tester which I've never bothered to acquire. Patrick Turner. |
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