Are the tubes going bad everywhere in the amp or linestage or does it tend to be a tube in a particular position in the gear? If it is the same position, there could be something wrong with the piece of gear causing excessive stress. Also, what particular brand and model is at issue here? Some gear simply push tubes, even non-output tubes, hard so that they don't have long lives.
It is apparently far more common for vintage 6SN7s to be microphonic, or to become microphonic, than for many or most other types of small signal tubes that are commonly used in audio equipment. There was a post here a year or two ago by Kevin Deal of Upscale Audio attesting to that with some rather striking statistics, and describing the troubles he has to go through to find and maintain a reasonable supply of non-microphonic 6SN7s.
Also, I’ve sent you a PM on this subject, **not** having anything to do with him or the tubes he supplies.
I'm using an Atma-Sphere UV-1 preamp which uses 2 6SN7's in it's line stage. It's an OTL and is paired with a SS amp.
Perhaps Ralph will weigh in regarding the biasing of NOS tubes.
@larryi , as far as tubes becoming microphonic, I have read that NOS 6SN7's do have a higher failure rate than other small signal tubes. The stock Chinese tubes, however, are trouble-free. But they don't stay in the preamp for an extended period of time.
I haven't been able to tell yet if a NOS tube is going bad in a particular socket. But, I've had two different tube types go to an early grave.
I have not heard anything about whether the Atmasphere preamp is hard on tubes on not. Other than a problem with the preamp, I am guessing you have had a run of bad luck. Is there a possibility the voltage from your power company is off? In a lot of places, voltage seems quite a bit higher than it should be and this makes some gear much more prone to failure. The maker of the tube amp and preamp that I bought is a custom builder living in Italy. Although I never specified voltage, he has made gear to match the specific voltage that the user is getting (he winds power transformers) from his utility; this makes a difference as to performance, and I would suspect, reliability.
It's been a while since I've last chased after 6sn7s. I never ran into highly microphonic versions of NOS 6sn7, although I bought quite a few weak tubes that supposedly tested "like new." Given the very limited stock of older tubes, the percentage of tubes that are bad must be rising as bad tubes are put back into circulation. But, I don't know of current manufacturers making anything like a Tungsol roundplate, which is why we keep foolishly searching.
... I have a new transformer on my street to correct for power swings from old grid and now line voltage is high, but stable at 120V.Actually, 120VAC isn't high household voltage at all, at least not in the U.S., where 123VAC is considered nominal voltage in many states.
Having a steady 120 volts is fine for almost any modern gear. There is some old vintage gear from the 1950's and 1960's that supposedly work better at 110 volts (I know people who use a variac for that), but, mdoern gear should be built to work with 120. In my area, I have seen spikes above 130, and someone not to far from me measured spikes close to 140. A friend of mine had a Tron tube amp that failed repeatedly. It was always the solid state part of the amp (bridge rectifier), and the manufacturer replaced the diodes several times before he finally decided to source beefier diodes for the repair; the manufacturer said he was unaware of how widely the voltage swings in the US and did not account for this. I read that an ultra expensive Japanese solid state amp made by Technical Brain also constantly failed in service in the US but never in Japan. It turned out that Japan has rock steady voltages and the manufacturer assumed the same for the US. It appears that builders in other parts of the world are slow to pick up on something we living in the US are aware of--the US is rapidly slipping into 3rd world status.
Sounds like you need to see what the voltage to the tubes should be and measure that to see if the preamp is within spec. If too high, that could be the cause of short tube life. What controls the tube voltage in that preamp I do not know. As far as micros' true with the 6SN7 tube but not as problematic as the 26/101D tubes for sure.
I have always noted that the old NOS 6SN7 GTs have a strong tendency towards microphonia. The fact that you have been free of it in those tubes for extended periods is exceptional. You may be forced to use current production to get what you want. BTW is the microphonia heard whilst just playing or do you have to tap on the tubes. If it's the later just don't tap..
Do the tubes that go microphonic sound bad? Mechans asked whether it is a problem with the sound or the "tap on the tube" issue and I think that is pretty important distinction. I run a different set of tubes in my linestage (310A and 311A), but, they too tend to have some that are microphonic when you tap on the glass and some that are not. For whatever reason, it could be coincidence, when we were auditioning a bunch of tubes, the ones that were slightly microphonic turned out to sound better.
I never actually checked if the 6sn7s I ran in my amp (Audio Note Kageki) were microphonic or not, as far as tapping the tube, mainly because how it performed under that test was irrelevant to me.
I had a 1950s Sylvania 6SN7GTB abruptly become severely microphonic a few years ago, in my VAC Renaissance 70/70 MkIII amplifier. It had previously been working well in my system for the better part of a year. I don’t recall the specifics of the sonic character that resulted, but basically the system just didn’t sound as good as it had previously. There was no obvious distortion, though, and nothing to suggest that anything was plainly defective. Eventually I isolated the issue to the microphonic tube by very gently tapping it with a pencil eraser, which produced a loud sound through the speaker in the corresponding channel. Replacing the tube, and the corresponding tube in the other channel (so that the two tubes were properly matched), restored the good sonics I was used to.
I’ve used a couple of other 6SN7GTBs over the years that have been **slightly** microphonic when gently tapped, but have sounded great and have worked reliably over time.
Three pieces of my audio gear run 6SN7's; Atma-Sphere MP-1 Mk3, MA-1 Mk3's and a Modwright modded CD player, a total of 22 6SN7's. After initial break-in, all 22 stock Chinese tubes in all three pieces were replaced with NOS. Needless to say, large improvements across the board.
The Atma-Sphere gear is almost ten years old and I have yet to replace a tube, not even a power tube. With all those tubes, I also thought it sensible to own a Hickok 533A tube tester and I check things periodically.
I'm not familiar with the UV-1 but I'd definitely check with Ralph. I remember talking to him early on about which tube positions in his equipment would bring the biggest improvements when rolling. He made it a point to say some circuits and some brands of equipment are more demanding on tubes. For example, a couple positions in the MP-1 require a 6SN7 GTB because it can handle higher plate voltages. Using a 6SN7GT, VT-231 or 5692 which can't handle the higher voltage will most likely cause an early death.
I've never counted but I must have over a hundred NOS triodes around here with a few nice rectifiers thrown in. Concerning 6SN7's, it seems to me that most of the noisy(kind of a spitty sound) and microphonic tubes I've encountered have been very old 6SN7GT's and their equivalents.
The red base RCA 5692 is a perfect example; some have a brown base and are branded Sylvania but were made by RCA. When they're quiet and up to spec, that tube can last beyond 10,000 hours and it can sound wonderful in the right gear. I've never had great luck with the various VT-231's either. They're an old tube and supposed to be a more "rugged" version made for the military. Most of the ones I've heard were microphonic.
The exception, and I'm guessing I was just lucky, is the highly sought after black glass Tung-Sol 6SN7GT sometimes branded VT-231. I found two pairs of them several years ago and was able to hear them in my own equipment before plunking my money down. They still sound and test fine; actually, they sound exceptional.
My advice, call Ralph. I'd also pick up a couple inexpensive RCA GTB's and give them a try. Some prefer the Sylvania's but to my ears they can sound a bit aggressive
I have had one microphonic tube that you could hear the effect thru the speakers and when tapping. Right now I have a matched set of 6SN7GTB’s that are microphonic when tapped only. They sound wonderful, and the problem is due to the luck of the draw from the vendor, but microphonic after only a few months is getting expensive.
For example, a couple positions in the MP-1 require a 6SN7 GTB because it can handle higher plate voltages. Using a 6SN7GT, VT-231 or 5692 which can’t handle the higher voltage will most likely cause an early death.
This is very interesting information. I’ve had a red base RCA 5692 go bad, plus a pair of VT-231 went bad. So far these are my favorite, and are expensive.
I think I will order a pair of new production tubes so that I have a spare set. Any recommendations for non-Chinese tubes with good imaging and smooth highs?
FYI, classical music.
Concerning 6SN7's, it seems to me that most of the noisy(kind of a spitty sound) and microphonic tubes I've encountered have been very old 6SN7GT's and their equivalents.That's a good observation. I'm not going to buy anymore tubes from the 1940s, and
maybe 6SN7'S from the 50's are a risk. Although, 1950s were the glory days for tubes.
For example, a couple positions in the MP-1 require a 6SN7 GTB because it can handle higher plate voltages.Just for the record, that's not the MP-1, its the MA-1 (I'm assuming this is a typo), which like all of our amps has a direct-coupled driver tube which has a fair bit of voltage, just at the upper limits of what a GT is spec'ed for. Despite being marked as GTs, the generic Chinese 6SN7 has no problem giving extended service life in this location but when people are using NOS tubes we make a point of letting them know that a -GTA or -GTB should be used in this location.
The UV-1, like all of our preamps (including the MP-1), does not have any high voltages that could threaten a 6SN7.
I can't think of a circuit malfunction that could cause tube to go microphonic! Excess voltage would not do it- that would contribute to a tube's loss of transconductance, perhaps going gassy, but not microphonics. The latter is as far as I can tell, a defect related to manufacture.
We do run into microphonic 6SN7s when picking tubes for the preamps. We just avoid them and rarely have to warranty 6SN7s on this account despite a 1 year warranty on our tubes.
Ralph, I actually meant to type MP-1 and my notes from a conversation we had some years ago indicate that the pair of 6SN7's between the 12AT7's on the left and the three 6SN7's to the right in each channel are Constant Current Sources and that they should be low noise and GTB's? Did I miss something?
The three in line on the right side of each channel should be Voltage Amplifier in front then a pair of Output/Power tubes to the rear. Rolling that Voltage Amplifier with NOS makes the biggest difference.
In the MA-1 amps, looking down on the five 6SN7's in each channel, the "direct-coupled driver" is the forward tube nearest the face plate. I do show a GTB in that position and I've found from experience rolling it can bring nice improvements.
A question, for those of us who have tube testers, do you think running the "life test" stresses the tube and perhaps causes premature failure down the road? From what I understand, when selecting "life test", voltage is reduced by approximately 10% so maybe not?
Confessions of a 6SN7 addict-
I collected a large variety of 6SN7s to serve as drivers in my first foray into modern tube amps (a Consonance M100 + ). In that role they demonstrated their differences some quite distinct and some only nuanced. The stock S tubes and EH tubes however were no match against the old tubes and thus the slippery slope of tube rolling.
Then I bought an inexpensive preamp which uses 6SN7s as the main amplifying element. All of a sudden some of those perfectly good tubes sounded precariously micophonic or otherwise noisy. Some of them were fairly expensive 'precious' old stock GTs. The tubes were still quite functional in almost all cases but I couldn't subject them to trauma. The upshot is that microphonia may be a problem in one circuit and not another, in addition, particularly in a gain section of a preamp.