Life expenctancy of 12AX7 tubes on a Phono preamp.

I'm a novice tube user, I only have a tube Phono preamp which uses three 12 AX7 tubes. One is for amping (V1) of both channels, the second one (V2) does RIAA de-equalization and some more amp, and the third one (V3) is used as a "buffer" tube to keep impedance low and reduce distortion, this V3 tube can also be changed for a 12AU7 tube.
I'm currently using two RCA 5751 tubes from 1962 with black plates for V1 and V2 and an English made Raytheon 12AU7 (most likely a rebranded Mullard) with small smooth plates for the V3. My Phono preamp is a "clone" of an EAR 834.
As an average, what is the audio expectancy of 12AX7 tubes on a Phono preamp? I've read on several websites like Audiogon that they can last around 8 to 10 years, I never thought tubes could last that long. Is this true?
Thanks in advance.

 

Most likely thousands and thousands of hours. With old production tubes maybe years and years. In relatively low stress preamp circuits, I tend only tonchange preamp tubes when they become noisy or something, or just when I want a different flavor. But, yeah, you can potentially get year and years out of a preamp tube in a phono stage.

 

As has been said many years
My Troughline tuner is running
On original tubes !
I would be wary when swooping tubes.anECC83 will have different
Voltage requirement to some variation
Tube,

 

Fifteen hundred to twenty-five hundred hours.

 

8-10K is likely for a 12v tube such as you've mentioned.

Best Sir,

Bob

 

Thank you for your reply. I've been told and also read that the RCA 5751 with black plates from the early 60's (1962) I'm using are iron cast and if well cared and not abused will last almost a lifetime, I bought two pairs just in case as I love how they sound.
One more question: I read on a couple of websites that running the tubes with actual signal when they are still cold (power the Phono preampon,wait for a minute or two and then start playing a record) will shorten their life, that not only they sound better when hot (that I knew before), that it's also better for tubes health two wait for 10 or 15 minutes before feeding any signal to them, is that true?

 

10,000 hours is the number I see thrown out for 12AX7/ecc83 tubes, however this is for quality NOS tubes (like Mullard 10M series, Telefunken, RCA, etc), definitely not new production tubes. But tube life depends much on the way the circuit was designed. Some circuits are designed in a way where the tubes 'loaf' along. In other circuits, tubes are more highly stressed, and ultimately the tube will have a shorter life.

If we do the math, 10,000 hours equates to leaving the preamp powered on 24/7 for 14 months! Or, leaving the preamp powered on for ONE hour per day for 27 years. Again, this is UP TO 10,000 hours, so you may not get this much life out of your tubes.

 

Thanks for your reply. I use to have my Phono preamp ON for around 3 hours a day, 5 days a week. Just in case this means something, my EAR 834 "clone" Phono preamp doesn't make my RCA (or others) tubes very hot even after several hours of use, I've seen on most Phono preamps tubes get very hot after an hour of use or less, this doesn't happen on my Phono preamp.

 

I don't know about cast iron. I doubt it. I think the black color is just a plate coating. I do think the black will help dissipate heat easier and that might have an impact on life. I will say, I've had so many RCA black plate 12ax7s go noisy and microphonic on me over time that I suspect noise will put an end to your use of a black plate RCA 5751 before heat will. But of course with any tube, they will get weaker over time, with a twin triode like a 12ax7, the triodes will probably become less matched, etc. They may last years and years but they won't be quite the same as they were when new. Nothing lasts forever. Personally I'm not a fan of RCA black plate 12ax7s and 5751s in hifi circuits -- I think they're too bassy and woolly and bloomy and fat sounding. I love 'em in Fender black face and silver face guitar amps, which can use the extra bass and low midrange girth, but not so much in audio applications. But other people seem to love 'em.

In terms of life and warming tubes up, really, I doubt any controlled studies were ever done over 10K hrs of tube life to determine if sending signal to a cold tube meaningfully shortens tube life. You won't get any signal if it's actually not heated up at all, and and when you do it might not sound its best until the tubes have reached thermal stability, so it always makes sense to switch the gear on and let it warm up for 10, 15, 20 minutes, but then of course you're also heating the tubes up for an extra 10, 15, 20 minutes, so you're adding hours. Which one is more likely to shorten or lengthen twin triode tube life in a phono preamp? I have no idea but my guess is there's no meaningful difference, but obviously the tubes need to heat up some to begin emitting, and I always turn on the tube gear and let it come up to temp before listening.

 

I have been using a tube headamp/tube preamp into two tube amplifiers since 1990 and I only had 3 tubes go, two power tubes (one 6550 and one EL34) and one headamp tube 6DJ8. Not bad for 27 years!

JG

 

Thank you both for your input. I always switch my Phono preamp (and the A/V receiver I use as an amp) 20 to 30 minutes before I start my listening sesion. It doesn't mother if this shortens the tubes life, I want them to perform well while using them, they don't sound right cold, if I have to change them before than I expected because of this I'll change them, I have another pair of NOS RCA 5751 just like the one I'm currently using (they are these: 5751 RCA VINTAGE BLACK PLATE NOS VALVE/TUBE (LC19) - Langrex ) so I have a spare pair for when the moment comes. And before that happens I may find more of these, 'tough they're becoming scarce, to buy or even another tubes I like more than the 5751 RCA, who knows...
I'm about to received a three set of matched NOS Matsushita 12AX7 which some call "the poorman's Mullards" as they were made with Mullard tooling and (i've heard) very much use the Mullard design of their 12AX7 tubes, but let's see how they sound.

 

Your experience mirrors mine with the RCA black plates. There are certain applications where these can provide exactly the kind of secret sauce that one is looking for when tube rolling. But those applications are rare because of how excessively colored these tubes sound. And when you find one, prepare to be disappointed relatively quickly when your tubes start to crackle and hiss.

As to earlier questions in this thread regarding tube lifespan, I ran one of my previous tube preamps 24/7 for several years thinking that I was being smart by avoiding all of the warming and cooling thermal stress on the circuit. I changed this practice a couple of years ago when I realized that I could no longer reliably attain genuine NOS tubes of the types that I liked best. And when I discovered just how dramatically their emissions had declined by being powered-on constantly.

Yes a number of dealers claim to sell "NOS" tubes, but I no longer trust that designation in the case for any non-JAN tube from all but the very-best dealers. IMO way too many used, or unknown condition, tubes which happen to test OK on a Hickok are getting sold as "NOS" these days. So now I only turn my preamp on when I intend to listen in the next hour or so, and I turn it off whenever I decide that I'm done listening for the day so that I am not needlessly burning-out my old-stock tubes.

Lastly, to the OP: 5751s are not a good substitute for 12ax7s in the majority of phono stages because these lack sufficient gain for the feedback-implemented RIAA equalization. In most other applications, substituting 5751s for 12AX7s is perfectly fine though.

 

That's only true if the tubes are part of the RIAA equalization circuit. Most inexpensive units and even some more costly ones use solid state RIAA equalization and the tubes are outside of the circuit like a buffer or cathode follower

 

I would personally call such a preamp either a hybrid preamp or a SS preamp which features tube buffering.

But you are correct that for such a circuit, or for a genuine tube circuit which uses a passive RIAA network (which is not very common in 12AX7 based designs) , a substitution of a 5751 is not a problem.

 

My Phono preamp is a "clone" of EAR 834, the gain of this preamp is INSANE, it sounds louder than CD's or SACD's playing using the internal D/A converters inside my Pioneer SC LX76 (also sold in the US in 2012 with a different reference and with the Elite badge), and yes, the second tube, the V2is inside the RIAA circuit, this tube is on the signal path. V3 is only used as a "buffer" tube or a cathode follower as russk said, the other two tubes are on the signal path and I don't think there's nothing "cheap" with the EAR 834 Phono preamp design.

 

Well, if a tube is inside the RIAA circuit and the circuit calls for a 12ax7 there, you probably need a 12ax7 in there, not a 5751, for accurate RIAA equalization.

 

I thought I could put a 5751 ANYWHERE I'd use a 12AX7, I thought they were 100% compatible. What about putting a 12AX7A on the V2 which does RIAA? Are these suitable on a 12AX design circuiy?
Anyway I think results is what matters and I like how my Phono preamp sounds with two RCA 5751 on the V1 and V2, I like its sweet and mellow sound, I find it very engaging.

 

All the "clone" EAR834's will have the RIAA in the feedback loop, so V2 & V2 do not directly affect the equalization.

jeff

 

I think the way these RIAA circuits go in an amp like that is that the plate resistance of the tube place a role in the eq, and if there's feedback in the eq circuit -- which I think there is in this circuit, then changing the gain of the device in the circuit is going to change the feedback and also change the eq response. I think the 5751 is spec'ed to have the same plate resistance as a 12ax7, but obviously not the same gain factor. So, I think for a phono pre that uses a 12ax7 as a gain device in the RIAA circuit that has feedback, you will alter the RIAA equalization if you swap in a 5751, though I'm sure someone more knowledgeable about me regarding these designs can correct me or clarify.

 

I'm curious about this. I know nothing about these circuits, so a question for my edification. If you change the gain of those tubes, don't you inevitably change the levels of feedback in the loop and thereby change the equalization?

 

This is incorrect if you care about achieving flat frequency response out of many phono circuits. 5751 is a very similar tube, but it has only 70% of the gain of a 12AX7, which makes these less than 100% compatible. In traditional feedback-implemented RIAA equalization circuits, the circuit is designed to use all of the tubes gain, not just 70% of it.

However, just as an FYI, there are some really great sounding 12AX7s out there that IMO can sound even better than 5751s. And this is coming from somebody who is quite fond of 5751s. If you like mellow and sweet, I recommend trying some of the current production Russian Gold Lions. My old ARC SP8 absolutely loves these tubes.

 

Good to know. Thank you!

 

Good to know also. I have a pair new with less than 100 hours of use 12AX7 Gold Lions (New Sensor new production) with golden pins, I'll give them a try.

 

This is the circuit design of my EAR 834 "clone". What do you think? What about the use of 5751, 7025 and 12AX7A tubes on this design?