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Old 19th August 2010, 05:20 PM   #11
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Default Tube lifetime

Those who are getting more than a few thousand hours on their tubes are doing pretty good. In the old days, the military often swapped tubes out every 1000 hours, regardless, and a "10,000 hour tube" was considered to be a super-premium type (like RCA's special red tubes or Philips/Amperex "PQ" tubes).

In low-power audio tubes, the main failure modes are loss of cathode emission (cathode wear-out or poisoning) and leakage due to evaporation of heater metals and insulation and cathode materials onto insulating surfaces. Loss of emission can be avoided by not running the tubes too hot and only applying B+ after the cathode is hot. Leakage is greatly accelerated if the heater is run over its normal ratings. Hence, it is important to check your heater voltage. If AC, use a true-rms meter, since the power line waveform is no longer a sine wave in most places. If it is too high, an easy way to reduce it is to insert a low-value power resistor in series with the heater. For example, let's say you have two 12AT7s running off of a supposedly 6.3V supply. The current draw is 2 x 0.3A or 0.6A. However, you find the voltage is actually 6.9V. To reduce it to 6.3V, use Ohms law to find that R = E/I or R = 0.6V/0.6A = 1 ohm. The power dissipated would be P = I * R = 0.6 * 0.6 = 0.36W. A 1 or 2 Watt resistor would work.

The cathode temperature of a tube is adjusted for the intended use of the tube by the original tube designer. Many low-level audio tubes draw low plate currents (a few mA or less) plus are sensitive to leakage, so tubes designed specifically for audio generally have a fairly cool cathodes - dull red or reddish-orange. You can see this on most 12AX7s, 12AY7s, and EF86s. However, tubes designed for RF amplifier applications, say the front-end of a TV or UHF radio, generally have quite hot cathodes - bright orange. This is because in RF, high transconductance is the name of the game, and transconductance is highest at high current. Also, running the cathode hot pushes the electron space charge that surrounds the cathode further away from the cathode and closer to the grid, also raising the transconductance. Things like microphonics and leakage are not very important in RF applications, so not much effort is made to address these issues.

The 12AT7 was designed by GE in 1947 originally as an RF amplifier and oscillator tube. It was soon replaced in RF applications by tubes like the 6BQ7 and 6DJ8, but it had a nice combination of high transconductance and high mu, so, despite its somewhat variable non-linearities, found some applications in audio. I've found that when using tubes originally designed for RF in audio applications, especially when they are run well below their spec'ed plate currents, that reducing the heater voltage so that the cathodes run cooler helps both the sound and longevity. You can do this by applying a variable heater voltage to the tube in a dark room and adjusting it so that the cathode appears red or reddish-orange. Then use this voltage in your design. I think you will be happy with the results!

By the way a lot of 12AT7s have quite inconsistent transfer curves and can often be non-linear. I've seen this when testing a lot of tubes on a Tek 570 curve tracer. I think this is because in the effort to get the most transconductance out of the tube, the elements are small and closely-spaced, with the result that manufacturing variations have a big effect. The best, most consistent 12AT7s I've seen are the later (post 1955) military types like the 12AT7WA and 6201. The Western European ECC81 types (Amperex, Philips, Mullard, Telefunken, etc.) are pretty good, too. The ones to avoid are the older consumer-grade tubes, such as the white-label RCA, early GEs and early Sylvanias.

- John Atwood
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Old 19th August 2010, 07:55 PM   #12
w5jag is offline w5jag  United States
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John,

While we've got you here, are you aware of anything in octal that is equivalent or nearly so to the triode sections in 12AT7?


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Originally Posted by JohnAtwood View Post

In the old days, the military often swapped tubes out every 1000 hours, regardless, and a "10,000 hour tube" was considered to be a super-premium type (like RCA's special red tubes or Philips/Amperex "PQ" tubes).
I just expect a lot longer tube life than that, unreasonably, perhaps, but it's still my expectation.

Anecdotal case: I have an R-274C/FRR (Hammurland SP-600) restored sometime in the late 1990's (1998 I think, maybe 1999), that has run 8-10 hours per day almost every day in my office since. A decade at only 8 hours per day, five days a week, would be over 20K hours. It's had two (2) tube replacements over that time - a Russian 6V6GT that was still good but had always sounded scruffy to me, that I replaced with a well used (can see through the getter) JAN GE 6V6GT four or five years ago, and a JAN 5726 that I replaced with another JAN 5726 when the AGC started to deteriorate.

I've never checked the tubes in it, but it seems to still be working fine.

Win W5JAG
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Old 19th August 2010, 09:58 PM   #13
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W5JAG wrote:

Quote:
While we've got you here, are you aware of anything in octal that is equivalent or nearly so to the triode sections in 12AT7?
Unfortunately, there is no octal equivalent. The 6SL7GT has the high mu, but has relatively low transconductance. The loctal 7F8 is kind of close, although it is more like a 2C51/5670 or 6BQ7. A triode-connected 6AC7 or 6SH7 starts to approach the 12AT7 characteristics, but these tend to be pretty microphonic.

You are right that often tubes last longer than 10,000 hours - it depends on how they are used and how tolerant the radio or whatever they are in is to degraded tubes. I've fixed shortwave radios that were basically working OK, but every tube tested weak. On the other hand, some radios and audio equipment are very finicky and falter even on supposedly "good" tubes. The so-called 10,000 hour tubes were ones that were guaranteed (maybe not literally) to last that long, if kept within the maximum ratings.

- John
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