tube life

[bonsai171]

How long do preamp tubes typically last?? thanks

Dave

[fdegrove]

Hi,

If kept within manufacturers specs about 10,000 Hours.

Cheers,

[Sch3mat1c]

Forever, unless you run a computer with them, in which case, once every five minutes.

10k hours is 1.14 years if ran continuous... if 8 out of 24 hours every day, 3.5 years... hmm I've been running Hept'AU7 almost that long with used tubes, far from preamp conditions mind you (i.e. at spec), and none of the 12AU7s are showing signs of letting up. Weird!

:Edit: Hmm you say if within spec... preamps are typically extremely within spec so I wouldn't hesitate to double or quadruple that figure (i.e. 20-40k hours).

Tim

[fdegrove]

Hi,

preamps are typically extremely within spec so I wouldn't hesitate to double or quadruple that figure (i.e. 20-40k hours).

Actually I've been too pessimistic with my figure: 100.000 hours according to Philips is what you can expect for CRT tubes with 60% failure beyond that.

Even if we'd halve that figure it's still very impressive.

Cheers,

[Sch3mat1c]

Ah, that's more like it! See, very long indeed.

Tim

[djmiddelkoop]

100.000 hours according to Philips is what you can expect for CRT tubes with 60% failure beyond that.

yes, I know an application where Panasonic CRT's are used, they are operating continuesly year after year, never switched off.

After 10 years some of them are degrading a bit, which is approx 10k hours ! No failures yet.

Dick.

[LBHajdu]

Having been won over by SE amps this past year, I would also like to ask some questions about tube life. (See the statement below)

1) If it is the case that KT88's out-gas because the plate is too thin, why not just make it thicker. Is the KT88 plate really more complex then a 300B. They both seem like two u-shaped bits of metal to me, with the KT88 being more boxy while the 300B has more round bends and is more flat.

2)Is having a red plate on a tube a bad thing because the out-gassing? Why can't they get all the gas out of the metal before hand, with all the bakeouts under vacuum they do (would it be too expensive, take too long)? Wouldn’t thinner plates be easier to output-gas?

3) Is all loss of emission caused by plate out-gassings contaminating the cathode?

3) Does keeping the outside of the tube cooler extend its life?

4) Do tubes with carbon plates last longer?
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I found this statement at:
Emission Labs - Introduction

For lifetime, directly and indirectly heated tubes depend on the same mechanisms, but in another order. Indirectly heated pentodes, like KT88 when constructed very good, will end their life by loss of vacuum, resulting from outgassing the plates (Anode). For KT88 and similar, this is unavoidable, and it is the natural way to come to an end, if no quality issue occured before during use. The occurrence and absorbing of very small fractions of gas gets visible by "eaten away" edges of the getter flash. Though such tubes perform good, they are getting near the end of life. Once the getters can not eat any more gas, the tube will show some sort of defect, and gets destroyed during further use. (So the previous text only applies for KT88 and similar).
Good quality directly heated tubes (DHT), like 300B normally have no outgassing problem. Their plates are made in less complicated shapes, therefore can be much thicker, and from less outgassing materials. Still all things come to an end. With DHT, the filament lower Emission and/or breakage normally ends the tube life. So great care should be taken not to underheat or overheat the filament, and switch on the tubes gently, possibly with pre-heated filaments.
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[JPeitzman]

Is having a red plate on a tube a bad thing...

Depends on the tube, but 9 times out of 10, YES. For instance, you may see the occasional 833A with cherry red plates. These tubes have 100 watt filaments plus a few thousand volts on the plate, they get pretty toasty. Otherwise it is not a good thing. It can damage the cathode, and plates can (and do) melt. Little odds and ends, like screen grids, also tend to melt.

I know leaks, gassing, etc.. can/do effect tube life, but there are other factors at play that can be somewhat more controlled. Filaments fail because of the repeated on/off cycles they experience in amplifiers. My father worked both around theater equipment and broadcast/radar equipment. The broadcast equipment (transmitters) NEVER had their heaters turned off, and they where run within their specs. Those tubes lasted forever! Theaters where a different issue, they where run very hard. He went into a drive-in once where the amps where pushing a short (squirrel made a snack out of the wires). The plates on the 807's where glowing about as bright as the sun! Anyways, I know we can't all just leave our tube running non-stop, but it is something that can contribute to a shorter life. I know some have found that using dropping resistors to drop the heater voltage to 6.2v can help extend tube life. My slightly limit inrush too, but they are very small values.
Tubes don't like being slammed with full B+ when they are still cold. This happens a lot with SS rectified B+, and a bit but not as bad with DH rectifiers. Standby switches prevent this completely and cost about a buck.
Long story short, be kind to your tubes and they will be kind to you. Keep them within their limits, take the extra 30 seconds or so to warm them up, under-volt them a tad (at the very least keep AT their heater voltage, not higher), and you will get thousands of hours out of them. They may even out last you!

[Cassiel]

[QUOTE]Long story short, be kind to your tubes and they will be kind to you. /QUOTE]

Hehe, so is it a reciprocal friendship? Nah, tubes aren't like women, be nasty with them and they will forgive us. I'm yet to meet a delicate tube. Tough bastards, that's what they are.

[Miles Prower]

2)Is having a red plate on a tube a bad thing because the out-gassing? Why can't they get all the gas out of the metal before hand, with all the bakeouts under vacuum they do (would it be too expensive, take too long)? Wouldn’t thinner plates be easier to output-gas?

Perhaps. The one failure I had was an 807 that went red plate when its bias supply failed. A few weeks later, the bias wouldn't stabilize, and you could see the blue glow filling the space between the cathode and plate. Some of the big RF finals, OTOH, must be run with the plates red. These types rely on the plate material (usually carbon) to getter the vacuum. Run these types too cold, and you will ruin them.

It's not possible to remove all the gas from metals by baking out. It's an exponential process like discharging a capacitor: the job's never really done.

3) Is all loss of emission caused by plate out-gassings contaminating the cathode?

This is caused by ion back bombardment. You typically see this problem with types that operate at extreme voltages (e.g. CRTs) and/or high current densities (e.g. RF/audio/horizontal deflection finals) Oxide cathodes are the most susceptible to this sort of fuxation, which is why the high powered RF finals that operate at 1000+ plate voltages use Th/W filaments, despite the much lower electron emission efficiency.

3) Does keeping the outside of the tube cooler extend its life?

Avoiding spec busting extends tube life. If you try to keep the outside of any sort of Hg-vapor tube cooler, you will ruin it in short order. These aren't transistors where you can pull more power by tacking them to bigger heat sinks.

4) Do tubes with carbon plates last longer?

Dunnow

How long do preamp tubes typically last?? thanks

Small signal VTs can last for decades of use so long as you don't spec bust, or have something go wrong, such as a Class C amp that derives its bias from the signal that goes red plate if the master oscillator doesn't start (which is why you should always include some form of protective cathode bias) or something like that. Of course, some "small signal" work is more akin to power amplification, with some types such as the 12BY7A pulling some pretty stiff currents. (The Pd= 6.5W for this type.) Other applications such as radio receiver IF amps also tend to run small signal types "hot" to maximize the gain in a situation where you don't care about harmonic distortion since the plate load is an LC tuner whose impedance drops sharply at frequencies off resonance.

Usually, small signal types last longer than large signal types.