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Leave amp on full time

9/29/1999 12:49 PM
Mike Leave amp on full time
Any guesses as to how long tubes would last being on all the time?
9/30/1999 1:40 AM

The devil's in the details again. There is no one succinct answer.  
It depends on where they're biased, the air flow in the cabinet, actual heater voltage, etc., etc.  
A preamp tube that is biased conservatively will probably last 10K hours or more. A power tube in something like an AC30 which is run very hot may not last nearly as long. Either one of them just barely tickled with heater current and running infinitesimal power and plate current may last almost as long as the shelf life, but that isn't very useful for amplifying either. However, there is a lot of conservatively designed tube equipment, old radios and the like, that are still running on original tubes forty or fifty years later. It all depends.  
There was a tremendous amount of data collected on tube reliability during the Golden Age, and some of it is still around. I've put a fair amount of the factors that affect tube aging in the Tube Amp FAQ at GEO. The link is on the entry to Ampage.  
Some of the things that shorten tube life:  
- power dissipation; there is a max Pd specified for every tube. Every time it's exceeded, the tube life gets shorter.  
- temperature of the outside of the tube; blowing air across tubes even at the same power dissipation will make them last longer  
- maximum current; there is only so much current the cathodes can produce. Running them above that even for an instant nicks a little bit of life off them.  
- heater voltage; modestly reduced heater voltage keeps them running longer. The Golden Age guys tried to keep them at -10% for high-reliability applications. Many amps today were designed for a 110 VAC line, and most utilities put out more like 122-125VAC, so the heaters are usually running at +10% all the time, sometimes more. That's why I wrote up the Vintage AC Adapter.  
There is one advantage to leaving tube (or any other electronic ) gear on continuously - it eliminated turn-on surges. The surge of heater current at turn on and the thermal shock of coming up to temperature and cooling down repeatedly is absent in gear left continuously on. It helps, but not much in the overall context of guitar amplifiers, which often violate every long life rule in the search for tone.
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9/30/1999 3:12 AM
Gil Ayan

R.G., and what about the transformer heating up? The filament supply is always working and will make a transformer hot after a while, even if it is on standby -- provided, of course, there is no fan blowing on it.  
9/30/1999 4:21 PM

Good point. In my answer, I assumed but didn't state that the transformers are properly sized so they don't exceed their ratings. If the designer relied on thermal mass to keep them under temperature and assumed some limited time on, shame on the designer. He should turn in his Iron Cross - or whatever they give journeyman amp designers.  
In class A amps, the maximum power dissipation exists at idle, so the no-signal case *is* the worst case for both tubes and transformer. A class A amp will actually cool down when driven just to distortion. In a class AB amp, the transformer load is less than max if there is no signal, and the dissipation/power used goes up with increased signal, to about 40% of max, and then goes back down, with the exact turnover point determined by the closeness to class A or B.  
An AB biased amp's transformer is kind of loafing with no signal.  
In either case, the transformer just contributes to the internal heating, but I would expect the basic design to have been such that the amp would work fine and not overheat anything just because it has been sitting for hours.  
To the extent that transformer, tubes, etc., are not designed to be thermally stable and within their specs, their lives will be shortened. That's why a fan is such a boost to life - it makes dramatic decreases in temperature, even for a piddly small fan.  
I really like the idea of putting a thermal switch somewhere inside the amp that turns on a cooling fan; also maybe thermal switches on the transformer and other places that detect overtemp and turn off the amp. Saves the transformers.
9/30/1999 6:37 AM
Ted Matsumura

I believe that if the bias is adjusted, and the voltages are not exceeded for the design of the amp, or the tubes, then sufficient cooling should keep the tubes lasting forever.  
By cooling, I mean when the ambient temperature is over 70 deg. F, you need to blow a fan across the tubes, and possibly the transformers.  
When the ambient temp increases, it's likely the electronics are heating up in a non-linear fashion.  
I have an air cooled car that is really oil cooled -- 13 or so quarts when you include the oil cooler capacity, and when ambient temps increase by 10 deg., oil temps increase by 2x or 20 deg. so hot days really stress both air/oil cooled vehicle's cooling systems, and tube amps.  
Bias the amp to ~10 % reduced voltages, and blow a fan across the tubes and transformers, don't operate or stress the unit if ambient temp is above 70 deg. Good tubes will last forever.  
I use tube amps (3 HK Award A300s) for my PC sound systems 24x7. They show no signs of degredation. Amps were designed for 7408/6V6's. I have biased for 6L6 WGBs.  
Tubes don't wear out, but they can break from environmental and electrical mal adjustments.  
9/30/1999 4:30 PM
Jim S.

I always thought that all tubes eventually wear out, no matter how good they are. Doesn't the cathode coating material degrade over time, from normal emission? I believe that's why tubes weaken after many hours of use, because the cathode becomes less and less efficient at boiling off electrons.  
"Good tubes don't die, they just fade away..."
9/30/1999 4:32 PM

"Tubes don't wear out, but they can break from environmental and electrical mal adjustments."
Not to be argumentative, but there are built in wearout mechanisms in tubes, even if they're properly cooled. Cooling is the biggest single external factor in extending live other than conservative use, of course.  
Left running in a properly biased condition, eventually the residual gas bombardment on the cathodes will break down or wear out the emitter coatings on indirectly heated cathodes, and the thorium will eventually evaporate from thoriated tungsten cathodes.  
Also, the heater filaments themselves in indirectly heated tubes and the heater/cathode in tungsten and thoriated tungsten tubes will evaporate enough metal to burn through. This is worse in high temperature directly heated tubes because the heater must run at much higher temperatures to get sufficient electrons out of the less-emissive thoriated tungsten or tungsten cathodes.  
In all tubes, the wearout of the filaments is a nonlinear function of the heater temperature. Like light bulbs, the life of a heater filament is inversely proportional to the 13th power of the applied voltage. In light bulbs (same technology, tungsten filaments) the temperature is such that the design life is about 1000 to 1500 hours. In tubes, with vacuum around them instead of nitrogen, and at substantially lower temperatures, the lives get much longer. Maybe MUCH longer.  
The wearout of the coated cathodes in indirectly heated tubes tends to be what gets them if the heaters are not subject to turn on surges. There's no way to get all the gas out of a tube, and eventually the cathodes give up. This may be decades for low power/low current tubes like 12A?7's conservatively biased. It can well seem like forever on a human scale, but they do eventually wear out.  
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