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Author Topic: Amp cool-down fan cycle  (Read 859 times)
WY3X
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« on: October 07, 2005, 07:53:09 AM »

This probably belongs in the ARTICLES section, but it takes so darned long to get things posted there, I thought I'd post it here instead.

This pertains to cool-down cycles after an amp is turned off- I was reading through a recent Commander amp review, and saw where someone was concerned about tube warmup because the cycle timer was dead in his new amp. This reminded me of an article I read about slide projectors in a photography magazine around a dozen years ago- the author of the article had done extensive testing, and had proven that running the fan after shutting off the projector bulb was HARMFUL and cost bulb life, and did not extend the life of the bulb as one would think. I thought this odd since about every slide projector manufacturer has hard-wired into their projectors a way to keep the fan running after turning off the bulb (perhaps a way to sell more lamps?). But the author of the article provided proof-positive by extensive test and result tables that proved beyond a shadow of a doubt that leaving the fan on to cool the bulb after shutdown contributed directly to the demise of the lamp in question (I use the term "lamp" and "bulb" interchangeably- please no arguments here....).

This is a highly subjective question, and I'm just seeking -opinions- because we all know we'll never agree (right?). Do you think this same thing applies to amplifier tubes as well? What do you think? If you turn off all voltages to the tube, filament, plate, grid, etc. and left the fan on for a few minutes after shutting down an amp, would it extend or shorten the life of the tube(s)? Anyone done any testing? Any amp manufacturer want to give me a ca$h grant to fund my private testing? <GRIN>

-KR4WM
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W8JI
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« Reply #1 on: October 07, 2005, 08:59:03 AM »

Don't believe most of what you read in articles or in advice solicited on the internet. Wrong answers are given all the time. Everyone wants to be helpful, but few people are straight forward enough to say when they are assuming or guessing. Non-peer reviewed articles are worth about what you pay for them.

As for lamp life, virtually all failures are due to filament failures. Filament falures have nothing to do with cool down time of the lamp envelope. They have a lot to do with dumb luck in getting a good lamp, mechanical vibration, and the quick ramp up of the filament temperature upon start-up.

Lamp cool down is only important for heat soak of other parts.

In an amplifier, virtually all failures are NOT related to controllable filament damage.

73 Tom

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KC8VWM
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Posts: 3121




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« Reply #2 on: October 07, 2005, 09:01:13 AM »

Any amp manufacturer want to give me a ca$h grant to fund my private testing?

-----

Firstly, I would like to say that I feel your question adresses a technical matter related to amateur radio most appropriately.

Secondly, what you have described seems reasonable. You are suggesting they incorporate a cool down cycle in amps after they are turned off. (Similar to projectors)

I have wondered this myself on occasion however, I don't feel this should only be limited to amps but might include most radios in general.

The question becomes, Should they incorporate a "cool down cycle" as a method of prolonging the life cycle of the equipment.

I dont see it hurting the equipment however I dont see damage caused by the lack of a cool down cycle either. Howvwer, one observation to consider when  evacuating all the heat from the equipment after it is shut off might end up resulting in a longer warm up period.

These are just a few thoughts I had to contribute.

73 Charles - KC8VWM
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NI0C
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« Reply #3 on: October 07, 2005, 09:03:28 AM »

I'm not sure the results of tests on the lifetime of slide projector bulbs would be applicable to vacuum tubes in power amplifiers.  Quite possibly vibration from the fan could have been the cause of premature bulb failure, offsetting the expected increase in lifetime due to the cooling effects of the fan.    

I usually let my amplifier cool down in the standby position for a few minutes rather than abruptly shutting it down after hard usage.  Probably most amateurs do that as a matter of course.

73,
Chuck  NI0C
 
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WB2WIK
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« Reply #4 on: October 07, 2005, 09:06:40 AM »

Tom (W8JI), I think you didn't really answer an important element of the question, and I'd like to hear this, too...

Most amplifier manufacturers recommend the user leave the amp "running" for a period (maybe a minute) after any transmission long enough to get the tube(s) really hot.  With glass tubes, you can see the anode color, and with ceramic tubes you can't, but you can feel the exhaust heat.

I note with my glass tube amps at least, if I power down (which also shuts off the cooling) immediately after making a transmission that gets the anodes red, the tube(s) seem to make a "noise" that doesn't sound very nice -- almost like cracking, or at least something shifting; the noise doesn't occur if I leave the amp/cooling turned on a minute or so and then shut down.

I have no data at all relating to tube life one way or the other, but the manufacturers do seem to recommend this.

What do you say?  What's the "noise?"

73

Steve WB2WIK/6
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KC8VWM
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« Reply #5 on: October 07, 2005, 09:33:20 AM »

You mean those little mechanical tube noises?

These sounds are quite similar in nature to the mechanical sounds an electric heater makes when it is cycling on and off.

I suspect there is some correlation related to heat and the thoriated tungsten wire used in tubes?
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W2AEW
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« Reply #6 on: October 07, 2005, 09:59:45 AM »

I always figured that a cool down cycle was good for the amp.  When the amp is running, and the cooling fan is running, air is always pulling the heat off of the tube envelope.  If you didn't have a cool down cycle upon power off, the air would stop immediately, and the heat from the tube(s) would then cause an increase in temperature thru the still air inside the amp.  My TL-922 features a cooling fan that stays on for a few minutes after turning the power switch off, and I intuitively felt (and still do) that is a nice feature.  

I haven't had that many different amps - so that leaves me wondering...  How many amps have a cooling cycle upon power off?  Is my TL-922 the exception, or the rule?
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K5DVW
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« Reply #7 on: October 07, 2005, 10:26:43 AM »

Re: the mystery of cracking noise of tubes. I currently have a set of 811 tubes that will do that after a long CW transmission. I don't have to turn off the amp! It sounded like the bi-metallic click of a automobile turn signal flasher, but it only clicks once. I figured it was a mica spacer in the tube sliding against the glass or something like that.
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KA0GKT
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Posts: 555




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« Reply #8 on: October 07, 2005, 12:14:41 PM »

In most commercial transmitters (Amplifier cabinets), there is a time delay relay to cool the tube after the filaments have been shut down.  In most cases, there is a flyswatter switch which disables the filaments and  B+ in the event of a cooling blower failure.

In the situation of a glass envelope tube, like the 811A, a few minutes in stand by with the fan runing ought to be enough.  In the case of external anode tubes (Glass or ceramic) I like a 5-minute cool down ands a 5-minute warm up.

In gthe end, check the tube manufacturers suggestions and err on the side of caution.

Cool-em off ans they'll last longer.

When it comes to projectors, one must remember that much more than just the projection lamp is being cooled.  There are infrared and Ultraviolet absorbtion filters and cold mirrors which are cooled by the fan, some of which are more expensive to replace than the little ELH lamp.

73 DE KA0GKT/7

--Steve
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N0TONE
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Posts: 173




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« Reply #9 on: October 07, 2005, 03:22:40 PM »

The only time that a forced-air cooldown is useful is if the tube involved has a very high power filament or heater, and the thermal conduction path to the outside air is poor.  This is true for some of the 100kW and larger tubes, but not true for anything used in the amateur service.

AM
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KA0GKT
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« Reply #10 on: October 07, 2005, 07:10:50 PM »

NOTONE wrote:>The only time that a forced-air cooldown is useful is if the tube involved has a very high power filament or heater, and the thermal conduction path to the outside air is poor. This is true for some of the 100kW and larger tubes, but not true for anything used in the amateur service.<

I beg to differ.  The 3CX1500 tube used in some legal limit linear amplifiers would most certainly require a cool down cycle following high duty-cycle communications like RTTY or digital modes.  the addition of a cool down timer is relatively small when compared to the price of a replacement tube.

I have seen lack of cool-down cause problems for 3-1000 tubes in chimneys where natural convection isn't sufficient to cool the still hot triode.

Considering the cost of some high power transmitting tubes, running the fan for a few minutes following operation is cheap insurance.

73 DE KAØGKT/7

--Steve
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W8JI
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« Reply #11 on: October 07, 2005, 08:00:19 PM »

I agree with NOTONE.

The only time a long cool down cycle might help a tube is if the tube has a huge element with large thermal mass that is running extremely hot (metal to the point of very bright color). If air is shut off with power down, latent heat from a very hot massive element MIGHT overheat seals.

While testing 3CX1200A7's for Eimac in prototype stages a blower was accidentally shut off while anode and filament voltages were on. The metal tube envelope was a crisp black color in the morning, but the tube was still running 1500 watts. I think ceramic seals are pretty reliable. As a matter of fact I can't ever recall seeing a seal failure in a ceramic tube.

As a matter of fact I've made extensive measurements of seal temperatures and the only time I've ever see a seal to go over temp on shutoff was with a 3-500Z that was running with the anode red to orange in color and a direct power down with the anode at color.

It certainly won't happen in an 8877 or any tube that doesn't run with extremely high temperatures on a very large metal mass.

The overwhelming 8877 problems in an amplifier with grid protection circuitry relate to internal shorts unrelated to temperature. They are failures primarily caused by poor grid alignment with bare areas of the cathode. The grid has something like 150 gold plated wires that have to be precisely aligned with gaps in cathode oxide. We are dealing in thousandths of an inch spacing being the difference between a long life tube and a premature failure.

I really can't recall seeing a tube seal failure (which is all heat soak might possible cause) outside of Chinese 3-500Z's that used crummy glass. Those tubes would suck air even at 400 watts dissipation with full airflow.

Other components around the tube might be another issue, but not the tubes.

It's probably better to not shut off hard right at the end of a high dissipation transmission (mostly for other parts, not the tubes). Of course I never pay attention to that in my amps. Extended air time in an amplifier **might** be good on parts around the tubes, but historically seals don't overheat after a tube is powered down. Unless you do something really dumb like run a big glass tube with the anode nearly orange and do a full-hot shutoff.

73 Tom
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