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Author Topic: Turning off tube amp after going QRT  (Read 1236 times)

KB8VUL

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Re: Turning off tube amp after going QRT
« Reply #15 on: March 24, 2023, 08:29:06 PM »

A lot of this is going to depend on the way your unit is constructed and what space you have to add stuff to it.
If you can shut off the high voltage and filament while the fans continue to run, that is the ideal situation.  Problem is most manufactures put the fan circuit and the filament circuit on the same power switch.  Which of course leaves the filament (heat source) under power when the fans are running. 

The way to do it so you can turn off the power and 'walk away' from it would be install a delay relay that is wired to the fan power that would continue to power the fans for a window of time after the filaments were shut down.  I would think five to ten minutes would be more than enough time.

But the other part of it is do you really need it at all.  And that is going to depend on how hard you are running the tube and what temperatures you are seeing on that tube.  Are you thinking that you are gonna get gabby on 80 or 160 meters running full out on AM for several minutes and then sign off and power down, or are you running SSB at 3/4 power and chasing DX with long pauses and when the band goes quiet listening for a few and then shutting down?  Big difference there.  But ultimately, it's what ever you want to do.  If you want automatic coll down that can be left unattended, the time delay relay is the direction to look.  And some amps are gonna have space for such a thing while others will not. 

The last thing with making any sort of modification to the amp, are you comfortable with doing the install?  Working with line voltages and understand the concepts of high voltage that exists in a tube amplifier and how you can mount things too close to that voltage and have arc over issues without even actually coming into direct contact with that high voltage?  If you are not well versed in these matters you need to either seek out someone that is, or simply not bother with it. 
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VR2AX

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Re: Turning off tube amp after going QRT
« Reply #16 on: March 25, 2023, 01:51:56 PM »

Is it more psychological than else? We just turn our cars/autos off full stop. Heat dissipates through the relevant medium, a bit of air when energy is no longer being supplied, does it imply not enough cooling before?
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K1KIM

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Re: Turning off tube amp after going QRT
« Reply #17 on: March 26, 2023, 06:43:27 AM »

Is it more psychological than else? We just turn our cars/autos off full stop. Heat dissipates through the relevant medium, a bit of air when energy is no longer being supplied, does it imply not enough cooling before?

Actually the heat will build in an engine after shut down if the water pump and fan no longer are turning.

We all do it, and the pressurized system is made to take it, but it happens nonetheless.
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KM4AH

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Re: Turning off tube amp after going QRT
« Reply #18 on: March 26, 2023, 12:30:55 PM »

Is it more psychological than else? We just turn our cars/autos off full stop. Heat dissipates through the relevant medium, a bit of air when energy is no longer being supplied, does it imply not enough cooling before?

Actually the heat will build in an engine after shut down if the water pump and fan no longer are turning.

We all do it, and the pressurized system is made to take it, but it happens nonetheless.


 The heat doesn't build up in the engine. The water temperature goes up because the engine oil temperature and block temperature were already higher.

Or, that's my story anyway.


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K6JH

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Re: Turning off tube amp after going QRT
« Reply #19 on: March 26, 2023, 02:03:24 PM »

Car analogies: I’ve always heard that turbocharged engines should be idled for a bit before shutdown, as it maintains the flow of cooling oil to the turbo bearings as it spins down. Running hard with a quick shutdown supposedly cooks the oil, leaving deposits which lead to premature repairs.

My Dad (KE6AR, SK Feb 21st) always taught me to run the slide projector cooling fan after shutting off the lamp. Supposedly to make the bulb last longer, but it also keeps plastic bits cooler. And have you ever seen film in a movie projector cook if it doesn’t keep moving and cooling?
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Jim K6JH

KM4AH

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Re: Turning off tube amp after going QRT
« Reply #20 on: March 26, 2023, 02:10:04 PM »

Car analogies: I’ve always heard that turbocharged engines should be idled for a bit before shutdown, as it maintains the flow of cooling oil to the turbo bearings as it spins down. Running hard with a quick shutdown supposedly cooks the oil, leaving deposits which lead to premature repairs.

My Dad (KE6AR, SK Feb 21st) always taught me to run the slide projector cooling fan after shutting off the lamp. Supposedly to make the bulb last longer, but it also keeps plastic bits cooler. And have you ever seen film in a movie projector cook if it doesn’t keep moving and cooling?


Yeah, but that has nothing to do with this.
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KB8VUL

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Re: Turning off tube amp after going QRT
« Reply #21 on: March 26, 2023, 02:34:50 PM »

Car analogies: I’ve always heard that turbocharged engines should be idled for a bit before shutdown, as it maintains the flow of cooling oil to the turbo bearings as it spins down. Running hard with a quick shutdown supposedly cooks the oil, leaving deposits which lead to premature repairs.

My Dad (KE6AR, SK Feb 21st) always taught me to run the slide projector cooling fan after shutting off the lamp. Supposedly to make the bulb last longer, but it also keeps plastic bits cooler. And have you ever seen film in a movie projector cook if it doesn’t keep moving and cooling?

This is the same thought process with a tube. Any device that has multiple layers and a heat source internally is going to obviously hotter in the center than the outside.  With active cooling (a fan running) the heat that is in the middle is transferred through the parts of the device to the exterior and then finally carried away by the flow of air around the device. 

When you stop the internal heating of the device, the heat source is gone, but the heat remains until convection can move that heat to a surface it can be dissipated from.  And we need to consider that a vacuum is a pretty good thermal insulator. So the heat will need to move through the vacuum of the tube, unless it's a ceramic tube where the anode is also the heatsink, and even with those, if you are actively removing the heat and then stop the temperature of the anode will INCREASE due to the heat internally that is hotter than the anode heatsink will continue to transfer to it increasing the temperature. 

So now lets consider the operational expectations of transmitter tubes.  NO ONE makes a 'ham radio' tube.  Other than maybe the glass envelope 3-500.  Anything that is ceramic was designed to be sitting in a commercial transmitter that is run 24/7 for months at a time.  Used by folks that don't get a hoot about a power bill because their transmitter EARNS them money or is otherwise funded by a company or the government.  The guy that has the ability to 'turn it off' simply doesn't bother.  So it's kept hot, and the fans run all the time.  And the tube is designed for that type of operation, not fired up in the evening for a couple hour's.  Then shut off to cool manually when the switch is thrown.  And even back in the day for AM stations that were daytime only operation, the tubes were glass (833's 811's and 572's).  Filament current wasn't the crap that was present in the 500KW transmitter at WLW (4500 amps) so the HV was turned off and the active cooling was left running with the tubes left hot. 

Now, a guy is NOT gonna want to leave his tubes hot and just pay the power bill in most cases.  BUT, that's when a filament will typically fail is on startup.  This is even present in incandescent light bulbs.  They 'burn out' when power is applied to them.  Leave them on all the time and they never fail.  But current limiting the filament power, bringing up a tube slow (which no one actually bothers with) would significantly increase the life of the filament.  But properly cooling it after it's shut off is certainly NOT going to hurt it and may also increase it's life.  Of course running it as it was designed to be run would be better, but that isn't in the cards for most ham operators and their electric bills.
 
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KM4AH

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Re: Turning off tube amp after going QRT
« Reply #22 on: March 27, 2023, 05:52:29 AM »

So, we turn the power off and the tube gets hotter.  I can envision a perpetual motion machine.
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K6BRN

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Re: Turning off tube amp after going QRT
« Reply #23 on: March 27, 2023, 09:59:23 AM »

So, we turn the power off and the tube gets hotter.  I can envision a perpetual motion machine.

The principle is pretty simple and is called "Heat Soak".

Essentially, when forced cooling stops in a machine, be it a liquid cooled car engine/turbocharger or tube amp cooled by air, thermal energy is still flowing from the hotter points in the system to the cooler, but the cooler bits are getting warmer because there is no more forced cooling and so parts of the path get hotter than they normally do.  Sometimes MUCH hotter.  With tube amps, those with glass tubes/frit pin seals are most vulnerable.  They can deform/crack and leak.

High output car engines have the same problem and they often have fan run-on thermostats to keep the cooling fluid temperature low and promote liquid convection cooling withinn the engine/turbocharger.

If this is NOT done, "wet" cylinder liner seals can fail or oil in turbo bearings can "coke up" due to excessive after-run heating.

No perpetual motion is required for this phenomena.

Brian - K6BRN

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VR2AX

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Re: Turning off tube amp after going QRT
« Reply #24 on: March 27, 2023, 12:18:39 PM »

Just go back to low school basics.. There is a fundamental principle of 'Conservation of Energy' (and Mass). We talk of 'power' rather loosely.
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KM4AH

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Re: Turning off tube amp after going QRT
« Reply #25 on: March 27, 2023, 03:08:54 PM »

Well, the heat does not build up is my point. It may move around.

Conservation of energy always applies.

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K6BRN

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Re: Turning off tube amp after going QRT
« Reply #26 on: March 27, 2023, 04:56:37 PM »

Well, the heat does not build up is my point. It may move around.  Conservation of energy always applies.

Regardless - allowing an amp with forced air cooling to idle and cool for a while after being run hard is usually a good idea.  Which is the point of the original post.

As to how long?  SS amps usually have a temp readout and I let them settle to below 30C.  For tube amps its usually just 3-5 minutes.  Longer does not hurt - so when in doubt, stretch it out. (cooling time, that is).

Brian - K6BRN
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KM4AH

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Re: Turning off tube amp after going QRT
« Reply #27 on: March 27, 2023, 06:27:29 PM »

Regardless of conservation of energy.  I love it..
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KB8VUL

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Re: Turning off tube amp after going QRT
« Reply #28 on: March 27, 2023, 08:53:58 PM »

You know, it just boggles the mind that as we sit here discussing a hobby that is technical in nature, based on a TON of scientific principals that we get the comments on here I have read.

Perpetual Motion?  Really?  I have to ask if you are trying to shine people on with that or you are serious.

Lets look at this from a different perspective.  Heat up a rock to 200 degrees.  Now remove the rock from the heat and put it in a metal can that is 70 degrees.  The can will get hotter because the rock inside of it is hotter than the can when you placed the rock it the can.  How does that happen?  THe heat from the rock leaves the rock and transfers to the can.  Simple enough.

Tube guts.  Fire up your 3-500 and lay down on the key on FM.  The Anode gets GLOWING hot. The filament is glowing hot.  Glowing hot is over 1000 degrees for most objects.  So where doe that heat go?  Outer space, another dimension, how about into the glass envelope surrounding the tube and physically connected to the hot parts.  Now when the fan is running, laws of thermodynamics tell us that air can act as a medium to transfer heat to. 

But we need to understand something here.  YOU CAN'T COOL AIR, you can ONLY remove the heat from the air, or anything else with heat in it, and move it somewhere else.  Air conditioning in your home or car doesn't cool the air..  It moves the heat outside.  Hence the reason the condenser outside gets hot and blows air hotter that the surrounding air, and in the winter, a heat pump will take the heat from outside and bring it inside to warm the house.  But the scientific part of it remains the same.  You remove heat from air, and transfer that heat to other air with air conditioning or a heat pump. 

All that being said.  Back to the can.  If you put a fan on that can, you remove the heat from it that is being transferred from the rock so it doesn't get as hot as it would if you put the can in some insulation and didn't blow air on it.  And that is what you are doing when you shut the fans down on a hot tube.  The tube WILL cool off, but the outer envelope will actually get hotter before it gets cooler.  Not because of free energy or some mystical nonsense, but because the tube contains things that are MUCH hotter than the envelope of the tube.  And that heat will transfer somewhere.  The only place it has to go is out of the tube, and to get there it has to go through the envelope. 
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KM4AH

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Re: Turning off tube amp after going QRT
« Reply #29 on: March 28, 2023, 03:01:28 AM »

You can write a book if you like.  But, the heat does not build up in a tube once the power is turned off.

Any part that gets hotter has an associated part that lost that heat.

The water in a car gets hotter, but that heat was lost by the oil and the block.

Don't use the term heat builds up and all will be cool.
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