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Author Topic: Turning off tube amp after going QRT  (Read 1242 times)

W9AC

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Re: Turning off tube amp after going QRT
« Reply #30 on: March 28, 2023, 04:55:51 AM »

This morning, I read through the cooling section in the 1967 edition of Eimac's "Care and Feeding of Power Grid Tubes." Although operating cooling requirements are given substantial treatment, there's no discussion about cool-down procedure. 

At the moment of amp shut-off when forced-air cooling stops, do envelope and seal temperatures momentarily rise above the normal operating temperature with cooling?  It would be an interesting exercise to record real-time tube temperatures across amplifier models with a Flir device.

Paul, W9AC
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K6BRN

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Re: Turning off tube amp after going QRT
« Reply #31 on: March 28, 2023, 07:05:28 AM »

This morning, I read through the cooling section in the 1967 edition of Eimac's "Care and Feeding of Power Grid Tubes." Although operating cooling requirements are given substantial treatment, there's no discussion about cool-down procedure. 

At the moment of amp shut-off when forced-air cooling stops, do envelope and seal temperatures momentarily rise above the normal operating temperature with cooling?  It would be an interesting exercise to record real-time tube temperatures across amplifier models with a Flir device.

Paul, W9AC

Hi Paul (W9AC):

Yes - measurement of tube pin/seal temperature over time after amplifier shutdown with a good FLIR would be interesting, but would also likely be different for different tubes and amps due to different thermal paths. 

But there IS some official guidance on this.  The CPI data sheet for the Eimac YC-179/YC-179A power triode states:

Quote
"Airflow must be applied before or simultaneously with the application of power, including the tube heater, and should normally be maintained for several minutes after power is removed for tube cool-down."

https://mgs4u.com/wp-content/uploads/2018/01/Eimac-YC-179-PDF.pdf

Eimac Application Bulletin #20 titled "TEMPERATURE MEASUREMENTS WITH EIMAC TUBES" might be helpful as well, if you can find it.  This bulletin is mentioned in many Eimac power tube data sheets.

Brian - K6BRN
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ZS5WC

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Re: Turning off tube amp after going QRT
« Reply #32 on: March 28, 2023, 08:56:24 AM »

 :)I have often thought about this , and where the heating of envelopes and pins that unsolder themselves like in the TL-922 I was wondering about a mod that will cure this problem.
Having a circuit, run off mains that will run fan and have a run on timer circuit, much like run on circuit for automotive radiator fans.
OR.. Having an Bi-metallic clickson temperature switch, like the ones used in microwaves to sense the exhaust air, and not turn off unless the air Temp is below 55 or 60 Degrees C.
The problem we face in South Africa is that the power utility can be turned off without warning and Having an TL-922 with 3-500Z tubes, this is a huge problem. If power goes off in the middle of an QSO and tubes are hot, the pins unsolder  themselves.
Still working on an ups or battery backup for the Fan on this issue.
73 De William , ZS4L / ZS5WC
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VE7RF

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Re: Turning off tube amp after going QRT
« Reply #33 on: March 28, 2023, 09:26:26 AM »

"Airflow must be applied before or simultaneously with the application of power, including the tube heater, and should normally be maintained for several minutes after power is removed for tube cool-down."

The above is good enough for me.

On one of my hb 2 x 4-400 amps, with bottom lid off...and rf deck pushed back on the desk,  with just enough overhang, such that I could take some V measurements at the sockets,  I put my finger on pin #1..... and just about took my finger prints off.  It was fn hot...and then some. Those pins run stupid hot.

On my hb 3CX-3000A7 amp,  initially I was using a SK-306 chimney, made for a YC-156 /  4CX-5000  tube...with their 4.94" OD anode cooler.   The 3CX-3000A7 tube anode is only 4.125" OD, so used layers of 1/8" thick silicone rubber and a SS hose clamp, to plug the gap.  We had the blower + heater on ( 7.5 vac @ exactly 50 amps, using a clamp on ac ammeter around one leg of the fil cable  going to the next shelf below).

Ran it for like 10 mins, then shut down the fil and blower...and removed tube.... so I could tweak the silicone rubber down a bit on the anode, which required 1st loosening up the SS hose clamp.

I was sitting in my office chair, with tube vertical, and  base of tube between my crotch, and a nut driver, tweaking the hose clamp etc.  Tube base inner + outer collets were good and warm.   But after 1 min, it was getting hotter and hotter + HOTTER. It was ready to burn my gonads, so lifted tube up...and gotta outa the chair asap.  Then felt the inner / outer collets..and indeed they are hot.....  but cooled off fairly soon.

And I had loads of air, with a pair of dayton  4C4006B blowers  running, and also a magnehelic gauge installed, which read 1.15" pressure.  70 cfm airflow...and a helluva lot of pressure.

On all this hb stuff, with expensive tubes,  when done for the night, amp in standby,  B+  shut off, fil shut off.... but blower stays on for 5-10 mins.   Depending on what u were doing beforehand, that anode can get stupid hot...with the requisite airflow.

Tubes cost too much, even to rebuild, so  I leave the blower on for  a while.  I also use the small Variac to bring up the fil V slowly to 7.5 vac.  I have a fil V meter...and also a fil ammeter.  Fil current comes up from 0-5-10-15-20.....50 amps, over a 20 second period...by hand.   Same deal when done for the night..and reducing fil V back to zero.   That's my..'new way' of doing 'step start' on directly heated metal tubes.  Cold resistance of these tubes is like only 1/10 of the hot resistance, and I was using a surplus fil xfmr, rated for 8.25 vac @ 79 amps.  It has a 208 vac input, so the variac has to be used, to make it work.  Just about all of the fil xmrs, ( except henry radio 8k ultra) are capable of one helluva lot of peak current, when u lay essentially a dead short on em...like > triple rated current.   I'm 90% convinced that's where a lot of grid to fil shorts come from.

On the L4B 2 x 3-500Z amps,  I just let the amp sit on standby for a few mins, then shut it down..which shuts everything off, no blower over run used,  but that may well change on my next round of mods.... more of an experiment.
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VE7RF

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Re: Turning off tube amp after going QRT
« Reply #34 on: March 28, 2023, 09:38:16 AM »

:)I have often thought about this , and where the heating of envelopes and pins that unsolder themselves like in the TL-922 I was wondering about a mod that will cure this problem.
Having a circuit, run off mains that will run fan and have a run on timer circuit, much like run on circuit for automotive radiator fans.
OR.. Having an Bi-metallic clickson temperature switch, like the ones used in microwaves to sense the exhaust air, and not turn off unless the air Temp is below 55 or 60 Degrees C.
The problem we face in South Africa is that the power utility can be turned off without warning and Having an TL-922 with 3-500Z tubes, this is a huge problem. If power goes off in the middle of an QSO and tubes are hot, the pins unsolder  themselves.
Still working on an ups or battery backup for the Fan on this issue.
73 De William , ZS4L / ZS5WC

The TL-922, already has fan over run built into it.  I think it runs for like 5-10 mins.  For frequent power outages, I would replace the fan with a 12 vdc unit.... and power it externally from a battery/ charger setup.

I have experimented with the Klixon  ( fixed temp)  sensors / spst-NO switches...which close on a temp rise. Problem is, u cant' adjust em.   Recently, I bought a few german, fully adjustable types, that also close on a temp rise when temp exceeds the dialed up level.  It's just a T stat..that is used for cooling, and trips on a temp rise. They are designed to fit a standard DIN rail setup, and they came with mating short DIN rails.   Din rail mounted 1st...the the T stat just snaps into the Din rail.   I use that on the HB  B+ supply, with it's pair of 10" slowed down fans.
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KM4AH

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Re: Turning off tube amp after going QRT
« Reply #35 on: March 28, 2023, 11:01:49 AM »

I repaired a TL922 a couple of weeks ago that lost the solder in one of the filament sockets in spite of a working time delay fan shut off.  The problem with many 3-500Z amps is the fan blows across the tubes rather than up through the sockets as they were designed to be operated. Eventually the filament pin connections get higher resistance from age and corrosion and melt the solder. Very common.
Sometimes the only permanent cure is replace the socket.

I let my amps run a considerable amount of time before I turn them off. Just out of habit.

I have a 3-1000Z amp and the tube is visible through a screen covered glass as was the style back in the day.  I would ask you to look at a glass tube and see how quickly it looses color either after a transmission or when the filaments are turned off.  A few seconds... the color is gone.

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W9AC

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Re: Turning off tube amp after going QRT
« Reply #36 on: March 28, 2023, 06:19:42 PM »

But there IS some official guidance on this.  The CPI data sheet for the Eimac YC-179/YC-179A power triode states:

"Airflow must be applied before or simultaneously with the application of power, including the tube heater, and should normally be maintained for several minutes after power is removed for tube cool-down."

Brian - K6BRN

I found a similar requirement under the seal cooling heading in an early 1950s issue of the Eimac tube catalog, courtesy of W9ADM who is long SK by now.  See p. 36:

https://worldradiohistory.com/Archive-Catalogs/EIMAC/Eimac-Tubes-Catalog-1950-1953.pdf

"Air and water flow must be started before filament and cathode power are applied and maintained for at least two minutes after the filament
and cathode power have been removed..."


Paul, W9AC
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W3SLK

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Re: Turning off tube amp after going QRT
« Reply #37 on: March 29, 2023, 07:00:46 AM »

VE7RF said:
Quote
I have experimented with the Klixon  ( fixed temp)  sensors / spst-NO switches...which close on a temp rise. Problem is, u cant' adjust em.   Recently, I bought a few german, fully adjustable types, that also close on a temp rise when temp exceeds the dialed up level.  It's just a T stat..that is used for cooling, and trips on a temp rise. They are designed to fit a standard DIN rail setup, and they came with mating short DIN rails.   Din rail mounted 1st...the the T stat just snaps into the Din rail.   I use that on the HB  B+ supply, with it's pair of 10" slowed down fans.
If air temperature is a premium detail in an amp then install a temp switch with the probe in the airstream. SOR or Ashcroft makes them and they are adjustable. The ones we had at work were a couple hundred dollars but they were for a Class I Div. I environment. I thought they made ones that were cheaper because we had a 'purchasing consultant' that felt we needed to save money not realizing they were being used within 5' of solvent charging.
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VE7RF

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Re: Turning off tube amp after going QRT
« Reply #38 on: March 29, 2023, 07:12:16 AM »

I repaired a TL922 a couple of weeks ago that lost the solder in one of the filament sockets in spite of a working time delay fan shut off.  The problem with many 3-500Z amps is the fan blows across the tubes rather than up through the sockets as they were designed to be operated. Eventually the filament pin connections get higher resistance from age and corrosion and melt the solder. Very common.
Sometimes the only permanent cure is replace the socket.

I let my amps run a considerable amount of time before I turn them off. Just out of habit.

I have a 3-1000Z amp and the tube is visible through a screen covered glass as was the style back in the day.  I would ask you to look at a glass tube and see how quickly it looses color either after a transmission or when the filaments are turned off.  A few seconds... the color is gone.

Typ  Johnson style 3-500Z sockets  all have ...'helper springs' on all their existing contacts, that provide additional tension.

Pins 1+5 are the fil...and pins 2-3-4 are the grid pins.   Folks have removed the sockets..and swapped pins 1+5  for pins 2+3.   Then you have a good set of contacts for the fil pins.   Then tweak the bad contacts before installing into pins 2+3. ..which will be good enough for the grid pins.   
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VE7RF

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Re: Turning off tube amp after going QRT
« Reply #39 on: March 29, 2023, 07:30:28 AM »

VE7RF said:
Quote
I have experimented with the Klixon  ( fixed temp)  sensors / spst-NO switches...which close on a temp rise. Problem is, u cant' adjust em.   Recently, I bought a few german, fully adjustable types, that also close on a temp rise when temp exceeds the dialed up level.  It's just a T stat..that is used for cooling, and trips on a temp rise. They are designed to fit a standard DIN rail setup, and they came with mating short DIN rails.   Din rail mounted 1st...the the T stat just snaps into the Din rail.   I use that on the HB  B+ supply, with it's pair of 10" slowed down fans.
If air temperature is a premium detail in an amp then install a temp switch with the probe in the airstream. SOR or Ashcroft makes them and they are adjustable. The ones we had at work were a couple hundred dollars but they were for a Class I Div. I environment. I thought they made ones that were cheaper because we had a 'purchasing consultant' that felt we needed to save money not realizing they were being used within 5' of solvent charging.

I looked at that method, and at the time, was too expensive, and also required dcv to operate it.   I wanted a dead simple T stat, that's bullet proof, and the german Din rail T-stats were cheap, and bombproof.   No DCV required to operate em, and contacts rated  for 300 vac @ 20 amps.  The T stat could be used to turn on a fan  (or blower), or could be used to shunt out a resistor... to speed up a fan.

I have since tried yet another method for cooling on the hb B+ supply, and simply wired the 2 x 115 vac fans in series, and ran em on 118 vac....so each fan gets  59 vac.   The pair of 9" diam fans gets  59 vac..and fans on all the time. Beyond dead quiet.  When bigger diam fans are slowed down, the noise level drops a bunch...and also the PITCH drops in freq a bunch.  Low level, low freq fan noise is something one can easily tolerate.  I also tested each fan individually with a small 0-130 vac variac, and found they would turn on with just  15 vac  applied.   I also tried both fans wired in series, and used the small variac to turn them on. They both start spinning with 30 vac applied ( 15 vac dropped across each fan).

Don't ever operate identical 120 vac blower's in series.....and apply 240 vac to the mess....esp if they have the usual start / run caps installed.  When T=0, at initial turn on, the V drop across each blower is vastly different, like  200 vac across 1st blower...and  just 40 vac across the 2nd blower.  Scott tried just that, using a pair of identical 120 vac Kooltronics blowers...and his line V was on the high side, like 250.  Smoked one of the blowers.
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VE7RF

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Re: Turning off tube amp after going QRT
« Reply #40 on: March 29, 2023, 07:33:48 AM »

AB-20...and all the application bulletins are on eimacs website.

https://www.cpii.com/library.cfm/1#22

It's the 3rd item up from the bottom of the huge list.  Scroll right to the bottom....then u will see it.
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W4JFA

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Re: Turning off tube amp after going QRT
« Reply #41 on: March 30, 2023, 04:12:58 AM »

For a pair of 3-500z, my theory is that you want the ENTIRE tube to cool as evenly as you can. If the amp is shut down soon after a long transmission is the cooling un-even? Meaning, if the base is cooled at a different rate than the glass envelope, that can't be good for the seal. This is just my thoughts, no scientific tests made.

Opinions?

On my 922, in addition to a stronger factory fan installed, I have holes below the tubes with fans that are mounted on the outside/bottom below which blow air up from outside air to the bottom of the tubes. This has gotta help. I leave them on for a while after use. Time will tell.

Bob
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W3SLK

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Re: Turning off tube amp after going QRT
« Reply #42 on: March 30, 2023, 06:45:02 AM »

VE7RF said:
Quote
I looked at that method, and at the time, was too expensive, and also required dcv to operate it.   I wanted a dead simple T stat, that's bullet proof, and the german Din rail T-stats were cheap, and bombproof.   No DCV required to operate em, and contacts rated  for 300 vac @ 20 amps.  The T stat could be used to turn on a fan  (or blower), or could be used to shunt out a resistor... to speed up a fan
I don't know what you mean by expensive but an Ashcroft temp. switch is nothing but a sealed capillary filled with a thermo-active fluid, (usually glycol). As the temp goes up, the fluid expands and pushes a plunger that activates a 10A micro-switch. No power whatsoever is required.

Edit: Jim I know you are a 'motorhead', here's a dirt cheap one for $24 from Amazon. Same as on a HVAC system. https://www.amazon.com/American-Volt-Adjustable-Thermostat-Temperature/dp/B01E52O5DG/ref=asc_df_B01E52O5DG?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80195746824459&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4583795273577896&psc=1
« Last Edit: March 30, 2023, 06:54:23 AM by W3SLK »
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