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Author Topic: Cooling a pairCooling a pair of 3-500zg tubes  (Read 481 times)

K8AXW

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #15 on: June 28, 2021, 05:58:49 PM »

Carl,

Frankly Carl, my pucker factor ramps up every time I power up the amp waiting for the "bang" from either the tube or vac cap seals having let go.

No ceramic vac caps available 38 years ago.  I've seen ceramic vac caps since then but hate to fix something that isn't broken...especially at 85 years of age.  Let the next owner do it!    ;D

73 - Al
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G3XLB

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #16 on: June 29, 2021, 02:40:41 AM »

I would like to thank everyone who have taken their time to reply to me.
Yes,Al  3=500zg tubes are very expensive and the thought of them burning up when first tested is not pleasant.Al, I read your dream amplifier,made nice reading.Would love more info on the filament step-start and the time delay relay you used.is the info in ARRL handbook 70? I have been thinking of putting in a delay circuit that prevents the filaments coming on until the blower has cooled down the tube bases adequately.
I was interested in the info on the 4-400A,The tubes I have are old used tubes swapped out of BBC transmitters so are probably on their last legs.I got them at ham fests down south. The tube that burnt up was one of these and your explanation could well explain the failure.
Thanks again to all and All if you have a schematic for your delay/step-start I would be most interested.73 and Hans again Mike G3XLB
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KM1H

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #17 on: June 29, 2021, 09:20:20 AM »

Quote
No ceramic vac caps available 38 years ago.  I've seen ceramic vac caps since then but hate to fix something that isn't broken...especially at 85 years of age.  Let the next owner do it!

Well understood Al, Im 80 and dont care for adding more work if I can help it.
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K8AXW

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #18 on: June 29, 2021, 06:51:37 PM »

Mike,

Unfortunately, the part of the do-it-all printer that allows me to copy and email information no longer functions.  I will happy to send the information by snail mail if you wish.

A highly regarded electronic engineer who used to prowl eHam has stated many times "that the 3-500Z type instant on tubes do not require step-start because the thoriated tungsten filaments self-limits the inrush current."  He further emphasized that step-starting thoriated tungsten filaments is actually detrimental to their longevity.

I guess I've been lucky for 38 years!   ;D

That circuit and others in my amp came from sources now unknown.

Both the filament and HV step-start circuits use commercial adjustable time delay relays to control resistor bypass relays.  Bought them new at hamfests cheap. 

Let me know your wishes Mike.
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KM1H

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #19 on: June 30, 2021, 08:53:02 AM »

Quote
A highly regarded electronic engineer who used to prowl eHam has stated many times "that the 3-500Z type instant on tubes do not require step-start because the thoriated tungsten filaments self-limits the inrush current."  He further emphasized that step-starting thoriated tungsten filaments is actually detrimental to their longevity.

Who was that?

In any well designed 3-500Z and similar amp Im aware of the filament transformer or winding is its own step start current limiter. Wasting $$ on a commercial unit is just more Hammy Hambone lack of knowledge IMO.

Carl
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K8AXW

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #20 on: June 30, 2021, 10:40:44 AM »

Carl,

"Hammy Hambone...lack of knowledge"....pretty harsh, don't you think?  This is why it's called Amateur Radio and why eHam.com and similar websites exist.

Why I remember after so many years the engineer I referred to is beyond me but it was Tom Rauch, W8JI.  Very knowledgeable man.

73 - Al
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W9IQ

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #21 on: June 30, 2021, 10:47:54 AM »

A highly regarded electronic engineer who used to prowl eHam has stated many times "that the 3-500Z type instant on tubes do not require step-start because the thoriated tungsten filaments self-limits the inrush current."  He further emphasized that step-starting thoriated tungsten filaments is actually detrimental to their longevity.

There is no question that a high inrush current into the filaments shortens the life of the tube. The broadcast industry has studied this issue for years as their tubes are comparatively expensive so wringing out every bit of service hours pays huge dividends. The broadcast industry has generally concluded that the reduction in service life to the tube is proportional to the cube of the inrush current. So a slight reduction in inrush current makes a large difference in service life.

There is a downside if inrush current limiting is not properly engineered. If a thoriated tungsten filament is held at 40% to 80% of its rated voltage (a rough proxy of its temperature) then it will act as a getter for gasses in the tube which will reduce its emission of electrons. This is the so called poisoning effect.

The cold resistance of a filament is often 1/10 or less than that of a hot filament - you can confirm this with an ohmmeter on a cold tube. So even a small transient impedance in the filament power supply can help with reducing inrush current. In at least one Ameritron amp design, it is claimed that the gauge of the wire used to connect to the filaments was chosen so as to provide inrush current limiting. Unless the filament supply is significantly over specified, the supply will generally cause some form of inrush limiting to occur. Whether or not it is sufficient will be very specific to the design in question. There is also the question of the total inrush current of the amp and its affect on power switches, etc.

If you are adding some type of soft start circuit for the filament so as to minimize the inrush current, a best practice is to ensure that it has a less than 1/10 second of engagement time (if it is not actively tracking the filament current) and that it is designed to fail in a disengaged mode so as to avoid the poisoning effect.

- Glenn W9IQ

« Last Edit: June 30, 2021, 10:55:48 AM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

K8AXW

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #22 on: June 30, 2021, 11:42:20 AM »

Glenn,

Thank you for the interesting information. 

I think a lesson to be learned here is, "build the damned thing and enjoy it!"  If we had to wallow trough all of the engineering studies on all major components we never would build anything! Ham radio would have never been.

It should also be considered that we operate IAS whereas we don't try to squeeze every minute out of tubes.  Watts, maybe.  But not time.   ;D

Do you think this is where the term, "Ignorance is bliss" comes from?  HI.

(BTW, I'm NOT minimizing the info you provided.)

73 - Al
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KM1H

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #23 on: June 30, 2021, 02:19:36 PM »

Quote
"Hammy Hambone...lack of knowledge"....pretty harsh, don't you think?  This is why it's called Amateur Radio and why eHam.com and similar websites exist.

Harsh? Nope and IMO quite deserved. They could have been called worse.

Quote
Why I remember after so many years the engineer I referred to is beyond me but it was Tom Rauch, W8JI.  Very knowledgeable man.

Thats who I was afraid you meant, he was so good....just ask him....that MFJ fired him.

These days he builds super power amps for CBers and is often discussed on the Amplifiers part of Eham.
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KM1H

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #24 on: June 30, 2021, 02:55:45 PM »

Quote
There is no question that a high inrush current into the filaments shortens the life of the tube. The broadcast industry has studied this issue for years as their tubes are comparatively expensive so wringing out every bit of service hours pays huge dividends. The broadcast industry has generally concluded that the reduction in service life to the tube is proportional to the cube of the inrush current. So a slight reduction in inrush current makes a large difference in service life.

That post has nothing to do with this thread and the broadcast plus short wave industry uses tubes from 50KW to megawatts where cost of ownership is very high. You might want to look up the Eimac 8974 as an example.

Also when we get to BIG tubes many can be rebuilt several times which reduces costs.

While the 3CX1000A7 is the smallest that can be rebuilt the 3CX3000 family is where most of us think is the entry level. Most of the real big tubes up to VHF are tetrodes. My 2M amp uses this tube that I picked up used about 32 years ago when the TV Translator amp it was in was shut down. It ran 1500W for me then and the power today is still the same.  To bad it wanst engineered for lower cost construction and the 8877 sent to the scrap heap.

Carl
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W9IQ

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #25 on: June 30, 2021, 03:46:05 PM »

Glenn,

Thank you for the interesting information. 

I think a lesson to be learned here is, "build the damned thing and enjoy it!"  If we had to wallow trough all of the engineering studies on all major components we never would build anything! Ham radio would have never been.

It should also be considered that we operate IAS whereas we don't try to squeeze every minute out of tubes.  Watts, maybe.  But not time.   ;D

Do you think this is where the term, "Ignorance is bliss" comes from?  HI.

(BTW, I'm NOT minimizing the info you provided.)

73 - Al

I am glad you enjoyed the data, Al. Yes, there is always the danger of analysis paralysis, but an experienced engineer tends to identify and crank through the issues intuitively and quickly. But 'cutting and trying' is a time tested method in many hobbies. It seems in ham radio, most things 'work'.

Incidentally the tungsten filament inrush current issue applies to many classes of tungsten filaments - even those used in (now less common) household lamps.

- Glenn W9IQ
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- Glenn W9IQ

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K8AXW

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #26 on: June 30, 2021, 05:16:35 PM »

My concern with discussing the technically correct but "ho-hum" practicality is that it may scare off homebrewers. 
They are a class of ham that is rapidly becoming extinct.

"Hammy Hambone and lack of knowledge" are somewhat insulting Carl.  That is the point I meant by being
harsh."  Especially to someone feeling their way through uncharted territory. 

I know I felt like an ass after installing the parasitic suppressor kits being sold by the now deceased California guru.  THAT subject really was a wildfire on eHam for a year.  But, what the hell...they were working so why dismantle them!  So I felt like a dumbass for a while.  I got over it. But, I sure felt bad for a long time.

73 - Al
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KM4AH

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #27 on: June 30, 2021, 06:11:46 PM »

I have a schematic for a grounded grid triode amplifier that K4POZ drew on a piece of typing paper and everything you need to build an amplifier is on it.  After a year of running it most every day I have not encountered a problem.
If anybody wants to arm wrestle about how smart they are try membrane theory or something, not 70 year old technology.
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KM1H

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #28 on: June 30, 2021, 07:08:57 PM »

Quote
"Hammy Hambone and lack of knowledge" are somewhat insulting Carl.  That is the point I meant by being
harsh."  Especially to someone feeling their way through uncharted territory.

Thats not an original saying and is primarily aimed at the nonsense I hear on the bands and saw at pre Chinese Flu hamfests. First read by me over on AMFone years ago.
These forums attract only a tiny percentage of the ham population and the majority of noobs come here to learn or are soon gone.

Carl
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K3UIM

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Re: Cooling a pairCooling a pair of 3-500zg tubes
« Reply #29 on: June 30, 2021, 07:57:06 PM »

Carl,
"These forums attract only a tiny percentage of the ham population and the majority of noobs come here to learn or are soon gone."

I was told by the local ham club president about this forum a couple of years ago and after visiting the site, I'm addicted to it.

I played with electronics, (mainly radio), for 15 or 20 years then dropped out for about the same amount of time, I then discovered that radio had become a whole different ballgame. (Scary, very scary!!!)

This forum has put the experimenter spirit back into my life. There's a world of learning to accomplish and at my "advanced years", I realize that it's nigh unto impossible to pick up where I left off. I'm so grateful for the help I've received here that I'm not feeling left out, just enjoying all the "new" changes that have taken place.

I've fallen in love with DCR's, which are probably considered "old hat" to all of you, but I'm having a ball with their performing and experimental possibilities. (I felt this way with my home built TRF back in the 50's.) LOL

Thanks to all of you for your patience and help with what I've managed to "re-learn" so far.

Charlie

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