CCA FM5000G eating PA Tubes?

FM does it with frequency!
FirebirdTN
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CCA FM5000G eating PA Tubes?

Post by FirebirdTN »

Loosing valuable hair. I rarely post, but thought I'd throw this out to the group:

I have an FM5000G that *might* have an issue.

When I started working for this station, they would always purchase rebuilt PA tubes. Always had great luck. Would get about 2 years to the day every time.

One day they switched to a cheaper PA Tube (National brand). Since then, I haven't been able to get more than 6 months out of them.

But I am most concerned about the last three tubes. The last 2 out of 3 have again been National brand, lasting between 4 and 6 months. I told them go back to the rebuilt tubes. So the most recent tube purchased was an Econco rebuild. I installed it Friday evening. I didn't get 36 hours out of it. Went off air Sunday around noon.

In all three cases, the failure mode is that they cause a HV fault. If I remove the anode from the tube, the HV comes up just fine. Put the anode back on the tube and it trips off immediately upon application of high voltage with the HV fault. Filaments all had PLENTY of remaining life.

In every case when I installed the tube and tuned up, ALL METER READINGS ARE NORMAL.

My question: Is there something that could be wrong with the transmitter that is causing this failure, or am I just having REALLY bad luck with PA tubes?

I basically sent a copy of this to VAN at CCAXmitter, but so far he only asked if I am running filament voltage below 7.5v. Said I should run it as low as I can. I thought that was a bid odd (I could be wrong, but don't see how filament could be causing THIS issue, especially since there are no signs of overheating). For the record, I always run @ rated filament voltage for 200 hours and THEN reduce it as low as it will go without affecting output power.

Thanks,

-Alan
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Shane
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Re: CCA FM5000G eating PA Tubes?

Post by Shane »

Others here will have more recent experience to draw from but from my own experience and anecdotal evidence I’ve been privy to, I’d say it’s quite possible this is part luck and part poorer QC from the tube makers/rebuilders and maybe poorer materials. The transmitter might be blameless...or not. It might be worth a try to do what the CCAXmitter guy says. It does go against standard practice but what have you got to lose?

My sources say Econco ain’t what it used to be but surely they would replace a tube with that much infant mortality?
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Re: CCA FM5000G eating PA Tubes?

Post by ChuckG »

I'd send the Econco back for testing, see what they say.
The Nationals are Chinese. I've had good luck with their 4CX3500's but I've heard of others having very short life with other types. Could well be a run of bad tubes...I had three Econco's in a row last year that would not tune. At all.

The filament voltage thing makes no sense to me either. If anything, running it too low will poison the tube. Too high will shorten it's life but not that short unless you were several volts high. Definitely check it at the socket to make sure one of those awful Filament chokes hasn't gone flaky.
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FirebirdTN
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Re: CCA FM5000G eating PA Tubes?

Post by FirebirdTN »

Thank you both for chiming in. I do appreciate it very much.

ChuckG, you mention something interesting...

Same company, different station-I installed a tube (National Brand) in one of their transmitters a while back...I can't remember when, but it was my first time at that particular site. I installed it in a CCA FM12000. I haven't had one bit of trouble out of that tube. Although that station is currently off-air as their line & antenna look like Swiss cheese after a tower climber went up to inspect after I was called in but could find no problems with the transmitter. The head scratcher on this one: Whatever interaction the harmonic filter had with the holy line and antenna, it still presented a perfect load to the transmitter, so it just kept cooking the line and antenna, yet no one could hear the station a half mile away. YES, the vswr detection circuitry was working. Verified with an exciter. Same thing: A near perfect load.

You just ever have one of those days where it seems like everything you touch goes to pot? I had one of those weeks when I posted.

I also had other issues I haven't even bothered to post about, such as relocating a solid state BE that didn't want to come back up after the move (only moved it 20 feet). The problem: The line. Transmitter shutting down due to VSWR, but was only made aware of this fact by probing the controller. The VSWR meter itself showed no VSWR (obviously defective). its a LCD meter, so the meter isn't bad, its either the selector buttons, or something in the controller.

Same site, same move, STL receiver didn't pick up a signal after the move in addition to the transmitter not coming back up. WHAT THE @#$%! Turns out, my fault. The previous engineer had tied a knot in the 3 foot jumper cable between the stl line and receiver (WHY???). I took the knot out LOL. Should have left it alone. Came back and replaced the jumper. STL problem resolved.

Then again, as an older fellow engineer pointed out to me, that is why he no longer does work for clients that neglect their sites. Disturbing equipment that has been neglected often results in frustrations like this.

Anyway, thanks to you both for chiming in.

-Alan
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Re: CCA FM5000G eating PA Tubes?

Post by grich »

FirebirdTN wrote: Wed May 05, 2021 8:56 am ...I also had other issues I haven't even bothered to post about, such as relocating a solid state BE that didn't want to come back up after the move (only moved it 20 feet). The problem: The line. Transmitter shutting down due to VSWR, but was only made aware of this fact by probing the controller. The VSWR meter itself showed no VSWR (obviously defective). its a LCD meter, so the meter isn't bad, its either the selector buttons, or something in the controller...
I have a BE FM20T doing the exact same thing. Meter is mechanical but exhibiting the same behavior...normal reading in forward power mode, zero indication on VSWR. VSWR OL circuit seems to work. Our consultant verified sample voltage to the controller. Controller was swapped, same behavior.

We have a Wattcher on the output side, so we can still keep tabs on reflected power.
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Re: CCA FM5000G eating PA Tubes?

Post by jthorusen »

For Alan:
One day they switched to a cheaper PA Tube (National brand). Since then, I haven't been able to get more than 6 months out of them.
What is the failure mode for these tubes? Do they all fail with HV faults? I understand that National tubes are not rebuildable. If the above mentioned tubes failed for other reasons (like lack of power out) and you still have them lying around, can you swap one in and see what happens?

Have you tried to remove drive to see if the PA will come up in that condition? If all the replacement reasons are HV faults, is there a calibration or check procedure for the sensitivity of the circuit? Back in my day, a HV fault was a tripped circuit breaker. :D

Regards,
James K. (Jim) Thorusen
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Re: FM-20T, RETC

Post by RGORJANCE »

For what it's worth, a few war stories:

1) FM-25K VSWR problem not showing up on meter. Swept antenna, coax, and finally went to check/ sweep harmonic filter. Dismantled connections and saw 3/1/8" bullet had very faint dark line on insulator. Carbon track across insulator to inner conductor. Replaced bullet...all OK.

2) Client had removed harmonic filter from position on xmttr. Mounted it on wall. Xmttr would not turn on. They had placed it at a hot node for their frequency. This is a "don't never" do it. Put it back at the xmttr. Now working.

3) Look for a "split bullet" connection on inner conductor.

Good luck. Hope the solution is easy and inexpensive.

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Re: FM-20T, RETC

Post by RGORJANCE »

Oooops! Fat finger problem, delete R! :oops:
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Re: CCA FM5000G eating PA Tubes?

Post by FirebirdTN »

jthorusen-

Yes, failure mode is always HV fault. Even trying low power results in HV fault once one of these tubes fail.

I received another rebuilt tube, and put it in yesterday. Tuned up no problem. Transmitter easily achieves 100% power. All readings normal.

But as a precautionary measure, I'm currently running it low power just to see what happens over the next couple weeks. They are supposed to get an exchange on the previous tube that failed after 3 days, so when that comes in and I have a spare, THEN I will kick it back up to high power.

My current line of thinking on running it at lower power, according to the data sheet, max plate voltage is 5KV. This transmitter runs anywhere between 5-5.2KV of plate voltage. Although it has ALWAYS ran at this voltage. Low power cuts the plate E down to about 4.3KV or thereabouts IIRC.

Time will tell...

-Alan
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Re: CCA FM5000G eating PA Tubes?

Post by Dale H. Cook »

FirebirdTN wrote: Mon May 17, 2021 7:29 amMy current line of thinking on running it at lower power, according to the data sheet, max plate voltage is 5KV. This transmitter runs anywhere between 5-5.2KV of plate voltage. Although it has ALWAYS ran at this voltage. Low power cuts the plate E down to about 4.3KV or thereabouts IIRC.
For many years I handled an FM5000G. Although I no longer have access to that transmitter I still have scans of the test data sheets. The plate voltage was 5400 volts on high power (producing 4.4 kW) and 3200 volts on low power.
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Re: CCA FM5000G eating PA Tubes?

Post by jthorusen »

It sounds as though it is a tube quality control problem. According to my Eimac catalog, back when they were making the 4CX5000A, its maximum plate voltage for 100% plate modulation was 5500 volts; 7500 volts in linear or FM service.

To have the tube show a consistent short even at reduced plate voltage sounds as though there is a serious structural problem inside the tube; perhaps it has even gone to air (lost its vacuum)?

You may be stuck if there is only a single-source supplier.

One thought occurs... if this is a seal failure and the tube is going to air, this can be due to overheating. You might wish to check the entire air path for the tube's cooling air... all the way from any filters or screens where air enters the building to where it exits the building. Pay particular attention to any screen wire covers over the entrance to or exit from the PA cavity. These can accumulate dust and become quite restricted. Of course, make sure the blower motor is running at normal speed and that any flexible ducting (if present) is intact and properly connected.

One other thing I might mention: In your original post you stated:
If I remove the anode from the tube, the HV comes up just fine. Put the anode back on the tube and it trips off immediately upon application of high voltage with the HV fault.
I hope you disabled the screen supply when you made this test. If not, the tube would certainly be bad after the test, whether it was good before or not. Operating a tetrode with screen voltage present and plate voltage absent will almost instantly destroy the tube.

Regards,
James K. (Jim) Thorusen
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NW Oregon Consulting Bdcst Eng.
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Re: CCA FM5000G eating PA Tubes?

Post by level42 »

CCA 5000G is a grounded grid triode 3CX3000A7, no screen supply.
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Deep Thought (Tue May 18, 2021 12:50 pm)
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Re: CCA FM5000G eating PA Tubes?

Post by jthorusen »

Thanks for the correction. I couldn't find any data on the 5000G on-line, so I used a brochure for the 5000F which mentioned the tetrode. I apologize if I muddied the waters.

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James K. (Jim) Thorusen
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Re: CCA FM5000G eating PA Tubes?

Post by FirebirdTN »

level42 wrote: Tue May 18, 2021 8:03 am CCA 5000G is a grounded grid triode 3CX3000A7, no screen supply.
Yup! :)

Thanks everyone for chiming in. Just for some clarification, no signs of overheating and both the blower motor and screen have been cleaned off.

Although its only been a few days since the most recent tube change (and running at low power), knock on wood all is well so far.

-Alan
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Re: CCA FM5000G eating PA Tubes?

Post by bradroybal »

FirebirdTN wrote: Wed May 19, 2021 6:55 am
Yup! :)

Thanks everyone for chiming in. Just for some clarification, no signs of overheating and both the blower motor and screen have been cleaned off.

Although its only been a few days since the most recent tube change (and running at low power), knock on wood all is well so far.

-Alan
Another thing to check is if that particular blower or blowers has a bad start capacitor. I had one in an old CCA start leaking oil and cause the RPMs to drop. So even though it looked like it was working, it wasn't moving as much air as it should have. Ended up with the same problem where tubes life was cut in half or shorter due to temps.

You can also drop an external temperature probe in the PA cabinet to measure temps to identify an exhaust issue. If you have an old probe that can plug into a remote interface for metering, great. But if not, you can get one of those BBQ thermometers off of Amazon for $20-30 for a pretty reliable on-site read.
The simplest explanation is usually the best one. Unless you're dealing with RF. Then good luck.
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