Avarage life of a 3cx15000a7

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GlowingTube
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Avarage life of a 3cx15000a7

Post by GlowingTube » Mon Oct 01, 2018 10:38 am

Since I while I am servicing tube transmitters, which is new (and pretty interesting technology) to me and I was wondering, how much hours can you get (avarage) out of a 3x15000a7 tube used in a FM broadcast transmitter? Is there a lot of difference between new (Eimac) or refurbished (Econco)?

Kelly
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Re: Avarage life of a 3cx15000a7

Post by Kelly » Mon Oct 01, 2018 11:29 am

Pretty much without fail, the lifespan of a tube depends on three main factors:

1. Installation and tuning of the tube.
2. Proper filament management beginning with installation.
3. Consistent voltage and proper operating temperature range.

New tubes operating with tuning maintained, utilization of filament management, and kept at a consistent temperature within designed parameters can last 6-8 years of continuous operation. I've found that rebuilt tubes, even if the aforementioned is done, only last about half the life of a new tube.
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Re: Avarage life of a 3cx15000a7

Post by GlowingTube » Mon Oct 01, 2018 1:06 pm

Thank you for you answer

1. What points are important at the installation? Last time when I replaced a tube, I cleaned the whole cavity room (plan to do this every year) and let the filament run for a hour before switching on the HV. Then slowly increased the power (tuning when necessary of course) until TPO and tuned it to the lowest plate current possible. After that I increase the output load slighlty to reduce grid current until the output power drops more than a few hundred watts. Idea about this is if the mains voltage drop slightly it won't become in under load situation.

2. On our 3CX transmitters there isn't a way to adjust the filament voltage. The 4CX transmitter(s) do have it, but never worked on those machines but definitely something to keep in mind when they need service.

3. The operating temperature is around 125-135F and our mains normally doesn't varies more than 4% (so do the HV/Filament on the readings). I think that this is pretty acceptable?

We had bad luck with a refurbished tube that knocked a transmitter off the air and blow out some parts. I ordered since then new tubes . .

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Re: Avarage life of a 3cx15000a7

Post by TPT » Mon Oct 01, 2018 1:38 pm

Econoco is now owned by CPI (e.g.=Eimac), although the refurbs go through a different plant. I'm getting about 18 months on the 3CX3000a7's in an ECO-4 and ECO-6's I take care of.

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Shane
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Re: Avarage life of a 3cx15000a7

Post by Shane » Mon Oct 01, 2018 1:47 pm

Don’t know if they’re available for the 3CX and the rig in question but a constant voltage transformer on the filament supply would take care of part of the filament voltage management. Drawback is you can’t adjust it either but it would get rid of the +- 4% variation.

At a much lower power level I had a Bauer 707 using 4-400As at 7v IIRC. The rig has a Sola constant voltage transformer in the filament supply which a Fluke meter always read at 7.07 v (how appropriate) at the tube socket. But it always bugged me that I couldn’t start off a new set at a lower voltage (after break-in at nominal voltage).

(For most AM ops at the 707’s power level, additional filament voltage mgmt could be more harmful than helpful anyway unless distortion is closely monitored; that’s not common in today’s minimal human intervention operating environment.)

Looking at specs for the tube in question, it appears a 4% variance is acceptable. Specs say 4.7% is ok. So you are probably good. You could closely observe line voltage variations compared to concurrent filament voltage variations to establish a nominal point where typically observed variations min and max out to be sure levels remain within that tolerance.

(This post last edited at 1535 MDT on 10/1/18)
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ChuckG
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Re: Avarage life of a 3cx15000a7

Post by ChuckG » Mon Oct 01, 2018 7:09 pm

What transmitter is it in and what power level? Armstrong used that tube in some of their 10kW's and it would last forever. It barely loafed.
In the older CCA's running nameplate power, maybe 2 years.
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GlowingTube
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Re: Avarage life of a 3cx15000a7

Post by GlowingTube » Mon Oct 08, 2018 10:31 am

ChuckG wrote:
Mon Oct 01, 2018 7:09 pm
What transmitter is it in and what power level? Armstrong used that tube in some of their 10kW's and it would last forever. It barely loafed.
In the older CCA's running nameplate power, maybe 2 years.
It are indeed Armstrong transmitters, between 10 to 20KW output power at around 7000V DC. We use mainly the modern types with LCD display.

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