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Author Topic: 3CX1500A7/8877 tubes -- where used?  (Read 6469 times)
WX2S
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Posts: 735




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« on: January 23, 2013, 11:09:40 AM »

Hi, all,

Does anyone here know where, besides ham radio, anything is being made that uses 3CX1500A7/8877 tubes? I saw an earlier post saying that the MRI machine manufacturers had switched over to solid state several years ago. Without some sort of non-ham demand driving production, there won't be much chance of getting replacements when the current stock runs out.

73,
- Steve (WX2S.)
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
W1QJ
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« Reply #1 on: January 23, 2013, 01:45:41 PM »

There was a large supply at one time from a fellow who serviced highway patrol radio equipment and apparently they used the 8877 in their equipment throughout that state.  I don't know what state it was but the felloow who serviced the whole state radio system used to change the tubes on a regular basis.  Naturally those old tubes made their way to us hams.  There used to be multi  sales of 8877 tubes on ebay weekly. You still see them often but not as much.  The thing to do is to pick up about a half dozen of them and you'll be set for life.  Rotate them once a year in your amp to keep them tuned up.
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WX2S
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« Reply #2 on: January 23, 2013, 02:45:53 PM »

For the price of a half dozen of them on eBay, I could almost buy a whole THP HL-1.5. Grin

But I still can't find anyone but hams manufacturing new equipment around them.


73,
- WX2S
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
WB2WIK
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« Reply #3 on: January 23, 2013, 03:18:13 PM »

One of my "neighbors" here in the L.A. area still builds amps using 8877s (as well as bigger stuff):

http://www.ampsystems.com/asiprod-old.htm

Most are industrial/medical/scientific applications.

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W1QJ
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« Reply #4 on: January 23, 2013, 06:06:23 PM »

Steve, it seems that this guy might be using 8877 tubes as drivers?
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WB2WIK
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« Reply #5 on: January 24, 2013, 09:31:49 AM »

Steve, it seems that this guy might be using 8877 tubes as drivers?

They use them both as finals and drivers, depending on the amp.  These guys make amps from about 1 kW output to about 100 kW output.  I see they go through quite a lot of 8877s and 3CX3000A7s; they're very close to my office (like 2-3 miles away), so I've visited a few times.

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KC4MOP
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« Reply #6 on: January 24, 2013, 04:25:37 PM »

I wonder if they use Peter Dahl transformers. Nice looking stuff.
They were a fascinating tube when first introduced, but if grid drive was too much the tube was destroyed. The newer amps have ample protection for these tubes now.
We had a Motorola 450 mhz repeater that used one of those tubes and the power out dropped, so we pulled it and installed a new one. A Ham op in the shop took the tube home and plugged it in his HF linear and the tube was happy again making legal limit power.
« Last Edit: January 24, 2013, 04:31:36 PM by KC4MOP » Logged
W6UV
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« Reply #7 on: January 24, 2013, 05:18:39 PM »

One of my "neighbors" here in the L.A. area still builds amps using 8877s (as well as bigger stuff):

I wonder how many of those amps listed as "Private Party in Japan" in 5KW and 10KW are being used by hams?

What is the power limit for JA hams?
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WB2WIK
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« Reply #8 on: January 25, 2013, 09:23:26 AM »

One of my "neighbors" here in the L.A. area still builds amps using 8877s (as well as bigger stuff):

I wonder how many of those amps listed as "Private Party in Japan" in 5KW and 10KW are being used by hams?

What is the power limit for JA hams?

They are permitted 1000W "antenna power" on HF; I presume that means 1000W at the antenna terminals, so if you know line loss you can figure that into the calculation for output power from the transmitter.

At least, that's my interpretation. Wink
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RUSS324
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« Reply #9 on: January 29, 2013, 02:47:51 PM »

Dont quote me but I thought they were used in hospital CT scan units. Again I have not verified this. I think I remember seeing 8877's listed as used medical pulls.
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W8JX
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« Reply #10 on: January 29, 2013, 03:58:43 PM »

Dont quote me but I thought they were used in hospital CT scan units. Again I have not verified this. I think I remember seeing 8877's listed as used medical pulls.

They used to be used in MRI machines. Newer ones are now solid state. Pulls will disappear in next year or two.
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NM3G
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« Reply #11 on: January 30, 2013, 09:54:55 AM »

The latest amplifiers I saw (about 4 years ago now) were for the 3T systems running 35 kW at 128 MHz. These used a 200 watt solid state pre-driver, an 8877 as the IPA and a hardware-modified 3CPX5000A7 as the PA.

There are still quite a lot of 1.0T (42 MHz) and 1.5T (63 MHz) systems using a solid state pre-driver, a 3CPX800A7 as the IPA and either 1 (42 MHz) or 2 (63 MHz) 3CPX1500A7 PA tubes. The 3CPX1500A7 is a pulse-rated version of the 8877, with additional ceramic insulator height to provide higher HV withstand.

With regard to solid state MRI amplifiers, the older generation of 1.0 and 1.5T systems (mostly GE) are replacing their Erbtec amplifiers, with a pair of 9 kW (nominal) solid state amplifier drawers and an integrated power splitter/combiner.

The amplifiers being replaced were available on the surplus market ... in pieces. These used a 3CX800A7 IPA and a 3CPX5000A7 (YC-156) PA. Even with the 15 kW rating (42 MHz) and 25 kW rating (63 MHz), the 5% duty cycle meant a relatively small power transformer coupled with a LARGE capacitor bank for both the IPA supply and the PA supply.

With the rise in PA tube prices (400% in the 2 years I worked on them), the end-users were pushing the tubes for all the operating time they could get. the 3CPX800A7s were good for about 8000 filament hours ... the 3CPX1500A7s were good for 12,000-15,000 filament hours. These tubes were biased very linear, and were only biased when RF was applied to reduce heating and power consumption.

Without accurate filament time the tubes were tested for absolute output and replaced once the emissions were 3 dB down from a reference tube (with the same bias).

Without exception, the tube filaments were run from a current and voltage regulated DC power supply. Current limiting prevented in-rush filament shock and the voltage regulation ensured the filaments were operated according to the manufacturer's data sheet (typically Eimac, but later other China-made tubes were used). NONE of the amplifiers used a filament transformer alone, and none were fed AC ... this contributed greatly to the lifespan of the tubes.

Some tubes had filament current monitoring to shut the amplifier down if the filament current dropped 50% (in the case of the 3CPX5000A7 there are two parallel filaments) or less.

Bias (and keying), interlock status, filament I/V, IPA and PA I/V, plate current, grid current were all microprocessor monitored. Exceeding set limits shut things down ... again, improving tube lifespan.

I no longer work in that industry, so as I stated above, my experience is a few years old. I keep reading about the demise of the tube MRI amplifiers, and I have to chuckle ... because the MRI manufacturers are still using tube amplifiers when they need, and are switching to solid state when they can.

The linearity requirements for the MRI amplifiers are getting tighter, and the solid state amps meet those requirements much better than the tube amplifers (the ONLY exception was a 4PR1000-based amplifier that was exceptionally linear ... and those tubes were getting a tad bit difficult to source).

Given the market value for power tubes now, I'd just purchase new tubes and run them according to the OEM data sheet and not worry about it. To put it simply ... 8000 filament hours is a LOT of ham radio time ... and if you NEED to keep your amplifier on hot standby 24 hours a day, you can afford to replace the tube on a more frequent basis.

73,
Rick
NM3G
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K1ZJH
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« Reply #12 on: January 30, 2013, 10:44:04 AM »

National Weather Service was using them in their 162 MHz band WX transmitters up until a few years ago when those old  exciters and PAs were retired.

Pete
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KK5DR
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« Reply #13 on: September 07, 2014, 10:13:51 AM »

Still a large number of GE built MRI units out there around the world that use the 8877.
To stay certified the tubes must be changed out every 100 hrs of ON time. So there are allot of them available. Buy a few of them, any amp you have will last a lifetime with a few spares to rotate annually.
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NM3G
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« Reply #14 on: September 07, 2014, 03:34:19 PM »

Um ... 100 hours was the typical burn-in time on an amplifier after I repaired it before shipping. Since the RF output is constantly monitored and the exciter level varied to meet the various scanning requirements (based on body mass, scan location, coil design, etc.), it would make sense to run a verification test with a test phantom every 100 hours (more like once a day) rather than replacing a barely used tube. An ER MRI will typically run 24/7 ... and I don't see replacing an amplifier every 4.16 days. You don't just pop a new tube in and run it back up ... the linearity requirements and the associated necessary RF tuning required pretty much means either a lengthy field engineer's day, or send it to the shop that can handle this kind of work.  As I said in my previous post ... I've been out of the field for a while now and can only speak to what was happening when I was servicing these amplifiers.

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