Are 3CX800A7 a good tube for longevity/SSB?

Discussion in 'Amateur Radio Amplifiers' started by KK4YDR, Feb 1, 2016.

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  1. W8NF

    W8NF Ham Member QRZ Page

    Highest volume I ever worked with were with 3CPX800A7s and 3CPX5000A7s. The 3CPX5000A7, when first developed for my employer, was called a YC-156. These were all used in pulse-rated MRI amps, very thoroughly instrumented, with auto-tune algorithms. We could tell if a tube's inter-electrode capacitance was off by 10%, and we caught an air variable capacitor manufacturer once who altered plate designs, thus changing the C versus angle curve. Just by reading stepper motor positions.

    In prior work, where my employer was an Eimac OEM customer, I worked with the 3CX5000U7, used at 50kW 405MHz pulse for weather radar...we made a few protos and then someone outbid the startup for the NOAA contract we were trying to get. Couple of years before that I was process/manufacturing engineer for a broadcast equipment supplier...TV, radio, translators, solid state and tube technology. We had UHF TV to a kilowatt using Thompson tetrodes. 100W UHF translators using Eimac Y-667A planar triodes (look like a 2C39 but much huskier), went through hundreds of those a year. On the radio side, our powerhouse of choice was the 4CX10,000D Tetrode, aka 8171. Even by the early 1980s, this was such a heavily-used tube that there were Russian and Chinese variants available at considerable cost savings. A station owner would often choose to have a main tube and a spare of non-Eimac origin, as this cost less than a single Eimac tube. Of course, domestic broadcasters wouldn't do this, because the cost per minute of downtime was too high. But in the third-world geographies that tended to buy our products, downtime was not so expensive, but tubes were! We also used 2 X 4CX15,000J for our 50kW FM rigs...but those tubes were more than double the '10,000 cost, so the only reason to do so was purely size. A trio of 10,000s would cost less, but the cabinet would have to grow!

    During one of my visits to Eimac in San Carlos, I got to see them un-mount an 8973 from its test fixture, which was simply a big oscillator. It had just finished a several-hour cool-down cycle. This 1 Megawatt tube has a separate water cooling system just for the filament.

    G8WRB has a data sheet on his website:

    Dave W8NF
    KM1H likes this.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Eimac was very innovative.

    The founders are likely rolling over in their graves about what happened to it.
  3. KK4YDR

    KK4YDR Ham Member QRZ Page

    Screw it man you have thread crapped enough. I filed a report with the admins here. They may not do anything but you have a record. I hope it leads to permanent banning one day.
  4. KK4YDR

    KK4YDR Ham Member QRZ Page

    Oh I will never buy an amp used if I can't see it in person, or test it, or validate several criteria. And brand new I will only buy one from HRO, GigaP, AES, DXEng, etc.... so it is a reputable sale with full vendor support and warranty.
  5. KM1H

    KM1H Ham Member QRZ Page

    Thanks for all that info Dave, WOW I am impressed and I believe this is the first time anyone with that level of first hand experience has been on this forum.

    Ive heard of the rapid emission decline of the YC-156 once it reached a certain point but had attributed it to hams that that were either pushing it too hard or had one already near the threshold.

    My big tube experience has been the 3CX3000A7 and 4CX5000A in BCB, FM and/or ISM minus the M servicing where gradual loss of emission was the norm.

    Who was that variable cap outfit if I may ask? Cardwell, OEP, or?? Ive noticed OEP plate differences when repairing them from arcing damage which is common in amps .

    Here is that original 3CX800A7 data sheet

  6. KM1H

    KM1H Ham Member QRZ Page

    You know something Eric...for someone who has been a ham for only 2 years and a General 4 months you seem more interested in complaining on QRZ than actually learning things that may help you make various decisions. Thread discussions have always tended to drift around and they dont revolve around you, me, or anyone else.

    I dont know when you were in the USN or what you did but during my 28 years we had a name for chronic whiners that I wont repeat here.

  7. KK4YDR

    KK4YDR Ham Member QRZ Page

    Been gen since day 1. It was nice to have met you. Please enjoy the rest of your weekend.
    Last edited: Feb 7, 2016
  8. AA7BQ

    AA7BQ QRZ Founder Administrator QRZ Page

    Cool it Carl. Name calling and personal attacks will not be tolerated here. If you can't make your argument without calling names then you don't have an argument.
    KN7S likes this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Hello Fred.

    I agree. Thanks for stopping by. :)

    Playing nice is easy when you try. In the day we used rubber and glue for name calling.

  10. W8NVC

    W8NVC Ham Member QRZ Page

    If I may add my two cents. I use a Commander HF-2500, I use the 3cpx800a7 pulse rated tubes. The tubes work great. I just keep my grid level as low as possible. And I believe in these two very important but easy rules. When your done talking for the nite. DO NOT SHUT DOWN THE AMP UNTIL THE TUBES ARE COOLED DOWN. 10 mins or so. If you follow the two simple rules, #1 keep grid current low, and #2 let the tubes cool down for 10 mins or so before shutting down, this is only if you have been talking. If you follow the two main rules, your 3cx800a7 or 3cpx800a7 will last a lifetime. Yes they are more expensive but they produce lots of power out with less drive. I have run both the 3CX and the 3CPX And I like the 3CPX. I drive them with 33 watts and I get 1,500 all day long with voice peaks of 2,000 watts. I have seen 2,900 watts into a dummy load with a 65-70 watt drive using a single tone pecker pulser and the LP-100A Digital vector wattmeter. But the amp don't even break a sweat with 33 watts drive and as I said with that drive it does in excess of legal limit
    However, I never run the amp like that when in QSO. I do that just for testing purposes. I run regular checks, Dummy load checks and On Air checks. I have a log and date my testing and wattage and grid and load also input and output are also logged. I have a checklist and keep track of it.
    My amp is built like a tank and I treat it with much respect. You take care of it and it will take care of you.. So in short, my opinion only is the Eimac 3cx800a7 and 3cpx800a7 are great tubes as long as you follow the simple rules.
    73 from w8nvc Nick

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