Are 3CX800A7 a good tube for longevity/SSB?

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

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  1. KA9JLM

    KA9JLM Ham Member QRZ Page

    They are good tubes. You just need to know what you are doing.

    If a person does not know what they are doing, They should not have a amplifier.
     
  2. AI3V

    AI3V Ham Member QRZ Page

    Why stop at 2500 watts?

    Rege
     
  3. W8JX

    W8JX Ham Member QRZ Page

    Problem is even some knowledgeable OP's have toasted 3cx800's. There are good tubes when used properly but they have very poor tolerance for tuning errors. Accidentally transmitting before tube properly warms up, if warmup timer fails, can permanently damage cathode in tubes (8877's too). Extreme care must be used. BTW ceramic power tubes have a finite life, how finite is debatable, and they age the same whether on standby or making rated power so being on standby a lot can subtract from usful service life. With instant on tubes like 3cx1200, 572 or 3-500z there is no need for extended standby times.
     
  4. W1VT

    W1VT Ham Member QRZ Page

    GM3SEK offers protection circuitry boards for both triodes and tetrodes. Given the cost of the tubes, I wouldn't build an amp using 3cx800s without the proper protection circuitry.

    http://www.ifwtech.co.uk/g3sek/boards/triode/triode-1.htm

    There was a time in which tubes were so cheap and readily available that you could argue that you didn't need protection circuitry--those days are gone.

    Zack W1VT
     
    NQ1B likes this.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Not at all. The reason modern (new on the market today) amps aren't using them is only due to their amazingly high cost up until very recently. Back in the 70s-80s when Eimac was making a lot of them, they weren't very expensive and lots of amplifiers used them. Manufacturers stopped using them when they became almost $1K each!
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    That can be a plus for some users and can save a lot of power. I can wait for tube warm up.

    I do not even bother with my 8877 amp unless I want to get serious. No need for that power normally.

    My 8877 amp auto tunes on the the first letter of my call. But I have to wait for tube warmup.

    My 572B amp comes on quick but takes more time to tune. It makes plenty of power for most band conditions.

    I would say a 3-500z would be a good choice, Now a days.
     
    Last edited: Feb 2, 2016
  7. W8JX

    W8JX Ham Member QRZ Page

    While not the biggest fan of some Amertron amps i think the AL 82 is a solid legal limit amp and a lot of bang for buck.
     
  8. AI3V

    AI3V Ham Member QRZ Page

    Can you provide a citation for this statement?

    I bet the real reason spare tubes don't work is cause the station engineer (read ham) swapped out his dud for the bosses working bottle.

    Opening up the factory sealed shipping box to "exercise" the spare is the cover story for why the box was opened in the first place.


    Rege
     
  9. W8JX

    W8JX Ham Member QRZ Page

    Simply physics. A tubes service life is based on the cathode's thermionic emissions which do degrade over time with use as such emissions are not constant. The rate of degradation, small it may be, is consistant as long as cathode is hot whether tube is idle or at full output. It could take 8000 hrs (there is 8760 hrs in a year) or more for it to degrade a noticeable amount but it will happen when enough run-time is accumulated. (even makers of tubes suggest a service life in hours and why they swapped MRI tubes frequently in past to maintain peak performance) A old trick with CRT's and some tubes in past was to raise the the filament voltage to increase emissions of a cathode that is loosing emissitivity but gain is short lived. (this was kinda common with early color TV CRT's). There is another factor called backward bombardment of cathode that can shorten life too. This seem to be the biggest issue with directly heated cathodes (572, 3-500z and so on) and some have suggested slightly reducing voltage on such tubes in some applications to increase service life.

    If you have trouble wrapping your head around these simple facts you should refrain from commenting on this subject in future.
     
  10. KK4YDR

    KK4YDR Ham Member QRZ Page

    So how is a ceramic tube ageing diffierent than a glass tube? Plus there are hams making full power on 30 year old tubes regardless of make and model. They report using them normally too.
     

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