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Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Jan 11, 2019.

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  1. SM0AOM

    SM0AOM Ham Member QRZ Page

    Well, I have academic credits in Applied Electron Physics from a renowned university,
    and was able to derive the relationship between heating increase of the cathode by diversion of electrons
    due to out of phase grid voltage and drive power from "first principles".

    There is also reason to believe that the Philips/Valvo tube design engineers that wrote the 1962 datasheet for
    the QEL 2/275 also understood these relations, and for this reason excluded linear operation from
    the requirement for reducing the filament voltage at frequencies above 300 MHz.

    Were those engineers maybe wrong just because they are not American?

    I have also supervised long-term operations of high-power fixed and shipboard HF and VHF transmitters as
    part of my 37 year professional career in radio engineering and I am also perfectly aware that filament control is the norm
    for thoriated tungsten cathodes
    , and that keeping the filament voltage within limits on oxide cathodes
    greatly increases tube life.

    This was the major subject of extensive correspondence in these matters between the engineering department of
    my then employer in 1985 and English Electric/Eimac, due to the fact that disappointingly short life-times often were encountered in the
    some 120 1600 W shipboard HF transmitters that we rented out.

    If their lifetime could be extended just by 20%, it would mean a saving over a year of some 200 4CX250B:s, not to mention
    the savings in general maintenance costs.

    It turned out that using ferro-resonant voltage stabilisers and setting the operating filament voltage at 5.8 V
    cured most of the tube life problems, and operating hours in line of what we got from shore-based equipment became the norm.

    Finally, I suggest that the clipping from HR 1971 above is read again.
    This text was obviously written by someone that understood
    electron physics, and who also have read the relevant application notes from
    the tube manufacturers where the reasons for back-heating were spelled out.

    N2EY likes this.
  2. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    I was testiing the meter voltage cal circuit and voltage went where it shouldnt!!!
    Zeners smoked!
    new ones orderd from MOUSEYWOUSER, only 48 cents each, but $8 shipping!!! So I bought 3 spares of each so I will have 4 chances to make it work
    As I always saidL " LOTS OF THINGS USED TO WORK!!!"
  3. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    OH I also smoked my Decade resistor box blowing away the 100 OHm resistor in that!
  4. KN4RME

    KN4RME Ham Member QRZ Page

    Wow! Is that veroboard?
  5. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Just ordinary perf board.
  6. KM1H

    KM1H Ham Member QRZ Page

    Reading and actually doing are quite different. Eimac engineers didnt jump to conclusions without some serious evaluations.

    The QEL2/250 had those filament specs in 1960-61 but not the 275 a year later. What was different between the two tubes and why a 275 designation if it was a 4CX250B clone?

    Did Philips license the 4CX250B from Eimac, or make their own version with a cathode change? Svetlana designed their own version from what Ive been told that mitigates the secondary emission issue. Did Philips copy that?

    BTW, It seems you had different things to say in 2003 about the subject in general and the 275 in particular. Did you learn more by actually doing? Have you ever dissected a 250 or 275?

    Maybe, and unless you can show design notes explaining Philips thinking back then Id say it was either an oversight, a consensus which seems to be the way things are done in parts of Europe, cathode changes ala Svetlana, or just plain not caring.
    Eimac, an American company, were the ones that designed the 4X150A in 1946 back when UHF SSB was still far off. It wasnt until the 70's that they included other modes in a "dynamic" catch all statement about uhf filament voltages which you continue to ignore since it does not fit your concept of reality. And as I said earlier the 4CX250 is still in production .....I guess those old crafty Americans understood the details which seem to escape many these days.

    Considering the actual uses of the 4CX 250 family in various critical mobile/aeronautical applications where the filament voltage was not steady dismisses your statement as well as RCA's +/- 10% spec. What did RCA do different?

    Nothing in that article indicates anything of the sort, nor does his backround. Grasping at straws again?

    If you want to continue with this discussion then try answering my questions with actual proveable facts.
  7. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    ZENERS due in this afternoon. Then back to testing. I got 3 spares each in case these get cooked
  8. VK4TI

    VK4TI Ham Member QRZ Page

    Jims gear is great , I buit one of the 2m 1kw kits and it proved to be reliable powerful and doesn't get driven over 400 watts so will last my life time i guess , it cost a little more than a junk box build but colour me happy
    and blade server supply used cost $50.00 for 2950 watts @51vdc.

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