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Which new or used amplifier design is representative of these design recommendations

Discussion in 'Amateur Radio Amplifiers' started by J85K, Apr 12, 2011.

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  1. W8JI

    W8JI Ham Member QRZ Page

    How it really works is like this......

    You won't see any statistical difference in life of tubes in amateur service by getting all worried about filament voltage. None. Zip. Zero.

    If you run the 8877 or 3CX800A7 somewhere between Eimac's minimum up to an extra 1/2 volt, you will never see a life history change.

    The only design problem in amateur systems is lack of fast electronic protection of grids in amps like the 8877 or 3CX800A7, lack of arc fault limiting with big filter capacitor banks, and as unbelievable as it sounds some amps do not even have filament warm up timers with indirectly heated tubes!!! Not having a fast grid trip circuit is a sure way to shorten tube life.

    It would be penny wise and dollar idiotic to include a variac while not having arc limiting and grid current overload, and a warm-up delay timer with a tube like the 8877 or 3CX800A7 or any heater-cathode power grid tube.

    Nearly ALL failures in 8877 or 3CX800A7 or other low-intercept tubes, that are not normal long hours end-of-life failures, are caused by not having grid protection or running grossly excessive or even slightly low filament voltages. Tubes like the 8877 or 3CX800A7 are ruined in seconds by excessive grid current, operation before full warm-up, or operation with low filament voltage. The rest of the failures, other than inadequate cooling, are almost entirely from tube manufacturing defects.

    Nearly all failures in thoriated tungsten tubes are caused by thermal cycling of the tube in well-manufactured tubes, although that now has been greatly eclipsed by manufacturing defects in tubes. Operating errors in thoriated tungsten tubes almost exclusively relate to over-dissipation of anodes. Some amps have inadequate cooling, which leads to seal failures, pin unsoldering, or glass failures. Other than amps with inadequate airflow causing glass or pin seal failures, the majority of failures now are manufacturing quality issues. Second to that is thermal cycling. Filament voltage as a general rule is not even on the statistical RADAR.

    We kept statistics on this stuff for years. The only 8877 ever returned for low emission had a filament mod to reduce voltage to 4.5 volts. The low voltage ate the tube up in about 3 months.

    For Ham use, anything right around the specified range is just perfect. You will never see a life change by getting all anal about filament voltage, but you sure will if you do things like not have a grid trip circuit in a metal oxide cathode tube or run the filament voltage low, or transmit before the cathode is at full temperature.

    Filament voltage is important in commercial BC transmitters. This is because the tubes are not cycled off and on, and they are not heated and cooled during operation while on. They are also operated at a fraction of ratings. This means most BC tube failures are emission life failures. This has never been the case in amateur service, except a few Dentron designs that ran grossly excessive voltage on filaments or amps that people have modified.

    73 Tom
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