Is there possibly anything that could happen to a amp

Discussion in 'Amateur Radio Amplifiers' started by N3JBH, Apr 20, 2012.

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  1. AG6K

    AG6K Guest

     To protect against things that can go badly in an amplifier, it needs:
    1. A sturdy glitch-R in the HV positive lead to the HV-RFC to limit peak-I during a glitch.
    2. 2 back to front 3A - 6A diodes across each meter movement and across the grid-I meter shunt-R.
    3. A step-start (soft-start) circuit to limit mains inrush-I to the ON-OFF switch.
    4. Parasitic oscillation suppressors with a Q of under 3 at 100MHz. ((813 type tube excepted)
    5. Heater-type tubes such as the 8877** should have a HV-rated fuse to protect against cathode flaking.
    6. A reverse-polarity protect Si diode across each electrolytic filter-C.
    7. A fusing device to protect against high grid-I.
    8. Ideally: adjustable, metered filament/heater V - otherwise a filament-V that is at the low-end of the mfg's rating- or, for heater-type tubes, a heater V that is near the middle of the mfg's rating.

    ** Additional precautions for heater-type tube amplifiers such as the 8877:
    --- #The 8877 is an unusual tube in that it is capable of oscillation well into the UHF range. #For an 8877 HF/MF amplifier I would:
    1. substantially reduce the VHF-Q of the VHF suppressor to reduce VHF/UHF amplification.#
    2. Connect a wire between the cathode and one side of the heater to prevent heater-cathode arcs during a glitch. Doing this necessitates using a 10A bifilar heater choke of c. 20uH. Also, neither side of the heater should be grounded.#
    3. #install a glitch R of at least 30Ωs in the HV+ lead that will safely withstand the Joules stored in the filter-C.#
    4. #test the 8877 for gold sputtering with the Funderberg loose gold test and take corrective action if needed.#
    5. to keep the HV neg. from spiking to several kV neg. during a +HV-to-gnd arc, connect two seriesed 3A diodes from the HV neg. to ground with the anode end grounded.#

    Editorial: IME, high power RF amplifiers are more complex than they first look. In my opinion there are no infallible amplifier design experts -- just some people who semi-understand them a bit and know to keep their mouths shut when there's something that they don't quite understand being discussed. As a result, there is no such thing as a perfectly designed high power amplifier ------ so warm up your soldering iron if you want to make yours work better and last longer.

    If you would like to discuss any of the above items, phone me.. If I'm out and about my cell # is 805.509.8314
    Last edited by a moderator: May 5, 2012
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