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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

    "The worrry I have is that the special Screen bypass caps in a socket are bad. An ohms check showed no problem at them, but it is unknown if they breakdown at high voltage (350V Screen)"

    My experience is that the screen bypass capacitors in the SK-600 series
    are quite robust for DC stresses, I recall that their static test voltage should be about 1000 V,
    but they may become damaged by transients, especially during a plate to screen flash-over.

    The classic remedy is to use two protective devices at each bypass capacitor;
    one quite hefty VDR with a knee voltage in the region of 500 V, and one gas-discharge tube with about 400 V ignition voltage.

    The VDR will absorb the initial flash-over surge until the gas-discharge tube has ignited.

    When the 4CX250s and 4CX350s in the Telefunken and Granger 30 kW HF and 5 kW
    MF transmitter drive chains became worn-out, it was common for them to blow the sockets,
    especially when there had accumulated dust and dirt in them.

    The addition of the VDR and gas-tube cured this for the remaining life of the equipment.

    Another addition that saves a lot of aggravation is a thyristor or thyratron "crow-bar" that shorts and disconnects the
    plate supply if there is a flash-over. This was standard issue on some of the larger vacuum-tube PA:s.

    A thyristor crow-bar designed for the 4CX250 family was described in
    DUBUS during the 90s (9002-7 Flash-over protection for PA's OE5JFL

    We used it to protect the drive plate and PA screen supplies in the Collins 208U-10s,
    which otherwise literally "ate" power zeners and regulator transistors.

  2. N2EY

    N2EY XML Subscriber QRZ Page

    Imagine the stress if a plate-to-screen flashover occurred with a 350 uF capacitor as the filter.....

    73 de Jim, N2EY
  3. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    FINALLY a Day of SUCCESS!!!!
    IT IS WORKING with the Minimal Switchbox. Of course there is NO WARMUP timer anymore, no LEDs or Whistles and Bells. I did find a wiring error on the Relay board which was why the PS was always in transmit and the logic to the TRANSMIT Light was backwards. Now all that is cured and it works like it should. PHEW!!!

    Too late to get this ready for the June VHF contest Saturday morning. Well, we came close!
    My 6M Killerwatt is DOA now too (the SB-226) It still works fine but th 10 to 100W driver is toast. I bought a 2nd Lunar Amp to drive it, but it cooked too. Not a problem with the SB-226. The driver amps have problems.

    The Little switch box is a different form factor than the big logic box. I can still work on that as the last configuration ALMOST worked ! The bare bones switchbox works fine. I just hafta remember to let it warm up before I transmit!
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    Yes, a "glitch resistor" is necessary even with lower values of filter capacitance. One needs to be careful when dissipating large energies in components that you do not know exactly the properties of.

    The larger transmitters I worked with had choke-input filters and reasonable amounts of capacitance, the 5 kW used 10 µF IIRC at 5.5 kV, so the stored energy was "only" 150 J.

    Compared to 350 µF at 2200 V it is small, five times less than almost 900 J. To avoid instant destruction of fragile components it takes a "glitch resistor" to absorb the fault energy.

    The 2o mm VDR:s used had a clamping voltage of about 700 V at a current 0f 100 A, and working out the glitch resistor value to keep below this value at a plate voltage of 2200 it takes (2200-700)/100 ohms or about 15 ohms.

    In practice the ESR of the capacitor bank and the series inductances and resistances serve to further limit the current. It is still advisable to protect the meter coils.

    Also, when the gas tubes finally ignited the current became shared between the gas tube and the VDR.

    A major requirement was to positively break the primary supply to the transformer, because otherwise the glitch resistor would explode.

    Larger transmitters used thyratron or ignitron crow-bars, which in essence short-circuited the power supply output when an overcurrent was sensed. A very clever form of AC coupling made this sensitive to the time derivative of the supply current, so flash-overs did trip the crow-bar very fast.

    I did never witness a test of the 30 kW supplies, which were specified to break the current before a o.18 mm silver wire melted off, but instead the same test of the supply (14 kV 16 A) in the 100 kW amplifier where a 0.22 mm silver wire should not melt.

    It took a lot of courage to perform the actual short-circuit, and the floor in the transmitter building trembled while the supply operated into a dead short for a moment before the primary power got interrupted...

    The remnants of an ignitron that had exploded during a test due to a manufacturing flaw was prominently displayed in the transmitter control room.

    N2EY likes this.
  5. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    the control panel mounts on the front panel with Velcro and can be remoted to the operation position. I really do need some LEDs though
  6. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    when first turned on, the HV is off until the RUN/STBY switch is thrown to RUN. Then the circuitry energizes the Big Contactor which turns on the HV XFMR. The screens then come up when the HV is running. The HV Lamp on the front panel lights. When we have a transmit situation (ground to the Amp Key jack on back panel) the Bias switches to the correct voltage for TX from RX. And we have the transmit light go on. CW and SSB switchng changes the Bias and screen voltages as required. The bias is precisionly set with the pots on the back panel. The screen and Grid current meters work. The Filament voltage and Relative Power functions still need design. But this is the final breakthru to getting it to work. The Arduino version will fit in a box the same size as the remote switchbox and will be remotable too. Just about time to plug the amp into it and see if it SMOKES!
  7. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    The OLD Logic box will be retired to the MUSEUM of FUTILITY
    K9ASE likes this.
  8. WA4SIX

    WA4SIX Ham Member QRZ Page

    For 6 meters, get a converted Larcan 1.5KW amp. I have abused mine for 5 years & it's still running great.

  9. NM7L

    NM7L Ham Member QRZ Page

    Would that be "Le Cylindrical Museum et Futility" (trash can)?
    N2EY likes this.
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    It's all about energy storage (for improved dynamic voltage stability at peak current). At nominal 3KV, 350uF isn't just a lot more than needed. It's ridiculous. FWIW, using LTSPICE, my FWD 4KV/800mA HV PS (using 47uF total) showed modest overall HV "sag" under load. That includes AC line & xfmr loss. :)

    The "Tim 'The Toolman Taylor" philosophy...

    "If a lot is good, WAY too much is just enough."
    ... is cute but, not the best idea. :p
    N2SR, AI3V and N2EY like this.

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