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  1. #11
    Beach Bum KA2PTE's Avatar
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    Once again Warren thanks for the very useful info.

    Turns out a new set of hurdles has arose. So plans for the 160 mod are postponed.

    Apparently one of the set screws in a 4-400A heat sink was loose, so that explains why one tube was more eager to glow red
    under low power levels - starving for plate current most likely. So I resolved that unusual issue I believe. But before I noticed that,
    had attempted a tune on 40m, and encountered some xfmr humm and other not sounding nice noises, so shut down.

    Apparently part of some kind of mod, this NP 600V / 0.5u F cap leaked oil -

    https://www.mediafire.com/view/olwny...l_cap.jpg/file

    It was hooked from ground effectively in parallel with C117 near the bias plug jack. Measures a bit high ESR, so tossed it.
    Someone speculated its an anti-humm mod on the screen supply but it is connected on the output of the VR75 regulator tube which I think delivers -75V to the system in certain modes.


    As this Amps been modified, I set out to update the drawing and have an updated version in this schematic:

    https://www.mediafire.com/file/c59nb...2PTE_.pdf/file

    As it stands now in CW or LINEAR mode the voltage regulator tubes go off, which means they are
    encountering an overload. I get just about the entire supply volts as a drop
    reading on R115 (4k) which concurs the tube string is seeing too much load
    or perhaps one or more of the VR tubes is weak.

    Default voltage readings as per the manual are in spec, except when you
    place tubes V1 and or V2 (4-400A's) in place.
    Somehow with them in the sockets, standby current is drawn from the VR tubes, but
    cant trace back to see which way. With this modded chassis, they are actually measuring
    cathode current on the meter now, not plate. At 75mA or so this seems awfully odd.


    Trying to figure out the mode switch is sort of painful - its got (3)
    commons - 1, 5 and 9.

    All tests by default are with only the filamant xfmrs powered, no HV 3400V
    applied.

    Even if the screen volts is making it to the tube pins 4 or 2, cant see how
    the plate mA meter is reading like 60mA.
    They modified the plate meter to be reading cathode current by lifting the 2
    common heater connections from ground.

    From what I have read, with no HV applied the screen is more or less a sub for the plate
    and some current flow through is expected. I'd think perhaps not as much as I am seeing now
    as there has never been this kind of current reading at idle ever. I have (4) tubes, and they
    all read the same thing so I dont think there is tube failure.

    The original owner of the amp said this of the mod:

    "Please note that the 4-400A parameters have been rejiggered for class
    AB1 operation. The original design operated class AB2, grid current on
    voice peaks approximately 2 ma. Absent an input signal the class AB2
    mode resulted in substantial resting plate current. After the internal
    power supply zorched itself, I was gifted a KS-1 power supply which
    puts out 3000V. The original PS ran around 2400V. This was an
    opportunity to switch from AB2 (grid current, distortion, etc) to AB1
    which for all practical purposes runs at zero grid current and requires
    less grid drive power while reducing distortion. The input to the
    T-Bolt gives a choice. Either use the tuned circuit or a 300 ohm
    termination resistor as seen on the diagram. In AB1 the driver doesn't
    suffer from the grid current induced non-linearity of AB2.

    Results: Grid bias increased and screen grid voltage increased."

    I seem to concur seeing this when I had ok results testing the amp before
    something went wrong. Though for some reason, the screen current never ever
    registered anything at all, till this odd situation.

    Now its got that "substantial" resting plate , er cathode, resting current
    he described for AB2 possibly. Could something have changed the config back toward AB2 ?
    That oil cap perhaps?

    Also another question is, the new resistors I found (R116A-B) in the updated schematic,
    do they look like they are modding to AB1 operation?

    Apparently you can swap out the VR75 tube with a VR105 and even a VR150 and that lowers idle current
    in the factory AB2 config.

    UPDATE:

    I concluded the NOS VR tubes must have just went over the cliff on me and cant supply much current under any load anymore. Supplier is taking them back for an exchange, but I have more on the way from another source just in case.

    If the tubes do not work, the raw filtered DC supply volts is applied directly tot he screen. The tubes only rated at 600V for the screen max,
    and I imagine theres likely 700-800V once the filter caps do their thing from the tube full wave diode. Its now acting like the plate and makes
    current draw on the meter of about 70mA or so. The meter is actually the cathode current meter , as per part of the mod, in the updated dwaring...so makes sense. Also tells me the transformer is able to put out ample current into the tube diode, up to that amount of current at least.
    Last edited by KA2PTE; 03-12-2021 at 03:58 PM.
    --
    "Get the US out of the middle east and get the middle east out of the US."
    (Vic Misek W1WCR, co founder of the Liberty net , SK)

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