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  1. #21
    Beach Bum KA2PTE's Avatar
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    Aug 2020
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    New choke in and Looking good

    Hi Warren

    Yes, good idea on the new name.

    I finished up the new coupling cap assy bracket and mounted the doorknob caps to it, wired it all up - looking pretty good:


    C10 is in the lower right part of the pic with the large copper braid putting 2 door knobs in parallel to give is about 800pF. Not exactly the 1000pF per stock design, but we arent exactly factory anymore as you indicated. (2) tan colored doorknobs are in parallel on the bracket near the left side to form about 840pF, and its tied to the last position of the coupling switch so in max level thats whats beig added in. Each other red door knob is about 600-650 pF or so which is close to the 620pF in the original spec.

    Fired up last night and today for some tests and things are pretty stable! Aside from the tubes being that active red color all the time, which seems strange but I am told is normal by you and others - as long as nothing is going nuclear I guess we are golden.

    Into a dummy load, the most I can read on the PEP vector meter was about 800W with a constant CW carier and the linear in LINEAR mode. I am doing some tests now on the air with about 500W or so and we are drawing 380mA on the "hacked"
    plate current meter.

    Another who had restored a factory Thudbolt, said you can up the 105V regulator tubes to 150 and you wind up with
    600V for the screen volts. 600V is the max rating for the screen on these tubes as per the spec sheet. I have (1) 105V
    tube with (3) 150's for about 550V on the screen. He said the more screen volts the better and I guess under a load you would have less than 600V on the screen when under load if you went with all 150V tubes.

    After some more emails , he said the -75V tube is not enough with the 550V screen setting I have, so I will have to wait till I get another 105V tube to try to bring the bias up.

    But clearly some very stable progress here since the new choke and caps are in. At some point wanna get that T103 connection in order so its on the 120V taps.


    UPDATE:

    So I learned the goal with a 3KV supply for AB1 is 750 screen voltage with a grid bias of -137v. Thanks to KB1VWC (Steve) who runs the same PS but on a Chippeawa that puts out nearly legal limit with the same tube setup.

    The previous test used -75 as the grid bias and 550v for the screen. KE7TRP who had restored a Thudbolt to original said that's not enough tube bias at -75V. Which probably explains why the tubes were glowing red, even with only 50w of amplification going on.

    After some doing, I wound up putting in all 150V type tubes as the regulators, and have I think made a step in the right direction. While I can only get about 550 or so watts into the dummy load now, not 800 , the tubes are clearly barely struggling with only a mild plate glow at 500w.

    Idle current is way down as well, as both of them indicated it would be. The -137V spec on the datasheet says to adjust that level for desired idle current. So looks like I have more figuring to do on the spec sheet to figure out the right spot.

    The Chippeawa has a bias knob on the front to set the idle current, makes this part alot easier.
    Last edited by KA2PTE; 03-24-2021 at 02:54 PM.
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