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Thread: Johnson Viking Thunderbolt

  1. #21
    Beach Bum KA2PTE's Avatar
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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.
    --
    Maybe covid19 is just a terminal case of Trump derangement syndrome

  2. #22
    'Grumpy old bastid' kb2vxa's Avatar
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    Hi Steve,

    The more you change voltages around the more I feel like Oblio and his dog Arrow exiled from the land of Point and wandering in the Pointless Forest. (Harry Nilsson - The Point, the song Me And My Arrow used to advertise the Plymouth Arrow) The name Firedrake may take on another meaning if it goes up in flames, keep the tubes at their proper operating parameters, 50V below max is too close for comfort. I didn't say that red glow all the time is normal, it's only normal all the time in an AM broadcast transmitter or CCS, they may only glow a bit on peaks in CW and linear operation. Both modulators and finals glowed in the K2PG re-tuned Collins 20V when transmitting, only the modulators glowed in the RCA BTA1R at WERA operated at half power. I never saw them in an Amateur amp where the duty cycle is only 20% unless an AM Gangsta pushes them to 100% and then they still have to be throttled back to 20% allowing for carrier expansion. This is all rather new to me so I'm mostly relying on the tube specs and a lot of good old common sense. A bit of that tells me unless you have enough loading capacitance it won't load down enough on 80M making the tuning dip broad and shallow as well as pushing the tubes too hard.

    "as long as nothing is going nuclear I guess we are golden."
    The Chernobyl Exclusion Zone won't be safe for human habitation for 10,000 years, but without people wildlife has returned and is doing remarkably well. "We are stardust, we are golden, and we've got to get ourselves back to the garden." Joni Mitchell - Woodstock, 1970

    "He said the more screen volts the better"
    Hoo Hee? Something tells me he doesn't have a tube spec sheet or most likely doesn't know shit from Shinola when it comes to tubes. That philosophy only applies to triodes! (Now think on it a moment and tell me what that means, hi.)
    "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.
    Not quite, control grid voltage has nothing to do with screen voltage per se, one does not affect the other. All voltages are measured under load, with dropping resistors they change depending on load conditions. Grid, screen, and plate voltages need to be adjusted for the class of operation, that's why the original design changed the fixed bias according to linear (Class AB1) or CW (Class C) on the mode switch, I don't know how the tune position lowers power so you don't send the tubes into orbit. Taking a good guess from my old 60W RCA VHF base station I modified for 2M, the tune switch lowered the screens in the 6146s to a very low value. Coincidentally it was the same make and model police transceiver in the original 1958 The Blob starring Steve McQueen.

    Showing my ignorance here, what is a "PEP vector meter"? I'm familiar with an RF watt meter like the trusty Bird 43 and a peak reading meter, but not that one. Then there's that hacked plate current meter reading cathode current. How do you know what your plate and screen currents are when you're reading both together? That's a hairy brain idea if I ever saw one.

    So THAT'S who has the hairy brain. Running with the screens 150V over maximum he's burning them up, when a grid short develops he'll be wondering how that happened. I wonder what +750V to ground on the control grids will do to a Chippewa besides make him wonder "Who peepee in my teepee?".

    " KE7TRP who had restored a Thudbolt to original said that's not enough tube bias at -75V."
    What makes him think he knows better than the engineers at Eimac who designed the 4-400 and those at E.F. Johnson who designed the amp around them? The tubes were glowing red at that low level because you're feeding the tubes much more voltage without changing other parameters accordingly. Remember with a 20% duty cycle my best guess is they should only glow on peaks when they're drawing appreciable current.

    "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."
    Interesting, the 4-400s in the BTA1R showed no color at the 500W RF output level. Since you're designing an amp from the ground up, at this point I only can tell you is watch those electrode voltages and make sure all of them are proper for the intended class of operation. That means defeating the safety interlock and testing with the amp upside down while you keep an eye on the voltages and currents. I guess you also know that means careful where you put the meter probes and keep one hand in your pocket, we'd like to have you around for a while. Oh, I said upside down because Eimac says base up or down, never sideways or you'll have sagging grids and something going BANG.

    Last but not least, the higher the fixed bias the closer you're driving the tubes toward cutoff, so naturally current goes down taking power with it. You know the score, RF output depends on DC input, the idea now is to get all of the voltages right according to the tube specs and you'll have a good working amp... I hope. (;->) Oh BTW, don't worry if the glass turns or has turned a slight smoky brown with age, that's normal. All of the pulls from the WERA transmitters were like that and still had plenty of life in them. Back in the day the Chief Engineer of a radio or TV station changed all of the tubes on a schedule and all of the pulls were perfectly good, that was standard industry practice. I had more 7025 low hum and noise 12AX7 twin high mu triode equivalents than I knew what to do with, the audio board in the main studio was bristling with them. They ended up in my stereo amp and other audio equipment. Broadcast quality home stereo? YOU BETCHA!
    "The universe is under no obligation to make sense to you."
    Neil deGrasse Tyson

    73 de Warren KB2VXA
    Station powered by atomic energy, operator powered by natural gas.

  3. #23
    Beach Bum KA2PTE's Avatar
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    Well been a while, but now getting back onto this epic project.

    I did find the neutralizing cap (C2) was dirty with a film of oil burner soot inside, and there were lots of ding marks in there from zorchings:


    c2.jpg

    So I cleaned all that up , and shined the aluminum with scotch brite and sanded down the burrs from the zorching. Also noticed the center movable electrode that moves in to that cup,
    was a little crooked. Turns out if you replace the doorknob caps, you stand a risk of mis-aligning that part of the cap, and looks like thats what happened also - as I had to replace those.

    I also made up a cheaper L8 with (5) 560uH axial half watt inductors, totaling a bit over 2.5mH but I am hoping 2.8mH is not much a problem from 2.5mH.
    Decided to put the plate on-off switch back in on the front panel so that in case of problems, its easier to throw it off, and kill the plate supply if the tubes somehow loose bias again.
    Had to route 115v back thru one unused pin of the input power jack, back into the Heathkit supply to the plate relay for that, but seems to be working as intended.

    I also have the spare set of 4-400A tubes in there now, as the last choke failure had one of the tubes sparking I think in the grid area. I put filament on and have all VR150 type tubes in the
    screen supply, with a VR105 acting as the operating bias tube, and when I apply plate, things appear to be ok / normal for idle once again.
    --
    Maybe covid19 is just a terminal case of Trump derangement syndrome

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