Yet another L4B rescue

Discussion in 'Amateur Radio Amplifiers' started by W9JSW, Apr 13, 2018.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The ZG tubes till use Zirconium deposited on the anodes as the getter material.

    "Reconditioning" inexpensive tubes like 3-500ZGs (after all, they're still <$200 MSRP, which, adjusted for inflation, is less than they originally cost back in 1972) is something only hams would do! But it's a fun experiment and if it works and prolongs the operational life a bit, what's to lose?
     
    WA7PRC likes this.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If it were me, and it's not, I'd get the HV power supply built, tested and connected, and verify operation before adding the Harbach kit or making any changes at all.

    IMO it's never a good idea to change anything from an original design before testing the original design, as you'll never know if a problem was already there or you caused it.

    Hams do stuff with variacs but I never bother, even with old tubes.:p The original L4PS had a 0.82 Ohm WW resistor in series with the B+ to act as a HV fuse (of sorts). If the new power supply includes the same thing (strongly recommended) and one or both tubes have an internal arc, the resistor will blow open before the rest of the power supply is damaged, and the resistor is cheap.
     
    WA7PRC likes this.
  3. W1QJ

    W1QJ Ham Member QRZ Page

    John, I am faced with this scenario all the time. People send me amps to bring on line often and I developed a system that works great for me. It's pretty simple. I use an SB-220 to bring unknown tubes on line. I keep the top screen off so I can see the tubes. I use a Variac, I put the meter in grid position and I slowly bring the line voltage up on the variac and I watch the tubes and the grid meter and plate meter, slowly raise the line voltage and see if there is any signs of plate or grid current ESPECIALLY NEGATIVE grid current, or any sign of gas in the tube (shown by a violet blue hue) the test stops there and there is a bad tube. I determine which is bad by elimination. If as I am raising the line voltage all looks well I keep raising it until normal HV is reached in the CW mode. I will then key the amp into transmit and again view the tubes for gas. If all looks good I will then excite the tubes with RF and once again check for gas. If all is good I would drop the Variac down a bit and hit the SSB switch and raise the HV from the CW 2400v to 3000v watching for gas. Once at 3000v if all is well once again I will excite the tubes with RF and check for gas. If this entire test shows normal, the tubes are gas free and they can go into normal use. This whole test takes me only a couple of minutes and its done. A 3-500 is either good or it's not. If the tube has a G-F short you will see it just about immediately when you bring up the line voltage, the grid meter will go negative. Soon as you see the meter go negative, test is over and you have a bad tube. If you have a hi-pot tester you could use that too. Grounding the grids is optional, you will get a tiny bit more power output with less drive. If that is not an issue for you then you may want to leave them as is.
     
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  4. W9JSW

    W9JSW Ham Member QRZ Page

    Ok, If I follow the suggestion on not installing the soft start board, shall I just implement a 100K resistor from ground to the center pin of the T/R relay so that I can operate properly without the 120V bias supply? I do not plan to implement that bias voltage capability. I have read that it is better to have a string of diodes as well and that is what the Harbach board also provides.

    Lou, your set of steps is well described and should be easy to follow. Just need to get cracking on the HV PS.

    I think I will eventually want the grounded grids. My SDR with current linear only puts out a max of 50W PEP. I run it with around 12-15 W of carrier. Should be interesting to see how well it does. I also have a Yaesu FT-757GXII that I can use to drive it. It is 100W PEP with 25W carrier. I hate to use it though. I like the visual capability of PowerSDR and the panadapter for seeing signals. It is a excellent receiver. Most of the time I am on my laptop upstairs talking to the radio in the basement shack. When I get the linear going, I will probably put a camera on the face to be able to monitor any anomalies during operation.
     
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    How much can it put out in CW keyed for 4-5 seconds?
     
  6. W9JSW

    W9JSW Ham Member QRZ Page

    50 watts on 40M, a bit less on 80M (probably need to adjust the filter).
     
  7. N8CBX

    N8CBX Ham Member QRZ Page

    When I use my homebrewed hi-pot tester on used (and new), questionable tubes, if there is much leakage on anode-to-grid, grid-to-cathode, I stop there and just throw it away. Now, some marginal, good used ones will always have some leakage and I might keep them.
    As a comparison, new tubes show about 1uA (or none) leakage at double the plate voltage. Bad ones peg the meter, at 25uA, way before then.
    Jan N8CBX
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    1 uA sounds high to me.

    I use a very old Biddle hipot tester (I think made in the sixties!) and test questionable tubes with that, too. One of these days I'll move into the 21st Century, but the 20th was so much fun...:p
     
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  9. W9JSW

    W9JSW Ham Member QRZ Page

    Going to throw together a hi-pot tester. Junk box has most of what I need. For the transformer, I have a 500-0-500 plate transformer. I know that this is not a "good" choice since it can sink around 200ma. If I use an appropriate bleeder on the secondary, can I make it safe enough to use? I think I would have to have a voltage doubler as well to get the voltage up. I would put a variac or similar on the primary to vary the voltage. I have a 50ua meter.

    This would just be used to check the 2 tubes I have in the amp. I have no further needs for it and would probably disassemble it when done.

    Comments? Going to look for a better trans on ebay that puts out 2kv at 1ma or so. Just wondered if this would do in a pinch.
     
  10. W9JSW

    W9JSW Ham Member QRZ Page

    Or I could just start assembling the harbach PM-400 board that I have on order then build an outboard circuit in a safe enclosure that I plug into the HV connector to allow me to slowly put power to the tube in a test fixture and see if I see uA leakage...while standing away from it with one hand in my back pocket. ;)
     

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