Yet another L4B rescue

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

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  1. K2XT

    K2XT Premium Subscriber QRZ Page

    KM1H has good advice.
    I have gettered tubes by running in an old homebrew amp to get full filament voltage on the tubes. Then use your homebrew power supply as you outlined, but put a small voltage on the grid to control plate current. As you lower the negative voltage the current will rise, and the tube will begin to get red. Let it run like this for a few hours. That is why running it with full filament and the blower is important.
    If the tube starts showing a blue glow, you got a gassy valve. Whether there is any hope in that case is TBD. Carl probably could tell us.
    I had my hv ps (only, the rest of the rf deck was run on full voltage) on a small variac when I did this so was able to get the voltage up into the 1500 v range. Sometimes a tube would seem ok at 1000 v but when moved up higher (readjusting the bias pot/battery to control current) the blue gas would appear. Sometimes when I came back to check on it the 5 a fuse on the variac had blown.
  2. W1QJ

    W1QJ Ham Member QRZ Page

    When tubes are gassy they could be VERY gassy or just slightly gassy. Usually the severity of the gas concentration in the tube can be determined by what HV turns the gas into a plasma. I have had some 3-500 tubes go plasma with as little as a couple of hundred volts on the plate. Those tubes are now ornaments made by a Vietnam Vet who I send them to. The higher the B+ voltage goes before the tubes show that violet plasma the less gassy it is. Some tubes are just so slightly gassy that they only show they are gassy when they are excited with a certain amount of RF. Others with various amounts of B+ and no RF. Some tubes may not show gas at normal plate voltage but then show gassy with normal idle current and no RF. So it can be all over the place. Degassing or gettering as some call it may or may not be successful. You just have to try and see if all the gas can be absorbed.
    WB1E and WA7PRC like this.
  3. W9JSW

    W9JSW Ham Member QRZ Page

    So, I can use this amp to do one tube at a time? I would verify that it gets full filament voltage and good air flow. Wire the grid so that I can apply a negative voltage from a variable voltage source. What would be the range of voltage I would need at what current. I read an article that suggested as much as 60V? Above 12v I would need to procure a variable voltage supply. Below 12v I could rig up a wire wound rheostat to vary the voltage. I would plan to use a variac on the HV supply and already have a 2.5kv meter that I plan to install into the shunt on the PS board. I also have a 0-500ma meter that I can place in the grid connection.

    Should I plan to hi-pot the tube first. I can check with the ham that I bought this from to see if he has a hi-pot fixture. He does a lot of tube amp work.
  4. W9JSW

    W9JSW Ham Member QRZ Page

    My amp looks like this - the sockets are on standoffs. I was planning to use copper braid to the 3 corners. With the gap should I still plan to use this copper strip method instead? Also note the tight quarters on the right hand side of the socket. If I use the copper strip, I will need a 90 degree twist to hit the offset hole that is already there, or do a Z shape to hit the hole. Not a straight shot in any case.

  5. W9JSW

    W9JSW Ham Member QRZ Page

    Looking at the picture closer, I see what you are saying. The short section of copper sheet will still be the shortest path to the major mass of grounded frame. I will go that route.
    K7TRF likes this.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Good plan. Save copper braid for situations where you really need flexibility, solid copper strap is a better option for low impedance RF connections. Personally I'd drill new holes if that's what it takes to run the shortest ground strap like WA7PRC's photo.
    N2EY likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    This sounds like you don't understand how tubes work.:)

    If you ground the grids and lift the filament transformer center tap 100K above ground, you won't be able to draw any plate current.

    Normally you might use a large resistor to ground for "standby" (tubes completely cut off, no chance of drawing current) which is then by passed by relay contacts inserting a Zener or string of rectifier diodes shorting out that resistor, to apply positive bias to the cathodes to "operate."

    With low voltage on the tubes as you originally suggested you might try, you probably don't need any diodes at all -- just ground the filament transformer center tap directly. 3-500Zs can operate at "zero bias" at relatively low anode voltage. Above about 2000V, we normally add a bit of bias (just a few volts), and above 3000V, we add a bit more (maybe 5-6V). At 4000V we usually add more than that (maybe 8-9V).

    Looking at the tube's dynamic curves reveal why.

    For grid grounding, what Byan showed in his picture is about the best approach. Don't use braid, use solid conductors and the wider (fatter) and shorter the conductors are, the more effective they'll be.
  8. W9JSW

    W9JSW Ham Member QRZ Page

    That is what I was speaking about. I mis-stated zero bias. What I was trying to describe is removing the 120V requirement from the PS, and using the 100K in standby. I think I have decided to use the Harbach combo board that gives me soft-start, soft-keying and bias.
  9. AI3V

    AI3V Ham Member QRZ Page

    Actually Steve, the scheme you describe is called "self bias" , in such a circuit a tube, any tube, can never be completely cut off.

  10. AI3V

    AI3V Ham Member QRZ Page

    And then, when you find out Sammy hammy myths of gettering ruined your amp, you can go buy a brand new one with a factory warranty.

    Just don't mention you gettered the new one when you ruin it.


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