Another Faulty Ameritron AL-11H

Discussion in 'Amateur Radio Amplifiers' started by G3YRO, Aug 5, 2017.

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  1. AF6LJ

    AF6LJ Ham Member QRZ Page

    I've seen 250V fuses explode with 800V when they opened and the arc made the glass explode.
    There are other ways of dealing with this problem.
    1. a fuse in the center tap of the filament transformer.
    2. a 25 or so ohm glitch resistor in the B+.
    3. low wattage low value resistors from the grid to ground bypassed with a cap that is rated for the B+.

    Not only is what you did bad engineering, if that fuse should fail while the cover is off someone could be blinded by flying glass.

    It is also going to leave scorch marks on the surfaces around the fuse holder.
  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    The holder isn't rated to stand off 2k volts to the chassis either.
    Using a Littlefuse 508 series fuse or better is a safer bet. They make this one in a 1A cartridge. I use the leaded one wrapped in heatshrink on a 10mm nylon standoff.

    I like Sue's idea of putting it in the CT. My amp's apart getting a facelift and I may do this one too.
    Last edited: Aug 10, 2017
    AF6LJ likes this.
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    That works because if the tube flashes over the the voltage drop across the inside of the tube is relatively high, the fuse opens and the voltage across the open fuse is relatively low. This also protects the filament transformer from damage due to the center tap burning open.
    KD2ACO likes this.
  4. G3YRO

    G3YRO Ham Member QRZ Page

    It's not mounted on the chassis . . . it's on a plain piece of PCB

    And putting it in the filament line doesn't protect it if it arcs over to the Grids. (wheich is what usually happens)

    Roger G3YRO
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Which is why the designer recommends doing this.
    AF6LJ likes this.
  6. AF6LJ

    AF6LJ Ham Member QRZ Page

    I addressed that already..
    Besides placing the fuse in the B+ line doesn't protect the grids ether.
    By the time the fuse explodes a hole has been blown in the grid and pieces of it may be shorting against the filament.
    That is what the low value low wattage resistors from the grid to ground are for.
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

  8. WB1E

    WB1E Ham Member QRZ Page


    Will you shoot a picture of the bad relay placement on the back side of the PCB if you get a chance?

    Out my PCB comes as well, still fighting the same issue over here. But the grid meter reads 20% and won't move with 40 watts drive, same with the plate current. May be this weekend until I get back to it.
    Cheers Roger!!

  9. G3YRO

    G3YRO Ham Member QRZ Page

    Well it's swings and roundabouts regarding the fuse . . . I've certainly used fuses on 1000V rails in the past, with no issues if they blow,

    I still think it may reduce further damage to the amp, so even if the fuse DOES explode when it blows, it's still the lesser of the evils.

    Some people recommend fitting a glitch resistor in the line, but that's even more likely to explode if there's excessive current. (plus it would take much longer than a quick-blow fuse to go open circuit.)

    Doug, are you sure your D16 hasn't blown? Otherwise you shouldn't get any readings on the two current meters. Have you checked the resistors on the board behind the meters?

    It's the lower relay on the PCB that uses one set of contacts to switch the antenna input, the other set completes the HT negative line to the filaments (via the transformer centre-tap).

    The easiest way to check if the relay contact that complete the HT- line has burnt out is to by-pass it externally. Disconnect the thick white wire low down on the RHS of the rear PCB that goes to the transformer (this is the Filament centre-tap, and connect it to a string of 6 diodes tp the blue wire on the back of the Grid Meter.

    This will then do what the relay does when it goes to transmit.

    Roger G3YRO
    Last edited: Aug 11, 2017
  10. WB1E

    WB1E Ham Member QRZ Page

    Awesome Roger, yea, the relay is moving, and I did clean the contacts. D-16 was blown, replaced with a 1N5408. Those metering resistors look good also. The meters do deflect, though not much into the dummy load. I tried driving it with external triggering and 40 watts CW. I need to set up better looking at any output, along with my SWR meter on the input. I was able to get a dip, but only from 3.5 to (dipped) 3.0. So still high input SWR, this with a brand new set of tubes. Have done the W8JI mods when my compadre purchased it with blown resistors underneath (filament to ground) now the grids are directly grounded. Checked at the socket and everything looks good to ground, next should start looking at (like you mentioned) the connection across the relay to filament CT. Also can't see any other semiconductors that haven't been checked good. This is an older version. No 2n3904 or any small signal transistors or second relay. Looked last night on the back of the input PCB. No doubt, the bias is pretty silly. But thanks to this thread, hopefully I can feel my way around much better!

    Will post any more findings on the weekend.

    AF6LJ likes this.

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