Ameritron AL-811 Mods etc?

Discussion in 'Amateur Radio Amplifiers' started by KJ4AQU, Apr 12, 2012.

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  1. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    Just obtained this and was wondering if this amp too, Like the SB-200 should have a "glitch" resistor added? Plus any other helpful hints? It works like it is though.
  2. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    Well I opened newly obtained but heavily used AL-811 and look what I found inside! Former owner mentioned he did switch bands in the middle of transmitting. Would that do this? I tested the burned looking 100Pf Cap and it tested good. But will probably replace it. Suggestions?

    Attached Files:

  3. K1ZJH

    K1ZJH Ham Member QRZ Page

    The caps are in series with the resistor. Sure looks like some heavy RF voltage hit that suppressor. I'd replace both caps, on either side.
    If one cap was stressed, then the other one was too.

    BTW, I would have kept the SB-200 over an amp using 811A tubes. :)
  4. KJ3N

    KJ3N Ham Member QRZ Page

    I would replace the entire circuit board. Carbon arcs have a tendency to create more arcing in the future. This is not a good thing.

    I would NOT use the amp until the board is replaced. JMO
  5. K1ZJH

    K1ZJH Ham Member QRZ Page

    Or you could flip it 180 degrees, and move the parts to the undamaged section of the board. It was designed to be used in the
    811H four tuber as well :)
    Actually, by the time you pay MFJ shipping charges, buying a new board probably wouldn't be that outrageous.
    Last edited: Apr 13, 2012
  6. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    I went ahead and did a 180 degree flip and put parts in unused area. New board is around $37.00 I'll use it as is for now.
    I sold the SB-200 because I needed the funds. Then I wound up getting this 811 for $300.00 so not a bad deal.
  7. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    Thought I would show an after shot of work done. 100Pf Caps on both side of previously damaged area replaced. Also 10Ohm 10 watt glitch resistor added in HV line as well.

    Attached Files:

  8. KM1H

    KM1H Subscriber QRZ Page

    Thats the circuit I developed in the 80's when MOX resistors were fairly new and detailed it in the mid 90's when the AMPS reflector started. My need at the time was for 6M conversions. Guess who tried to poo-poo it and then copied it?[​IMG][​IMG]

    Lately Ive been experimenting with Ohmite OY series 2W resistors and have been pleased so far using them in suppressors just like carbon comps. Using a pair of 100 Ohm in parallel gives some overhead especially with 572B's and others where the parasitic is closer to 10M than later designs. Mouser has them at $1.28 a pop.

  9. AG6K

    AG6K Ham Member QRZ Page

     It is a mistake to assume that the Tune-C arced because the previous owner switched bands while transmitting since as soon as a bandswitch is moved the tank immediately goes out of resonance and there is little V in the tank. A more likely reason that a Tune-C arcs is the presence of VHF energy. If you are curious to what's going on here, couple a dipmeter to either side of the DC blocker cap and observe the sharpness of the dip c. 95MHz. This is the AL-811's anode's parasitic resonance. - note - G-G amps are only self-neutralizing below their grid-resonant freq. 4, 811As have 800Ωs of feedback XC at 95MHz and their grids resonate c. 75MHz (KM1H) so there's enough feedback to allow them to oscillate at their parasitic anode resonance of 95MHz. To reduce the chances of VHF oscillation, reduce the VHF-Q of the VHF suppressors. This lowers the VHF RL on the anodes of the 811As and that reduces their VHF gain. You can expect 28MHz power out to drop c. 2%. . . . An arced Tune-C is a good thing because it saved your bandswitch. . . . Parasitic osc. puts high EMF stress on filaments due to the unloaded condition. I would add a glitch R to the HV+ circuit that will limit peak I to 150A or less. I have seen an AL-811 with shattered 811A filaments because it did not have a glitch-R when it oscillated at 95MHz.
    Murphy was right.
  10. PP8DA

    PP8DA Premium Subscriber QRZ Page

    Hello, my name is Nick - PP8DA
    - Please can you explain me what for is a 10 ohm 10 W glitch resistor is good for in the HV line.
    I run the same amp for more than a year with the orginal valves up to 550 W but never
    above Grid I more than 140 max 150 mA.Thank you for your time and your explanation.
    Direct replay please including a smal drawind if possoble
    Vy73´s de Nick
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