AL-811 non-usable on 80/160 meters

Discussion in 'Amateur Radio Amplifiers' started by N7RF, Oct 17, 2016.

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  1. N7RF

    N7RF XML Subscriber QRZ Page

    I picked up a used AL811 from an estate sale and tried it out on 20/15/10 with fairly good power out. I had no idea how many hours the tubes may have had on them, but I started noticing drooping output power so I bought three new matched 811's and installed them. I also borrowed a friend's 1500W dummy load so I could go through each band to see what the pi-section control settings are.

    With the dummy load on the output, I can tune everywhere from the high end of 75-meters to 10 meters. But it will not tune down to the low end of 80-m, nor the 160-m band. The LOAD control, where it does work around 4 MHz, is set to approximately zero. It's like the LOAD side of the pi network needs more capacitance. At these frequencies, I cannot get the PLATE to resonate for any setting of the LOAD control.

    If I'm reading the schematic in the manual correctly, the band switch appears to be in the 160m position. It shows a C24 (500 pF, 1kV) and C25 (360 pF/1kV) that appear to be switched in on the LOAD side of the pi for lower frequency operation. I am suspecting that one or both of these caps is gone. Or the switch is bad. I suppose this is an easy thing to investigate. Does this sound like I'm on the right track?
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yep, it does. Probably blown caps, open bandswitch connections, or some combination of both.

    If a user rotates the bandswitch (even once!) while transmitting -- Poof.
     
  3. WQ4G

    WQ4G Ham Member QRZ Page

    IMO it does not sound like you are on the right track. Your post is a bit confusing to me when you say "At these frequencies, I cannot get the PLATE to resonate for any setting of the LOAD control." Isn't it the TUNE control that adjusts the resonance of the Plate Circuit?

    I am not familiar with your amp.... But, the situation sounds similar to my SB-200 when I am not driving it hard enough.

    Have you reviewed the manual section that covers the tune up procedure for your amp? What is leading you to believe you can not get the Plate to resonate? No power output?

    Most amps out there you must get the Plate Circuit resonate FIRST using an initial low drive input from the exciter and the TUNE control. Once resonance has been achieved then the drive level is increased and the LOAD control is used to keep the Grid Current under control.

    For example with my SB-200 (with Plate resonate) when I increase drive level (from the exciter) the Grid Current (in the amp) begins to climb. Then the LOAD control is adjusted to lower the Grid Current back down. With the Grid Current down I can increase drive again - which in turn increases the output of the amp in watts.

    As far as I know the LOAD control adjusts the coupling between the amp and the exciter and has nothing (or not much) to do with resonance of the Plate Circuit.

    Dan KI4AX
     
  4. KH6AQ

    KH6AQ Ham Member QRZ Page

    The AL-811 is tuned by adjusting LOAD and TUNE for maximum RF output while making sure the grid current is not too high. Reduce the RF input power if the grid current or RF output is too high.
     
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    The Load control in an amp adjusts the coupling between the tuned plate circuit and the output load (the antenna or dummy load). In a grounded grid amp design there is incidental impact on input coupling but the Load control is first and foremost an output coupling tool. If you under couple the amp's output power you will see a sharp rise in grid current in a GG amp which isn't a good thing but the Load control is really about coupling the amp's output signal, not coupling to the exciter.
     
  6. WB1E

    WB1E Ham Member QRZ Page

    I just finished doing a repair on this model amp. An opposite situation occurred. Had the load control bottomed on 80m. Happened the tuner was not fully setup yet on the amp's output. Still had a large SWR. The issue went away when the amp's output tuner/matchbox (Murch UT2000) was brought down to 1:1.
    So having said all of this, pray tell...what is YOUR SWR at the amp's output?
     
    Last edited: Oct 19, 2016
  7. N7RF

    N7RF XML Subscriber QRZ Page

    Thanks all for the comments...
    @WB2WIK - I haven't dug back into it to see, but I'm certain this is what's wrong. At least in that general area.
    @KI4AX - I know that sounded confusing. If the LOAD capacitance (antenna side of the PI network) isn't right, adjusting TUNE capacitance will never find a resonance.
    @WX7G - I always start out with low RF input (30-40W) to find the resonance, then bring up the power, and set the load. I can get reasonable output power 40-15 meters (500-600W). 10m is a bit low. But I cannot get it to tune at all below 4 MHz.
    @N5DMC - The SWR at the amp's output is 1:1 (it's a 1500W dummy load). The reflected power meter on my Daiwa cross-needle VSWR bridge doesn't even twitch.
     
  8. KH6AQ

    KH6AQ Ham Member QRZ Page

    It sounds like you are looking for resonance at full power; that is, dipping the plate control. I never found much of a dip at rated power with my AL-811. The Ameritron tuning procedure is to use an RF power meter and adjust LOAD and PLATE for maximum RF output while keeping plate and grid current to the limits specified in the manual.

    Once the settings for a band are found they can be logged (LOAD and PLATE settings and exciter power). Tuning then consists of presetting LOAD and PLATE, applying the logged exciter power, and touching up the PLATE control for maximum RF power.
     
    Last edited: Nov 4, 2016
  9. K7MH

    K7MH Ham Member QRZ Page

    It is not uncommon that there is not enough padding capacitance on 80 and 160 with the AL-811 amplifier. If you Google about it you will find a few forum articles about it. I had to add some capacitance on an Al-811 I had for 80 at the bottom of the band but I don't remember how much.
     
  10. KH6AQ

    KH6AQ Ham Member QRZ Page

    The symptoms point to C24 and C25. Are they installed? Are they not being switched in? They shown in the schematic on page 9 of the manual.
     

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