ad: SDRKits-1

AL-811 Not Loading Correctly on 80/160

Discussion in 'Amateur Radio Amplifiers' started by AD7I, Nov 20, 2019.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: Left-2
ad: Left-3
ad: Subscribe
ad: l-BCInc
  1. AD7I

    AD7I Ham Member QRZ Page

    I have some issues with an Ameritron AL-811 for which I hope that members here might be able to give me some advice. I bought this amp from a friend at a good price, knowing in advance about these problems. The date on the sticker on the transformer is 7-94, so it’s about 25 years old.

    My problem has to do with the Load setting for the amp at lower frequencies.

    The amp seems to work well on 20, 15 and 10 meters, slightly lower output on 10 meters, but works as expected.

    However, at lower bands and in particular 160 and 80, the load control must be at the zero value (max capacitance) to get output, and although as the load control is rotated CCW toward zero (smaller dial numbers) the output power begins to peak, it does not reach peak before the load control reaches zero and stops, which leads me to think the amp is not properly loaded. It’s like it needs more C at the output of the pi network.

    I should add that on 30 meters the load control is also at zero, without the output power reaching a peak. I don't really consider using the amp on 30 meters due to the power limit on 30 meters being 200 watts, but I mention the loading issue on 30 in case that's helpful in diagnosing the problem.

    All of the testing listed above was done on a Cantenna as the load. The plate and grid meters always look “reasonable” but on the low side. It’s just that the output power can’t reach an output peak via the load control.

    I’ve read Tom W8JI’s comments at the bottom of https://www.w8ji.com/loading_amplifier.htm

    I’ve also tried temporarily adding extra C to the output as Tom had mentioned on his website (but I can’t find the reference although I’m sure it’s in there somewhere) by putting a coax TEE at the output of the amp, with one port of the TEE output going to a PL259 connected to some fixed capacitors. I’ve tried values of 330 pF, 660 pF and 1100pF bridged onto the output of the amp. No improvement of the load control at 160 or 80 meters.

    Can anyone offer some suggestions as to what I should try next?
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd take a very close look at the band switch contacts perhaps with a magnifying glass under a bright light. There are several possibilities for what you're reporting but one is poor output circuit coupling on the problem bands and that could be as simple as burnt or oxidized band switch contacts.
     
    AD7I likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Could be, or just another AL-811 problem -- there are many.:)

    If extra outboard padder caps don't work, it does sound like the bandswitch may not be working properly.

    However...lack of adequate "drive power" will result in very similar symptoms. If that's the problem, possibly a problem with the tuned input circuit. Low drive creates the need for MAX loading capacitance, and increasing drive creates the need for less.
     
    AD7I likes this.
  4. W9RAC

    W9RAC Platinum Subscriber Platinum Subscriber QRZ Page

    For whats its worth...... Rob at Ameritron who is the tech there told me on the phone that the 811H is not to be used on 30/15 meters without tuning the coils. If they are tuned for 30 or 15 then you can not use 20/17. He said its a either or. The 160 behavior you mention is not uncommon. Having owned 4 811h's, none of them liked 160. I use a dedicated 160 dipole and a Palstar HF auto tuner. They still do not like 160. I have not noticed such behavior on 80. 73 Rich
     
    AD7I likes this.
  5. AD7I

    AD7I Ham Member QRZ Page

    What you describe re the input pi network for 30/20 is another issue that I have that I didn't mention. It may seem nuts to use an amp on 30 meters, given the 200 Watt PEP limit, but I use an amp on 30m to take my radio operating digital modes from 20W to 200W. And with the input tuning network straddling 30/20 I get an amp input SWR of 2.5:1 for both 20m and 30m, which is a less than ideal match. yet the specs stated for the amp say the input SWR on all bands should be less, but it isn't.
     
  6. N8FVJ

    N8FVJ Ham Member QRZ Page

    My AL-811 amp did need more capacitance added on the load variable cap as applied to the band switch to the original values. After I added the correct amount of fixed capacitance the amp tuned 1.8 to 2.0mHz and 3.5 to 4.0mHz.I remember the amp would not tune correctly below 3.8mHz, forgot what the factory stock tuning range was on 160 meters.
     
    AD7I likes this.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    You can't load that amp with a small amount of drive or else the load control would be at zero and you would be looking for more capacitance that you don't have. The trick to loading on 160 is to start with 20 watts drive with the load on zero and peak the plate tune for max output. Once you have the proper setting of the plate tune you can then hit the amp with 50 or 60 watts of drive and then peak the output with the load control. The load should then stat lifting above zero, more drive will lift it even further off zero. When the amp is at normal operating output, about 500 watts carrier, the load should be above zero indicating there is just enough capacitance to support normal power. I have repaired and upgraded many of these amps and in every case this is what I see.
     
    W1TRY and AD7I like this.
  8. N3DT

    N3DT Ham Member QRZ Page

    I have an 80B that would do the same thing on 80 and 40, zero load at lower than full output. Not sure the output circuits are the same but I ended up doubling the doorknob load cap that's switched in at 40 and lower from 170 to 340. I just added a 170 in parallel with the other one. I found one on ebay for a few bucks. I did get one from ebay that was not what was advertised, but got a complete refund. It worked for me, now I can use 40 and 80 at 20W input. It works fine at full output too.

    There was a much older thread I found on this same subject, but he had problems at full output.
     
    AD7I likes this.
  9. W1QJ

    W1QJ Ham Member QRZ Page

    Oh, I guess the issue is because some hams want to run 150 or 200 watts on FT-8. Yup that would be a problem.
     
  10. KJ6ZH

    KJ6ZH Ham Member QRZ Page

    The AL-811 from 25 years ago did not have enough fixed capacitance for 160 and 80 meters.
    Check the schematic from then and now. The newer ones added a 180uf fixed capacitor in parallel to the existing 180uf one at the 80/160 m position on the band switch.
    I did this mod for my 2006 AL-811 and this gave me more range on the load control for 80 / 160. No longer bottomed out at Zero when tuning.
    You can get the part from Ameritron for a couple bucks. Easy to install.
     
    AD7I and K7TRF like this.

Share This Page