Driving An Amplifier

Discussion in 'Amateur Radio Amplifiers' started by N9CUC, Nov 5, 2016.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It's good you've got some power out of the amp now. Based on the numbers above you should be seeing approximately 300 watts out of the amp now. That's a lot better than no output and no current rise above bias levels like you saw a while back.

    The next question is whether the rig is actually driving normal power into the amp or is perhaps folding back its output power when driving the amp. Sure check the cables as recommended in previous posts but bad cables would more likely be related to no power at all from the amp as you were previously seeing.

    I'd temporarily put a power meter in line between the rig and the amplifier. You're checking for two things, does the rig put out normal power when the amp is in line and active, and if not is the SWR between the rig and amp high? You can use your new tuner for these measurements, just make sure the tuner is in bypass mode so you're only using the metering sections and not trying to tune the amp's input circuitry.

    The most common causes of low power out of an amp (that is reflected both in low plate current and as low RF output power) are:
    - Insufficient drive power from the rig which could be caused by the rig folding back its output power in response to high SWR looking into the amp's input stage
    - Some problem in the amp's input stage including dirty relay contacts or mistuned input circuitry which could lead to the problem above
    - Or bad amplifying tube(s) in the amp.

    So start by making sure the rig's output power doesn't drop when the amp is keyed. IOW, test output power with the amp connected but in standby mode and then test again with the amp in active mode and keyed. The rig's output power should stay the same when the amp is keyed.
  2. WZ7U

    WZ7U Ham Member QRZ Page

    It seems some store bought coax patch cords give guys fits. I would hope you have made a few yourself so you can effect repairs should that be the necessity.

    Something that just crossed my mind; are you sure that the center pin of the 259's fit the center pin socket of the 239's tightly? I had that happen many moons ago and was the dickens to diagnose. Just my 0.02, adjusted for inflation.

    Remain methodical in your inspections. Good luck!
  3. G0JUR

    G0JUR Ham Member QRZ Page

    He stated he is getting a high swr reading on the ft950, he should look at what causes that before he goes any further.
    the high swr is most likely causeing the rig to fold back output so he cannot get full drive to the amp.
    Having read through most of this thread, leading a horse to water comes to mind, I really think he needs a visit from someone who has a clue about driving an amp the OP doesnt seem to understand whats needed to successfully use the equipment and advise over the "net" isnt getting through, unfortunately its rather a long swim for me to call on him but surely there must be someone close by with an hour to spare o_O
  4. N9CUC

    N9CUC Ham Member QRZ Page

    I had an electrical engineer/ham come down 2-3 weeks ago. Plus he is going to come back, hopefully this week. He believes, as you, the high swr into the amp is whats causing the lack of power.

    So, how does the high swr between the FT-950 and 811H get fixed? Again the exact same transceiver, amp, and jumper cables worked down at the Ham Station in Evansville but not at my place.
  5. G0JUR

    G0JUR Ham Member QRZ Page

    First thing I would do is place a good power meter between rthe rig and the amp a good power meter on the output of the amp and feed that into a dummy load
    then I would investigate what is causeing the high input swr and look for ways to reduce it either by tuning the input tuning circuits on the amp
    and possibly removing all tubes and soldering a non inductive resistor across the valve socket somewhere in the region off 100 to 150 ohms to check that the input circuit tunes that, if it does and it wont tune with the tubes in their sockest then 1 or more tubes are bad.
  6. N9CUC

    N9CUC Ham Member QRZ Page

    My electrical engineer/ham is coming over Wednesday morning. He is bringing jumper cables, swr/power meter, 811A tubes, antenna analyzer, dummy load, and resistors and diodes to see if we cant get this 811H to work.

    Hope we can at least isolate the EXACT problem as to why I cant get any power out of the amp. At least that would be a start.
    K7TRF likes this.
  7. KS9Q

    KS9Q Ham Member QRZ Page

    One other possibility - the tubes should have surge suppressors at the sockets. If one of those fired and went bad, it would throw off the impedance of the input circuit and your SWR would be out of whack. Usually there is a pretty good pop sound associated with such an event though.


    Jim K - KS9Q
  8. N9CUC

    N9CUC Ham Member QRZ Page

    All appears well.

    Thread closed.
  9. W6GRD

    W6GRD Ham Member QRZ Page

    No Fair! Tell us what the outcome was please?
  10. N9CUC

    N9CUC Ham Member QRZ Page

    The operator was following the instructions in the owners manual to the letter. Once everything was checked out, including checking the cables as well as removing the cover from the amp and testing all the tubes and looking for any other issues, we decided to go through the entire tuning up process. Apparently its not quite like the owners manual says. That being increase the drive until you get 450ma plate current or 100ma grid current. Which the operator couldn't ever achieve, which he stated here many times.

    So, once the operator learned to not quite follow the owners manual, per direction from Ameritron, and with the help of the electrical engineer........well, lets just say the 811H seems to be working properly now.
    KG5ILR and W6GRD like this.

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