Driving An Amplifier

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

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  1. KR2C

    KR2C Premium Subscriber QRZ Page

    I'm not sure how this is useful or what it even means.
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Is the VSWR going INTO your amplifier still high on the higher bands? (if it IS, this is probably why you can't achieve full power)

    This indeed CAN be due to a faulty valve (tube) . . .

    If one valve has changed its grid capacitance to earth slightly, this would affect the higher bands' input matching tuned circuits, but probably NOT on 40/80/160.

    Roger G3YRO
  3. N9CUC

    N9CUC Ham Member QRZ Page

    Well, Dan at the Ham Station got it to work and tune up just fine. He got the 450ma plate current and 100ma grid current. Just not sure what the difference is between his place and my place.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Exactly how do you have your station configured? Is it set up as shown below:

    RIG----->AMP----->Power/SWR Meter------->Dummy Load

    Or with the tuner in line as:

    RIG----->AMP----->Tuner including Power/SWR Meter------->Dummy Load for Testing/Antenna for operating

    And similarly how were things configured down at the Ham Station shop during testing?

    Were there any differences in:

    - Rig driving the amp
    - Load the amp was working into (as in into a dummy load or into a known well tuned antenna)
    - Jumper cables between the components (did you test down at the shop using the same coax jumpers you use at home)

    There's a number of reasons you could see different results but based on the posts in this thread it's not clear how you have things configured during testing nor how that configuration may have been different while testing at the shop.

    The good news is that the shop testing validates your tubes and basic operation of the amp so you can focus on other variables.
  5. N9CUC

    N9CUC Ham Member QRZ Page

    It was RIG--->AMP--->SWR/Power Meter--->Antenna (Alpha-Delta DXCC)

    Now its:
    RIG--->AMP--->MFJ Tuner--->Antenna

    The difference at the Ham Station was that there was a dummy load being used down there as opposed directly into my antenna. However, when my electrical engineer was over 2-3 weeks ago he brought his Cantenna dummy load and we still couldn't get it to tune.

    Jumper cables were the same as I took them down to the Ham Station with me.

    And nothing is working on 40M now either. I get the 1700V on the amp as well as the 75ma in the first part of the tune up stage. But then the manual says to increase the drive up to no more than 450ma plate current or 100ma grid current. When I increase the drive (i.e., increase the watts from around 10/20 to up around 30/40) nothing happens to the plate or grid current.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    A couple of thoughts:

    - You should definitely add a dummy load to your shack, especially if you plan to run a tube amp. Even after this issue is sorted out, it's best operating practice to tune up your amp into a dummy load so that you minimize high power on the air tuning signals. But it also reduces unknowns when troubleshooting problems like the amp problems you're seeing.

    - I assume the 75ma current you're referring to is the bias current measured at the plates prior to keying a signal. If you see the correct bias current but it doesn't increase with drive signal then it does imply a drive level problem. That could be an intermittent coax jumper or a problem with the amp's signal input such as dirty or pitted contacts on the amp's T/R relay that provides good contact sometimes but not at other times.

    It sounds like you're not actually getting drive signal into the amp on 40 meters. Does it work properly on other bands? If so it could be dirty or bad contacts on the amp's band switch, particularly on the wafer that switches the tuned input circuitry.

    I'd start by double checking all of your connections and try swapping cables if you have some spares as a coax jumper with an intermittent connector could easily lead to the kind of problems you're seeing (but it would be on all bands, not just 40m). You might also temporarily connect a power meter between the rig and amp to validate that the amp is actually receiving drive signal. IOW, temporarily connect the system like:

    RIG ---> Pwr Meter ---> AMP ----> Dummy Load (or Tuner----> Antenna)

    If you see power on that first meter but still don't see plate current increasing above bias level then it's likely you have an internal amp problem in the input stage (e.g. T/R relay or bandswitch contacts or related circuitry). If you don't have another meter then you could temporarily set the system up like:

    RIG ---> Tuner/meter (in bypass mode)---->AMP---->Dummy Load

    Again the goal is to make sure you're actually getting drive power into the rig at times when the plate current does not rise while transmitting.

    Basically try to divide and conquer and break the problem into parts that can be tested individually.
  7. WA4SIX

    WA4SIX Ham Member QRZ Page

    Correct me if I am wrong, but there should be an SWR meter built into the rig. You should be able to monitor the input SWR with that.

  8. N9CUC

    N9CUC Ham Member QRZ Page

    No, it doesn't work on any of the bands now. It used to work on 40M but not anymore. Its never worked properly on 80/20/15/10. And yes, you are correct about the 75ma.

    And like I said, its odd that everything works fine at the Ham Station but not at my place. So, bad jumper cables, bad signal input, low drive signal, etc, why would they work at the Ham Station but not at my place?

    There must be some simple explanation for all of this.
  9. KI6LZ

    KI6LZ Ham Member QRZ Page

    I'll bet it won't work there now. Possible intermittent or gradual failure.
  10. N9CUC

    N9CUC Ham Member QRZ Page

    I've had it at the Ham Station twice and both times it worked there.

    Does anybody know of a "Traveling" amateur radio expert that could come and help me out here?

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