AL-80b tuning procedure clarification

Discussion in 'Amateur Radio Amplifiers' started by KN4LGK, Jan 12, 2020.

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  1. KN4LGK

    KN4LGK Ham Member QRZ Page

    I just purchased my first amp and I have a question regarding the tuning procedure - probably a dumb question but I want to avoid a mistake. The instructions indicate in the steps to increase the exciter (IC 7300 in my case) to full output and then adjust the plate and load to full output. Am I interpreting correctly to increase these to the full published output of the amp? Also is the operating output regulated by adjusting the exciter output? I realize these are really elementary questions but I’d appreciate the advice.

    Glen
    KN4LGK
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    A couple of clarifications on the procedure you wrote above.

    - Yes for final tuning you should drive the amp at the highest power you plan to use. IOW, don't tune up the amp at low power and then increase the rig's power when actually using the amp. You can do the opposite, tune up at highest expected power and then decrease your rig's driving power but don't tune up low and then transmit high.

    - You should generally tune an amp for peak output power but then increase the Load control (Clockwise direction) a bit until the total output power drops a bit to ensure linearity and reduce the chance of splatter, especially in SSB mode. A monitor scope or spectrum analyzer is the best way to ensure you've loaded for proper linearity but an amp tuned for maximum possible power is almost always under loaded and linearity/distortion won't be ideal without some additional loading.

    - Keep an eye on grid current, not just total output power or plate current when tuning your AL-80b. You don't want the grid current to exceed 175mA and FWIW I've found I get my best linearity and rated power output when I load the amp to see 150 mA of grid current when driven by a 100 watt rig. That's confirmed with both a scope in trapezoid display mode and with a spectrum analyzer but after watching the monitor scope for the first month or so I had the amp I've found that Loading to 150 mA of grid current when driven by a 100 watt rig coincides with good linearity and gives me the rated 800 watts in CW mode or 1kW PEP SSB.

    - You can actually do the entire tuneup procedure while just monitoring grid current. Assuming your AL-80B behaves like mine and you drive it with a 100 watt rig you can just peak the grid current with the Plate control (same as dipping the plate current) and then adjust Load to see around 150 mA of grid current while maintaining that peaked grid current with the Plate control.

    My actual tune up procedure using a rig that supports a low power output Tune mode (e.g. Ten Tec Orion II, Kenwood TS-830s) is:

    - Warmup the amp for at least a couple of minutes before driving it
    - Run the amp into a dummy load
    - Switch the amp to appropriate band
    - Rough adjust Load control to about where I expect it for band in use or if not certain run it further CCW on low frequency bands and center to further CW on higher frequency bands
    - Key the rig in Tune mode (or CW mode low power)
    - Quickly peak the grid current (same as dipping the plate current)
    - Increase the rig's power to full power output in CW mode
    - Adjust Load control as necessary until I see roughly 150 mA grid current, re-peak grid current with Plate control as neeeded
    - Switch to active antenna and operate normally

    That takes somewhere around 5 to 10 seconds particularly if the Load control is in the ballpark for the band in use.

    With practice you can skip the low power tuning step or tune up in SSB mode driven by voice but start by tuning in a constant carrier mode into a dummy load before switching to an active antenna.

    And yes, the easiest way to run lower power is to reduce your rig's power setting if you're not running ALC feedback from your AL-80b (which I don't) but you can set up an ALC control loop and use the AL-80b's front panel adjust feature to manage total power output though that is also managing the rig's power via the ALC line.
     
  3. KN4LGK

    KN4LGK Ham Member QRZ Page

    Thanks so much for the help. I’ll head to the shack tomorrow night and finish the set up.

    Glen
    Kn4lgk

     
  4. KM3F

    KM3F Ham Member QRZ Page

    The repeated questions of how to tune an amplifier are based on no technical knowledge beyond turning the knobs.
    They don't have any knowledge of what is happening or what it aomplishishes.
    Therefore directions may get it close but still not understood.
    .
    A try at this:
    A tube amplifier operates within ohms law. A voltage and current at any level is equivalent to a resistance R= E/I.
    This is the tube load resistance at any given operating condition. Since under SSB drive, this resistance changes between limits and is why the tune at a specified drive needs to be considered.
    Even tuning at max output still does not guarantee the best settings at a low audio voice to a high audio voice loudness levels as one talks. Therefore some linearity is lost that can't be accounted for in any way but tune for max power then reduce the drive if necessary.
    Further, given the varying tube dynamic resistance over the drive level changes; this changing R value is higher than the 50 ohms used to feed the transmission line.
    What the tuning of the plate and load does is an impedance 'transformation' from the tube R to the output R required to match 50 ohms. (it won't stay the same over loudness drive level changes from the radio)
    The output tuning sets a narrow band pass 'transformation' for the band selected that s only good for a practical 50khz +/- limit before retuning should be done.
    You view this by observing the Plate current, the Grid current and the Power output meters that results.
    .
    Again, a try to fill in what's missing to the plug and play operator that there needs to be a bit deeper understanding of what and why.
    .
    For instance if tune at one part of the band and QSY to another without retuning is asking for splatter as one result.
    Or tuning at low power then raising the output with more drive results in the same splatter. The transformation above no longer is optimum for settings.
    The only time you tune at a lower power is under drive conditions you >> absolutely know << the peak power tuned at will not be exceeded. An example of this is AM operation where you know the radio percentage of modulation will not be above about 90%.
    Example a carrier level, >>the 90%will not result in a peak power beyond the amplifier tuning level <<.
    For example a carrier drive of 100 watts x 4 = 400 watts x .9 = 360 watt peaks on a 400 watt amplifier tune. This offers a 40 watt protection. Read it on a PEAK READING WATT METER or a Scope display waveform.
    The amplifier should stay more linear with this tune than if tuned at max possible then reduced way below that point. If you do both a max tune and a limited tune, the Plate and Load settings will be different showing up the difference. It shows the tube really is not a 'fully' linear device 'especially' when a tube gets what we call soft (a bit slumpy).
    Bottom line is, there is more to tuning than simple instructions can convey.
     
  5. N3DT

    N3DT Ham Member QRZ Page

    Glen,
    The thing is everything says to tune for max PO. But that doesn't mean you have to tune the amp for max power and then reduce it. I run mine at about 30-40W input all the time and don't tune for max (70W input) all the time. I do, however, tune for max output given the Power in. You will find that the Load control will have lower numbers when tuning with lower power inputs. The Plate will be nearly the same. the thing is you want to tune for max PO at whatever P in you use. The Ig should peak while the Ip should dip (min) and the PO (on the meter) should all happen at the same time. I actually find sometimes the Ig peak to be a bit more sensitive than the Ip dip. If not something's not quite right. Find the spot where they all coincide, Max Ig, min Ip, Max PO. Then make a note of it along with the Pin and keep it. You don't want to be searching for that min Ip all the time, it's hard on the tube. It should only take a second or 2 to get the Ip at min, then you can play with the load control and get the others in line.

    If you reach a condition where the load is at 0, that's not especially good and it may happen at low power on 40M and below. I had to add 170Pf in my circuit to get mine to work right at low P in. My 80M Load is still very close to 0 at low power.

    It's also not bad to increase the load a bit (higher number) once the sweet spot is found. It makes for a bit more linearity. You'll get the hang of it.
     
    N4NYK likes this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Using the AL-80B's ALC output is a great way to prevent overdriving it in the event of operator error or almost anything else, assuming the connected transceiver has a compatible ALC input that works.

    Unlike many early amps that just use a diode detector to sense drive levels, the AL-80B doesn't. It monitors grid current and creates a control loop that really works well and isn't frequency, band or load sensitive. Set it up for 150mA max Ig and it will limit it there. And then, indeed, the ALC SET control on the front panel works like an RF OUTPUT control on the connected transceiver.

    Thankfully, the 80B uses a rugged tube and sufficient components to allow for a lot of operator error without damage.:)
     
    N4NYK and K7TRF like this.
  7. KN4LGK

    KN4LGK Ham Member QRZ Page

    Thanks everyone for the help. I think I can get it tuned. I appreciate it very much.
     

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