AL-1500, metering accuracy?

Discussion in 'Amateur Radio Amplifiers' started by NZ9Y, Dec 5, 2011.

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  1. NZ9Y

    NZ9Y Ham Member QRZ Page

    I have an AL-1500 about a month old and I'm having doubts about its meters.

    The needle that acts as a multi-function meter seems to disagree wildly with everything else in my shack.

    On PO it reads about 500w higher than my other 2 power meters.
    On Ip it reads at or above 1000 mA when tuned to 1.5kW.

    I thought this thing was supposed to "protect" on high SWR? I tuned up into a dummy load and accidently switched to the wrong antenna. An arc (less than a second) was all I got, no protection.

    I keep expecting the grid current to jump up but it doesn't. I feel like I'm beating on the thing to get it to 50mA. The tuneup instructions say to increase drive to 100mA grid current, but I can't get it there without taking the plate current really high.

    I believe everything is reading much higher than it really is. Is there anything I can do without a precision power meter?

    My AL-80B meters agree both on its own needle and my other 2 power meters.

    I've tried calling Ameritron tech for a week, to no avail. Any help here will be very appreciated.
     
    Last edited: Dec 5, 2011
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I would never run an 8877 at 100mA grid current, at least not intentionally.

    In the AL-1500 manual, under "Metering Functions" the first paragraph states, "Do not exceed 75 mA grid current."

    Under the tune-up instructions, step 11 states, "Final grid current should be around 50 mA." (At full power, after tuning up.)

    The only way the Ig would be higher than this is if the amp is underLOADed and/or overdriven. The grid protection circuit kicks the amp into standby at 175 mA Ig, and frankly IMO that's pretty high.

    My own 8877 amp, a monobander for 6 meters (3500V Ep) achieves 1500W PEP output power with about 25-30 mA grid current. I can drive it higher, but then the output is >1500W and it's silly.
     
  3. NZ9Y

    NZ9Y Ham Member QRZ Page

    Sorry, I stated the grid current from memory. I have a few amps, so to prevent mistakes from one to another, I use the manual to tune up EVERY TIME. The differences in grid current from a 3-500 to a 8877 are too great for me to trust my memory. :)

    At 1500w regardless of which power meter I use, the grid current is really low, like 10-20 mA. I never have needed to get the Ig over 50mA when tuning.

    I'm also very carefull not to run underloaded. I learned how important it is from a guy here. :)
     
    Last edited: Dec 5, 2011
  4. W8JI

    W8JI Ham Member QRZ Page

    Have you read the manual?

    Read the section on metering functions, page 5, I think.

    It should say:
    That is not a directional coupler meter. It is a simple RF voltmeter with peak hold in the reading. In later amps, there is a directional coupler that is load impedance independent.

    In 1982, I initially thought it was a better idea to calibrate the common relative voltmeters used in all amplifiers in peak power, because the rules were just changing. In hindsight, I learned people don't read manuals and expect any relative voltmeter that is only good at 50 j0 loads to be like a directional coupler. Had I realized that in 1982, the scale would just have been 0-10, or some other relative number. Hindsight is 20-20.

    :)


    As for grid current....

    At 100 mA, with the HV used in the 1500, grid dissipation is 8 watts.
    At 275 mA, grid dissipation is 20 watts.

    20 watts is the rating.

    175 mA sets grid disconnect at 13 watts or so. That is 65% of the CCS grid dissipation limit, which is more than conservative enough. 175 mA is safe for the tube, and prevents annoying false trips while tuning or if the exciter has overshoot.

    The only rig I ever owned that had so much overshoot it would false-trip the 1500 off was my Icom 775 DSP.

    73 Tom
     
  5. NZ9Y

    NZ9Y Ham Member QRZ Page

    Tom,

    Yup, the divisions are there on the meter, but the meter still clearly says 500,1000,1500,2000 watts. Do the divisions really matter? It doesnt say the scale changes, just the divisions.

    I dont expect perfect accuracy, but it would be nice if it wasn't off by 500+ watts.

    One other kind of silly question. I never noticed before, but staying keyed down for more than a few seconds shows the power drifting upward on all my meters. I suspect its my load heating and the impedance drifting. Is this what Im seeing?
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    What kind of load?
     
  7. NZ9Y

    NZ9Y Ham Member QRZ Page

    Oil can. I'll bet there's something I can do to disperse heat better. Aquarium pump :), more resistors spread throughout the liquid. I'll put it on my todo list.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Which "oil can" load, and what kind of oil?

    I visited a local ham two weeks ago who had a brand new MFJ "oil can" load, but didn't have transformer oil so he used automatic transmission fluid in it.

    It "boiled" the fluid easily with less than 1 kW output from his SB-220, and quite quickly. Smelled like a car that needed service.:p

    With ATF that load can probably handle 500W for a little while, 1 kW for seconds, and maybe not 1500W at all.

    With mineral oil, should be a bit better.

    MFJ rates theirs with the proper oil at 1 kW for 10 minutes, but I'm not so sure about that rating...

    My older Heathkit version ("Cantenna") with real, honest-to-god high temperature transformer oil in it can't really handle 1 kW for 10 minutes. Maybe for 2-3 minutes. At 1500W it gets quite upset.

    For testing big amps I use a bigger load, like my Bird 8251 which can actually handle 1000W for hours, and at 1500W for several minutes it doesn't seem to care. It's twice as big and heavy as the Cantenna, though.
     
  9. W8JI

    W8JI Ham Member QRZ Page

    The manual clearly says the meter accuracy will be off unless the load is 50 j0. This is right out of the manual, but I'll put some stuff in bold:

    That sentence explains it is an envelope following voltmeter.

    That explains the load has to be 50 j0 for accurate readings.

    That explains when the load SWR is off, the meter will be off.

    Depends on your load, but it was never intended to be an accurate power meter. It is a tuning aid. It is a relative voltage meter like almost every amplifier from that time era used, except it is calibrated in watts instead of 0-10. The shortfall is explained.

    It sounds like you have a poor load, which will cause meters to read differently. It is unlikely that a pump will fix it. It takes a load about 2 foot square with fins and a very large resistor to really handle 1500 watts or more. I just had an AL1500 up here, and it ran 2.6 kW PEP into the dummy load on most bands.

    73 Tom
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Bird 8251 will do it. I got mine at the local Swap Meet for $75. That was a good deal.

    It looks like an old fashioned steam radiator. It's oil filled (with "real" dieletric/transformer oil) but also has a lot of fins.

    It's big and heavy, but it has a carrying handle.:eek:

    One nice thing about the Bird load is it can not only handle a kilowatt all day long, but it's good to >500 MHz. I think its rated VSWR at 1000 MHz is 1.3.
     
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