AL80B tune up

Discussion in 'Amateur Radio Amplifiers' started by N5YPJ, Apr 5, 2013.

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  1. N5YPJ

    N5YPJ QRZ Moderator

    The Ameritron manual's tune up is a little confusing.This is my first amp but I have had tube final transceivers. Can anyone tell me the tune up procedure you do?
     
  2. W9JEF

    W9JEF Ham Member

    I don't have that particular amp, but here's what I do with my 4 x 811A home-brew amp: First, drive it with just enough power to get about 100mA of grid current. As you increase the drive, alternately tune PLATE and LOADING for maximum output. CAUTION: Check the manual for maximum grid current and never exceed it.
     
  3. WB2WIK

    WB2WIK Premium Subscriber

    Do you have the ALC line connected, to a compatible transceiver? If that's done and you make the ALC adjustment, it can be impossible to overdrive the amp and exceed the grid current rating...
     
  4. N5YPJ

    N5YPJ QRZ Moderator

    Yes I have the ALC connected and made the adjustment according to the manual.
     
  5. WB2WIK

    WB2WIK Premium Subscriber

    Then it's easy.

    I find zero reason to start out "tuning" at lower power, it's just a time vacuum.

    Drive the amp fully (assuming the bandswitch is in the correct position!) and just adjust PLATE and LOAD for maximum output power. They are interactive, so you have to do this 2-3 times. Takes maybe 5 seconds.

    Once they're both peaked for maximum output power, observe Ig (grid current) and turn the LOAD control slightly more "clockwise" to make the Ig go down slightly. You're done.

    I tune my AL-80B from a "cold start" in 5-6 seconds max. On the higher bands like 10m, tuning is more critical. On lower bands like 80-160m, it's very broad and not critical at all.

    The built-in wattmeter is quite accurate.
     
  6. K5AMH

    K5AMH Premium Subscriber

    Where did you learn about this trick? My AL-80B manual doesn't exactly indicate this last step (by monitoring the grid current) but does suggest a slight clockwise rotation of the LOAD control after maximum tuning to improve linearity.

    Gary
     
  7. WB2WIK

    WB2WIK Premium Subscriber

    I learned it the first time I used a linear amplifier, probably in 1966.:eek:

    Heavy LOADing does improve linearity while maintaining good efficiency. The "trick" works with almost any kind of vacuum tube RF amplifier, even my dual 4CX250B 2m amp. I always tune for max out then adjust LOADing for less grid current. In the case of an AB1 amp like the 4CX250B job, adjusting the LOAD current for "zero" (literally) grid current improves linearity and prolongs tube life -- a lot.

    I thought everybody did that.:eek:
     
  8. AF6LJ

    AF6LJ Ham Member

    When you read some of the tune-up instructions that come with amplifiers, you sometimes have to question who wrote them and why...
     
  9. AH6RR

    AH6RR Ham Member

    Here we go again!!!
     
  10. WB2WIK

    WB2WIK Premium Subscriber

    From http://www.w8ji.com, and the guy who designed the AL-80B which is the subject of the OP here:

    The most common amplifier tuning or loading error is adjusting an amplifier at low or reduced drive power as a last amplifier tuning step. When we load a radio or amplifier at reduced drive as a last tuning step, we establish that power level as the absolute ceiling for drive and output power. Final loading at reduced drive results in a loading control too-far meshed. This can causearcing, splatter, and excessive grid current.
    Ideally (if possible) we should make the final tuning and loading adjustments at or near maximum exciter drive power. Some amplifiers drive too easy to do this, so we should always pay attention tofactory instructions and avoid exceeding factory amplifier tuning current limits, especially for control and screen grids. Grid current isespecially important to watch because grids often do not have sufficient thermal mass toabsorb large overloads even for short time periods. Excessive grid current in metal oxide cathode tubes (ceramic tubes with indirectly heated filaments) like the 8877 and 3CX800A7 can damage tubes inless than a few seconds; whereas most anodes will tolerate severeoverloads for 15 seconds and longer. It is better to let the large anode or plate in a tube take the brunt of any mistuning heat, which means with any mistake it will be better to over-couple or have the load control capacitance slightly lower than optimum.
    The last few tuning steps should always be:

    • Load the amplifier to maximum obtainable output at full exciter drive (without exceeding amplifier short term overload ratings)
    • After that, advance the loading control very slightly beyond that point (towards less capacitance).


    But I had been doing this for more than 30 years before reading about it (although Bill Orr did describe loading vs. linearity, with some test data, decades ago and that was somewhat helpful) mostly to save my tetrodes and pentodes used in grid-driven service. In all of those amps, I could achieve the same output power and the same efficiency with "overcoupling" as with "peak for max" coupling...the difference is virtually zero. But with overcoupling, I could always adjust for literally zero grid current, keeping them AB1 where they were intended to be (like the 4CX1000A, which can't handle any grid current). Tuning for max suds and ignoring the final step of loading for 0 Ig shortens the life of such tubes a lot, sometimes down to minutes instead of years.

    What blows my mind is some companies actually manufactured and sold such amplifiers with no grid current metering at all!:p
     
  11. AF6LJ

    AF6LJ Ham Member

    I totally agree with that...
    Like I said some manufacturers instructions are a bit goofy.
    Not these....
    Everything I have read says linear amplifiers do good with an increase of five to ten percent more loading.
    :)

    WE are on the same page in regard to this subject........
     
  12. K5AMH

    K5AMH Premium Subscriber

    Maybe the manufacturers want to sell replacements for cooked linear tubes and make more money that way??
     
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