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AL-572 Low Grid Current and Output

Discussion in 'Amateur Radio Amplifiers' started by K1DNR, Feb 6, 2013.

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  1. K1DNR

    K1DNR Ham Member QRZ Page

    This is an area where my experience is badly lacking - I have an AL-572 amp which is about two years old now.

    It has served me well, but over the past few months I've seen the grid current go down (the meter shows 10-20mA) and power output is down to 300-500W on most bands.

    Tuning is the same on the Plate control, but the Load control is now lower and lower on all bands, and nearly zero on some.

    This is into a 50 Ohm dummy load, or my antennas (which are all well matched to 50+j0 Ohms at this point).

    I've found a few posts, and some info on W8JI's site. I've ordered a set of replacement tubes.

    Today I opened the amp up, and made sure the tube sockets were all tight. All the filaments light up.

    The amp is wired for 120V at this point, but output is low even for this supply voltage. (HV is in range)

    Other than putting in new tubes and trying it out - are there any suggestions? The amp is usable. Intuition tells me that one or two tubes are bad, but I can't tell by looking.

    Re-seating the tubes and sockets did not help. Visually, everything looks fine, solder connections good, etc.

    100W Drive

    Any advice appreciated, thank you.
    Last edited: Feb 6, 2013
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I agree. 572Bs aren't what they once were back when Cetron was making them.

    3-500ZGs, on the other hand, seem to be pretty good coming from China. I don't know if they're as good as the originals from Eimac, but they're pretty robust. It's the reason that I recommend the AL-80B over the AL-572 to almost anyone.

    I have a "Chinese" 3-500ZG (RFP branded) for six years now in my AL-80B and it works exactly the same today as it did six years ago when I plugged it in, and gets used daily at full power.

    I can only hope they keep cranking them out like that. Nobody knows.
  3. K1DNR

    K1DNR Ham Member QRZ Page

    Ok, thanks for the responses. In retrospect the AL-80B would have been a better choice, but we've certainly gotten a lot of good use out of the Al-572.

  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I think the AL-572 is a good amp, it's just the quality of the tubes that is always questionable.:p
  5. PA1ZP

    PA1ZP Ham Member QRZ Page

    Hi Charlie

    How about the swr at the rig?

    My tubes went soft and the power output got lower and lower.
    And the input SWR got higher and higher.

    Bought new Chinese tubes and they work wel , but i only use my amp every now and then.
    Maybe an hour a week.

    The original tubes lasted about 3 yrs.
    But at that time I was much more active on HF and used the amp often.
    But the new are in use now for 3 yrs and still work very good.

    Better knock on wood hihi.

    73 Jos
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Good story.

    But I know guys with Clipperton-L's (4 x 572Bs) from 1979 who use them every day and still have the original tubes in them!

    They made them better "back then.":eek:
  7. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    If the power is still low when you replace the tubes remove the cover on the bottom of the amplifier and check to see if there are 100 ohm resistors from the grid lead to ground along with bypass caps from the same pin to ground.
    The grid pin is easy to identify it will have those parts on the same pin of each tube....
    If your amp is old enough it will contain those parts....

    There was a mod from Ameritron that required the removal of those parts and the grid lead connected directly to ground.

    If that is how your amplifier is configured and you still have low output you have another problem and unless you have someone in your aria, you will have to send it back for repair.
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would check the ALC and make sure it is working properly.
  9. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Good idea or maybe unplug the ALC line from the radio for now......
    See if the power comes back to normal.
  10. K1DNR

    K1DNR Ham Member QRZ Page

    It has gotten higher - but not higher recently (within the past 6 months). I hadn't noticed because I use the tuner on rig to match the input these days (although I did manually tune the slugs originally).

    I did remove the bottom plate and cleaned the sockets of dust, and tightened them up and made sure all the pins were firmly in contact. I noticed the caps there, didn't see the resistors mentioned but I wasn't looking for them.

    Tom has a Gettering process for these tubes - not sure if this makes a small or large difference in the longevity of the tube.

    Even if I don't follow that process, my plan was to run the new tubes with much lower drive for a week or so as a break in. Again, not much experience in this area yet.

    As for the amp - I think the case is the same as the AL-80?? So perhaps - next time around we'll plan on using the chassis and power supply if appropriate to rebuild the amp. I realize the potential pitfalls in doing it - but if well thought out and researched it might make a good project!
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