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 Ham Member 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 Platinum Subscriber Platinum Subscriber QRZ Page

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

    AF6LJ Ham Member 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!
  11. KA9JLM

    KA9JLM Platinum Subscriber Platinum Subscriber QRZ Page

    If there is a problem in the ALC circuit in the amp, It could cause problems even if it is disconnected.

    But disconnecting the ALC is a great idea, for starters.
  12. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I think it is the same chassis, but re-purposing for a 3-500ZG probably isn't worth it. For one thing, the power supply is different and the power transformer is different. Note the power transformer in the 572 costs $142 (from Ameritron) and the amp weighs 40 lbs. The power transformer in the 80B costs $180 (from Ameritron) and the amp weighs 48 lbs. The power transformer in the 80B is beefier than the one in the 572.

    The 572 power supply provides 2700V, the 80B power supply provides 3100V. The filament winding would need to be different as well, since the 572 filament supply is 6.3V at 16A, while the 80B is 5.0V at 14.6A.

    I think this is "more work than it's worth.":eek:
  13. K1DNR

    K1DNR Ham Member QRZ Page

    Yes I suppose you're right again Steve. Maybe solid state down the road... thanks for sharing the info and wisdom.

    Oh yes ALC... dont generally use it and mostly cw. On ssb i set the drive power lower and use a fair amount of compression. So it always seemed like more trouble than it was worth. I am not married to that opinion. Just saying...
    Last edited: Feb 7, 2013
  14. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    On CW, there isn't any "IMD," so do what you will! The only thing that can happen if you overdrive the amp is possibly higher harmonic content and shortening tube life.

    I think the AL-572 is a perfectly good design, the only weakness is the quality of the tubes. If you get good tubes, you're good to go; and if not, they can fail. But the amp itself probably won't.
  15. K1DNR

    K1DNR Ham Member QRZ Page

    okey dokey... so I put 4 new tubes in, one set of Taylors (they came "matched") and one set of PRC from two different sources.

    Checked the bottom and made sure pins all had good contact with the sockets.

    Buttoned up the case, powered it on: Ip reads 200mA with amp in standby, no RF. On xmit (10-15W), no change on Ip, 0 Ig - very high SWR indicated by the exciter.

    50 Ohm dummy load on the output. SWR fine in standby
  16. K1DNR

    K1DNR Ham Member QRZ Page

    I put the old ones back in and all went back to normal, began swapping in the new ones - one of the tinned caps on the anode of one of the tubes came off. The glass seal looked ok. I applied some heat and a little solder inside, and the tube seems ok. Output on 20m with 70W drive into a dummy load is now 600W. Much better. SWR on the rig is fine.
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