Ameritron AL-811H just went pop

Discussion in 'Amateur Radio Amplifiers' started by NI2M, Oct 11, 2021.

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  1. NI2M

    NI2M Ham Member QRZ Page

    Hi y'all!

    Was in the middle of a contact when all of sudden the AL-811H went pop, there was a smell that lingered for about a minute, the amp went completely dead and the power to it went off as well. I did notice something was afoot while I switched it on this morning as the RF power out was lower than "normal" with the same roughly 50W drive that I have used to excite the amp for years. Truth be told I thought that I had observed similar behavior in the amp for a few days prior as well, but thought it may have been just a "fluke." Obviously it wasn't.

    Last time I had to fix it was when I wasn't getting much power out of it at all. Turned out one of the caps in the power supply had popped (the pop I hear today was more loud). Decided to replace all caps and bleeder resistors while I was at it and the amp worked flawlessly ever since (for about 4 years).

    I try and limit my drive to max 50 W and am ordinarily getting 500W-600W out from the amp and leave it there, not trying to push it.

    I haven't yet taken the enclosure off the amp.

    Best 73 de Juha
    NI2M
     
    NQ1B likes this.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think you should open it up and look to see if anything burned.

    4 years is a long lifetime for 811As used in that particular amplifier.
     
  3. NI2M

    NI2M Ham Member QRZ Page

    Will do. The “tell” was similar to what happened earlier so it might be “just” a cap that popped. Let’s see.
     
  4. NI2M

    NI2M Ham Member QRZ Page

    Opened up the amp. Nothing "burned" jumped out at me........ One of the two fuses had blown (obvious from the fact that the amp powered off at the time of the "pop") and one of the 4 811H's has gone bad. When shaking the tube I can hear rattling made by a loose part as well although whatever is loose remains "hidden" and the tube shows discoloration and looks distinctly different from the other 3. Caps/bleeder resistors don't appear to be at fault based on cursory visual inspection.

    Lazy man's approach:
    Now, the question is if other components have also contributed to the tube blowing (possible) which would be prudent to establish before anything else. I think I have a "spare" tube that I could pop in to see if the amp will work "normally" and no fuse is blown. If amp works normally for a few days then I may acquire 4 brand-new tubes. The amp is at least 13 years of age (I am likely the 2nd owner if not the 3rd) and I have had it for about 7 years (bought 4 new tubes several years ago, prior to the cap damage in the power deck).
    Juha
     
    KA0HCP likes this.
  5. VA3DXV

    VA3DXV XML Subscriber QRZ Page

    You can power it up and run it with the suspect tube removed, just make sure to secure the anode cap well away from anything it could arc to.

    If all is well, replace them with 572B's (won't get much more output but they will be much more rubust to tuning mistakes)
     
    N3RYB and WA4SIX like this.
  6. WA4SIX

    WA4SIX Ham Member QRZ Page

    Ditto...

    Ed
     
  7. NI2M

    NI2M Ham Member QRZ Page

    Powered up amp with 4 tubes (the "spare" is from another manufacturer which isn't ideal but may help at this point).
    Plate voltage at 1round 1.6kV and no plate current. Amp on standby. Rocked toggle switch from plate voltage to plate current and it read
    around 35mA (it should be zero when there is zero watts of RF excitement). Before "rocking", it read zero as it should. W8JI writes that "If the amplifier operates normally but shows some bounding or changing plate current in all operating configurations, it is possible and common that S4 (HV/IP switch) has dirty contacts." Perhaps this is the culprit. Will clean the switch for a look-see.

    Will obviously need to establish whether the amp actually amplifies next and if it does it with any sort of consistency (assuming no other
    components contributed to the tube failure which I don't know as of this writing).
    Yes, I am aware of 527B's but they are much pricier than the 811A's so I don't I will fork over that kind of money but use will that money
    towards a brand-new amp. "Tuning mistakes" are fairly rare an occurrence as I know the amp well and I know which cap/inductor values will
    get the amp in the ball-park before I put any kind of power through the amp. They were more frequent when I used my home-brewed extremely
    high-Q mag loop (40/30 meter bands) although I was able to get in the ball-park there too with time. Tuning into the mag-loop may have precipitated
    the demise of that one tube though is my theory.

    Thanks!
    Juha
    NI2M
     
    Last edited: Oct 13, 2021
  8. NI2M

    NI2M Ham Member QRZ Page

    Thought "all was well," but apparently that is not the case.

    I made some contacts with the "spare 811A tube" from another maker which likely threw the neutralization out of whack. Was monitoring a band and had put the amp on stand-by mode.
    All of a sudden I observe the noise floor go up quite significantly, moving up the band by about a 100kHz I note a tremendously strong "interference" with the bandwidth being around 20 kHz,
    but the signal was +60dB/S-9 and seemed like the s meter scale wasn't enough. I decided to switch the amp off for a look-see and the interference disappeared.
    Today I received a matched set of new 811A tubes (I had a presentiment that these would make any difference to what I observed with amp on stand-by), put them in and had the amp
    on stand-by mode. After a few minutes I observed a tremendously strong noise that was very wide-band (I was on 20 m just like I was previously) and at least +40dB/S-9. Switched the amp
    off and the noise disappeared.
     
  9. N3RYB

    N3RYB Premium Subscriber QRZ Page

    Out of curiosity what band were you tuned up on?
     
  10. NI2M

    NI2M Ham Member QRZ Page

    20 m. All of what I have observed took place on that band. The very wide-band noise will likely show up on all bands although I haven't bothered checking it.
     

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