[Amps] AL-1500 'thump'

[Gary Schafer]

An 8877 can arc and burn away a small point where it arced and be ok after that. But this usually happens with a new tube.
If the grid was over heated it could happen.

Good luck with it
Gary k4FMX

Gary Schafer


> On Feb 15, 2026, at 3:33 PM, Steve Bookout, NR4M <steve at nr4m.com> wrote:
> 
> Good suggestion, but don’t think so.  Solid AC line and nothing dimmed.  Computer would have reset.
> Didn’t see any type of glitch from the amp meters as I was facing the amp.
> The other fellow, Gary, mentioned tube arcing.  Surely, could be that.  I can pull the tube and hi-pot it. Unless, some really marginal voltages before a fault, that might not tell me much.
> I’ve got several tubes and I could swap one out, but does not PROVE anything.  With this tube, I may never see it happen again.
> Of course, I’d love to never see this become a REAL problem.  It would really define the problem, but may not be good for the equipment.
> I had a Millen HV connector arc over 20 years ago.  The problem was easy to find, but it took months to find all the damage it caused.
> I appreciate you and Gary sharing your thoughts.
> 
> Steve NR4M
> Sent from my iPhone
> 
>> On Feb 15, 2026, at 4:08 PM, Tim Duffy <k3lr at k3lr.com> wrote:
>> 
>> AC mains "brown out"?
>> 
>> 73
>> K3LR
>> 
>> -----Original Message-----
>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Gary Schafer
>> Sent: Sunday, February 15, 2026 3:53 PM
>> To: Steve Bookout
>> Cc: amps at contesting.com
>> Subject: Re: [Amps] AL-1500 'thump'
>> 
>> Sounds like the tube arcing.
>> 
>> 73
>> Gary K4FMX
>> Gary Schafer
>> 
>> 
>>>> On Feb 15, 2026, at 12:40 PM, Steve Bookout <steve at nr4m.com> wrote:
>>> 
>>> Hello all,
>>> 
>>> Had a friend using the station this weekend, and while sitting there with him, while operating on 40 meters, we both heard a 'thump' or 'thungg' from the amp while transmitting.  Happened two or three times or a period of a few hours.
>>> 
>>> Reminded me of the sound the current surge makes when you first turn the HV on in an amp.
>>> 
>>> Puzzling to me cuz if that was it, why and why now?   And, no other follow up issues.  I don't think a PS filter cap would 'sorta, kinda' short, then not and be OK.   It would only be a small part of a larger 'event'.  (Been there; done that and still remember the pucker factor.)
>>> 
>>> Amp was being operated in a typical fashion, with resonant load, 25 watts of drive, ~ 40 ma grid current,  +/- 1200 watts RTTY.
>>> 
>>> Dealt with many amps before, but have not heard anything like this in the past.  'If' it had a soft start to deal with initial current surge, I might think there was a problem there, but there isn't one.
>>> 
>>> Ideas?
>>> 
>>> 73 Steve NR4M
>>> 
>>> 
>>> 
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