[Amps] AL-1500 'thump'

Sun Feb 15 17:29:03 EST 2026

Before Christmas, I had a YC209 (8877) go west in my AL-1500 over the 
course of te year, I would hear the occasional arc coming from the amp. 
Well, it didn't actually sound like an arc but there was definitely 
something that was not happy in there.

As you know, there are a million screws to remove and put back if you 
want to look inside. That, plus the fact that the amp resides in the top 
shelf of my amp rack which is home to 5 amps and I need my scissor lift 
stand to get it out and put it back in, deterred me from investigating 
to any extent.

Then, that fateful day, as the HV came up, the fireworks happened. 
Someone mentiond "pucker factor." Yeah, well that certainly happened. 
Anyway, turns out the tube was shorted and it took out a few items when 
it crashed. I fixed the broken bits and put in a good YU158 that I had 
kept for this day and the AL-1500 is working better than I've ever known it.

Oh, I read somewhere about the warm up timer in the AL-1500. My HV was 
coming on after 1 minute and 58 seconds. I changed a resistor value and 
now it is 3 minutes and 20 seconds. Perhaps the short warmup time over 
25 years contributed to the eventual failure of the tube. I mean, when 
the DX is there and you're waiting for the warm up timer to light up the 
HV, you hit it with RF as soon as it comes up. Maybe that was bad. 
Anyway, if it did contribute, then I should be OK from now on.

As it happened, If I'd taken it out, I wouldn't have found anything and 
I probably wouldn't have dragged out the very heavy Biddle hi-pot tester 
to test the tube.

So, your thumping could be a similar event, with more pronounced 
symptoms, warning you of the upcoming demise of your tube. It is not 
something you can see physically. The only way you'll know is to hi-pot 
test it as you mentioned. It might be worth doing. Enjoy all those 
screws and don't lose any.

73, Alek VK6APK.

On 16/02/2026 5:33 am, Steve Bookout, NR4M 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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