I have 38 years of experience with 8877 amplifiers - they are wonderful.
Everything I learned came from others (good engineers from Ohio) and by reading
everything I could about this wonderful tube and how to use it properly to
achieve bullet proof 100% duty cycle home brew amplifiers (mostly single band
RF decks) at 1500 watts output. I have 16 8877 amplifiers here.
For years I used (same as a SB-220 and L-4B) a 0.82 ohm two watt resistor as a
HV fuse. Any momentary short or glitch (plate to grid short) caused by a 8877
would instantly destroy (bang) the resistor and safe guard HV components. After
I built a 8877 tube tester (I was tired of blowing up good amps to find bad
tubes) with four meters and two variacs, I noticed that the tubes that blew the
HV resistor in the amps were OK in the tube tester. I surmised that the
short/glitch might be quick and "temporary" for these tubes in running
amplifiers and that they were still OK. So I have revised my HV fuse to be 25
ohms at 20 watts for these amplifiers. If there is a "real" big short (bad
tube) the resistor still blows, but those temporary/glitch plate grid shorts no
longer cause the amplifier to go bang with the 0.82 ohm version. I have some
8877s that are 40 years old - that are still running fine.
Always remember that your situation is likely to be different from mine. This
is just my experience.
Remember you are dealing with a huge HV voltage and you must always respect
SAFTEY first!
I never pull the cover or work on ANY amplifier/power supply alone. Never.
I love this reflector - I have learned so much.
73
Tim K3LR
-----Original Message-----
From: Steve Bookout, NR4M [mailto:steve@nr4m.com]
Sent: Sunday, February 15, 2026 4:33 PM
To: Tim Duffy
Cc: Gary Schafer; amps@contesting.com
Subject: Re: [Amps] AL-1500 'thump'
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@k3lr.com> wrote:
>
> AC mains "brown out"?
>
> 73
> K3LR
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Gary Schafer
> Sent: Sunday, February 15, 2026 3:53 PM
> To: Steve Bookout
> Cc: amps@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@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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