[Elecraft] KPA3A Failure & related

[Bob McGraw K4TAX]

This discussion has risen its nasty head earlier.  Wes and I exchanged 
several private e-mails on the topic.  Here is what I've learned:

My purchased new from the factory my K3S -100 in August of 2015. With it 
I drove my AL-80B with 60 to 80 watts for some two years.   Absolute no 
issues of concern.    In April of 2018 I purchased a new KPA500 and put 
it in place of the AL-80B.   I drove the KPA500 with my K3S at 15 watts 
for some 2 weeks.  Then reports of "fuzzy audio" surfaced.  The 2 tone 
test revealed excessive IMD.  I contacted Tech Support and they arranged 
to send me a LPA3 and a KPA3A.   These were installed and all was well 
for about one week.  Then again the reports of "fuzzy audio".  I 
contacted Tech Support again and they said "send it in".   The service 
report indicated they replaced the KPA3A with a rev E which is reported 
to be more stable.  An interesting comment on the Elecraft Service 
report; "Due to failure of the stable rev E amp, this indicates 
something in the shack.   Possible kickback from and amplifier.   It is 
likely to happen again if the root cause is not found. ........"    
UGH...folks, the amp is a KPA500!     >> HOUSTON - DO WE HAVE A PROBLEM?>>

In the discussions via e-mail and here on the reflector which followed, 
I learned of others which experienced the same type issues.  I asked 
"what's the solution".   Interestingly the answer came back [I don't 
recall from whom}  "stay away from 15 watts". Since then I have driven 
my KPA500 with 20 watts .....PLUS  I have connected and engaged the ALC 
from the KPA500 to my K3S.  This is a menu item in the K3S.  Now some 
may not like the ALC connection and activation.

My thinking is that IF there are any spikes originated with the rise of 
the CW signal or voice signal this may overdrive the amp.   With my P3 I 
see this A LOT on SSB signals on the bands which exhibit initial ALC 
overshoot.  This is both on CW and SSB.    Makes me wonder what does the 
input impedance of the KPA500 does when hit with a millisecond or so of 
excessive drive?   Not long enough to activate the attenuator or throw a 
fault signal. I do know that tube amps reflect to their input the 
effective tuning of the output.   Could this be an issue with solid 
state amps where the input impedance goes to zero thus placing in effect 
short on the driving transceiver?   Mind you not long enough to throw a 
fault signal but........what does it do to the driving stage?     I've 
not measured it and am not exactly sure how to accomplish this.  It may 
not be the issue at all.   Perhaps some of you more technical gurus can 
expound further on the topic.

I do know factually since the change to the KPA3A  rev E, and I 
activated the ALC between the KPA500 and my K3S the issue has not reared 
its ugly head.   Lets hope it doesn't.

All in all, I am VERY pleased with Elecraft Tech support and extremely 
pleased with my K3S, P3, KPA500 and KAT500.  A winning combination.   
But I'm still walking on eggs  and hoping none break.

73

Bob, K4TAX



On 2/16/2019 10:24 AM, Wes wrote:

> Unfortunately, you're wrong.  My K3S has failed multiple times.  
> Fortunately, my KPA500 has not.  I'm not in the market for a KPA1500.
>
> Wes  N7WS
>
>