KPA 500 voltage drop on key down

[Matt Murphy]

I just set up my KPA 500 and the HV on standby is 72.5 volts.  On key down,
however, it drops to 48 volts and the lights in the shack noticeably
flicker a bit.

Would changing a the tap on the transformer potentially help things? Or is
the real issue the voltage drop between the pole and the wall socket?

Suggestions/ideas would be much appreciated, as well as tips for debugging
any issues with the household wiring.

Per the KPA 500 manual, the amp may fault if the voltage under load gets
below 60VDC, so running in it with reduced drive may be necessary. While
doing that wouldn't be the end of the world, I'd like to address the root
cause if possible.

73,
Matt NQ6N
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[Cliff Frescura]

Sounds like the amp is plugged into an outlet that is potentially overloaded
or underrated.

Changing the tap won't solve the problem and may damage the amp.

73,

Cliff K3LL

-----Original Message-----
From: Elecraft [mailto:[hidden email]] On Behalf Of Matt
Murphy
Sent: Friday, November 20, 2015 7:46 AM
To: Elecraft Reflector
Subject: [Elecraft] KPA 500 voltage drop on key down

I just set up my KPA 500 and the HV on standby is 72.5 volts.  On key down,
however, it drops to 48 volts and the lights in the shack noticeably flicker
a bit.

Would changing a the tap on the transformer potentially help things? Or is
the real issue the voltage drop between the pole and the wall socket?

Suggestions/ideas would be much appreciated, as well as tips for debugging
any issues with the household wiring.

Per the KPA 500 manual, the amp may fault if the voltage under load gets
below 60VDC, so running in it with reduced drive may be necessary. While
doing that wouldn't be the end of the world, I'd like to address the root
cause if possible.

73,
Matt NQ6N
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[Jack Brindle-2]

I agree with Cliff. You want to use the largest power cabling possible when feeding the KPA500 (or any amplifier). The KPA500 goes from very little current to 10 amps or more very quickly. Resistance in the power wiring gets translated into heat loss and voltage drop, which you see as the lights dimming. This is one of the reasons many amp owners run their amps on 230V instead of 115V. It’s also the reason we ship a very beefy power cable with the KPA500. Unfortunately we can’t do anything about the wiring in your wall. You might want to have it checked or use a non-loaded outlet for the KPA500.

I don’t believe that the manual actually states the lower limit for the 60V supply. For the record, the KPA500 will fault if that supply drops below 40 volts. This is really an overall power supply check - I don’t think I have ever seen the supply drop that low except in testing. The upper hard limit is 90 volts, which is where the transformer taps come in to play. You don’t want the supply to go above 90V when idle, so we have you set the tap according to your line voltage to make sure this is handled properly. If you do see a high voltage fault, then you should actually change the tap for a lower voltage to make sure we stay within limits.

73,

Jack Brindle, W6FB
Elecraft Engineering

> On Nov 20, 2015, at 8:13 AM, Cliff Frescura <[hidden email]> wrote:
>
> Sounds like the amp is plugged into an outlet that is potentially overloaded
> or underrated.
>
> Changing the tap won't solve the problem and may damage the amp.
>
> 73,
>
> Cliff K3LL
>
> -----Original Message-----
> From: Elecraft [mailto:[hidden email]] On Behalf Of Matt
> Murphy
> Sent: Friday, November 20, 2015 7:46 AM
> To: Elecraft Reflector
> Subject: [Elecraft] KPA 500 voltage drop on key down
>
> I just set up my KPA 500 and the HV on standby is 72.5 volts.  On key down,
> however, it drops to 48 volts and the lights in the shack noticeably flicker
> a bit.
>
> Would changing a the tap on the transformer potentially help things? Or is
> the real issue the voltage drop between the pole and the wall socket?
>
> Suggestions/ideas would be much appreciated, as well as tips for debugging
> any issues with the household wiring.
>
> Per the KPA 500 manual, the amp may fault if the voltage under load gets
> below 60VDC, so running in it with reduced drive may be necessary. While
> doing that wouldn't be the end of the world, I'd like to address the root
> cause if possible.
>
> 73,
> Matt NQ6N
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html Message
> delivered to [hidden email]
>
>
>
>
> ______________________________________________________________
> Elecraft mailing list
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[Guy Olinger K2AV]

Hi, Matt,

On Fri, Nov 20, 2015 at 11:13 AM, Cliff Frescura <[hidden email]> wrote:

> Sounds like the amp is plugged into an outlet that is potentially
> overloaded
> or underrated.
>
> Changing the tap won't solve the problem and may damage the amp.
>

Precisely so!

Put an AC voltmeter on the AC line in a spare socket where the KPA500 is
plugged in. Key it and watch the AC voltage. If it sags any more than 2 or
3 volts, you have a wiring problem of some sort somewhere. Amplifiers
really should be on their own circuit (or a good stiff circuit if shared)
back to the breaker box. *Especially* so if its 120V, even a KPA 500, which
will draw a KW from the AC key down.

Precipitous keydown drops in AC voltage at the socket means a fire hazard,
and possibly some nasty problems in your AC wiring. No fooling around with
this problem. Sometimes these kinds of things come from a problem in the
power line neutral somewhere. That is really dangerous.

73 and Good Luck,

Guy K2AV
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[Elecraft mailing list]

Is the amp running off of 240?
The amp does draw quite a bit and really works best on 240


      From: Matt Murphy <[hidden email]>
 To: Elecraft Reflector <[hidden email]>
 Sent: Friday, November 20, 2015 10:45 AM
 Subject: [Elecraft] KPA 500 voltage drop on key down
   
I just set up my KPA 500 and the HV on standby is 72.5 volts.  On key down,
however, it drops to 48 volts and the lights in the shack noticeably
flicker a bit.

Would changing a the tap on the transformer potentially help things? Or is
the real issue the voltage drop between the pole and the wall socket?

Suggestions/ideas would be much appreciated, as well as tips for debugging
any issues with the household wiring.

Per the KPA 500 manual, the amp may fault if the voltage under load gets
below 60VDC, so running in it with reduced drive may be necessary. While
doing that wouldn't be the end of the world, I'd like to address the root
cause if possible.

73,
Matt NQ6N
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