From: David Kirkby <drkirkby@gmail.com>
Subject: Re: [Amps] PA efficiency
To: "Carey Lockhart, KC5GTT" <kc5gtt@gmail.com>
Cc: amps@contesting.com
Message-ID: <BANLkTik7gt5eZmp_R9Pp3DiTQveq8FrCbg@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
On 18 May 2011 13:20, Carey Lockhart, KC5GTT <kc5gtt@gmail.com> wrote:
> i was affraid of that. 58.3%. my gs31 2 meter amplifier i use for a standby
> isnt much more efficent than my gs-35 that just croaked. i have been using
> these yu1aw designs. have yet to get the rated operating paramters listed. i
> dont even have a clue where to start. i am wondering if there is a more
> proven design better suited to my skill level. any suggestions?
>
> Carey, kc5gtt
It's totally pointless stating the efficiency to 3 significant figures
(58.3%). Whilst if you use a DVM you can read DC voltages and currents
to well under 1%, you wont be able to measure the RF power to anything
like that accuracy. So your efficiency is probably somewhere between
48% and 68%. So worrying about the odd 1 percent is totally pointless
unless you use professional, laboratory quality test equipment to
measure the RF power.
In fact, I doubt your high voltage probe is sufficiently to measure
the voltage to better than 1%.
Dave
## I'm sure it's very easy to calculate eff a lot better than the..."48% -
68%"
range you mentioned. If you were really adamant about it, you could use a
calibrated fluke or hp dvm... and measure the loaded B+ across each
electrolytic
in any HV supply. And ditto with measuring plate current.. [ just insert the
dvm in series
with existing plate current meter].
## as for the wattmeter end of it... the bird is spec'd at +/- 5% of full
scale.....
anywhere on the scale. Which implies that unless the meter is nailerd right
to the
top end... you won't get +/- 5 %. IE: try using a 5 kw slug to meausre a
625 w
signal from an hf amp. It reads 700 w. Switch to a 2500 w slug... and it
now reads
675 w. Switch to 1 kw slug..and it now reads just 625 w. Which one do you
think
is the most accurate ? A 1 kw slug implies +/- 5% of 1 kw = 50 watts.
## So if the wattmeter with the 1 kw slug is used to measure a 100 w xcvr, the
meter could read say 100w. Now the real power could be 150w..and the meter is
reading 50 w low. or the real power could be just 50w... and the meter is
reading 50w
high. That's a huge margin of error..and obviously way more than +/-5%.
Point is..if
the bird meter isn't in the top end of it's scale... [ ie: use the smallest
slug possible]....
then your readings will be out to lunch.
## A few yrs back... I ordered up several NEW 2.5 kw HF slugs from Coaxial
Dynamics.
I shoveled them into the CD 83 wattmeter.. hot.. with amp running a cxr.....
and resulting
readings were close... but still all over the map..with some slugs high..and
some low.
Which one is correct... who knows? Put a heat gun or hair dryer near a
bird slug..and you
can watch the readings change 10% in front of your eyes. Take a reading when
room temp
in the shack is cold..like 60 deg F..then do it again when shack temp is 85
deg F.... and you
will be in for an eye opener. Same deal in a mobile set up..in summer time...
with car windows
rolled up..and sitting in the hot sun.
## I gave up on the bird/cd meter's..and now use the array solution's
'power-master' wattmeter.
It's not affected by heat, and if you don't like the calibration, you can
change it in software....in
+/- 1% increments..from a max of +15%.....down to a low of -15%. They
calibrate each coupler by
hand..and provide individual calibration number's to use for HF... one for
fwd..and a 2nd calibration
for rvs. Then 2 x more just for the 6m band. Same deal for their
vhf/uhf couplers. That's as good
as it gets..and good enough for me. The other problem with the bird/cd units
is severe lack of resolution...
esp at the upper end of each scale.
## You could also use a CALIBRATED scope across a dummy load... and measure
the rms voltage. Then
use V squared / 50 ohms.
## either way.... you can get something a bit more accurate than just..
48-68%. If I built an amp that I thought
had 68% eff.... then found out it was really only 48%... I'd go nuts. Not good
enough.
later.. Jim VE7RF
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