Date: Sat, 28 Jul 2012 11:30:38 +0930
From: "Leigh Turner" <invertech@frontierisp.net.au>
Subject: Re: [Amps] Overdrive protection / simple remedy
To: <amps@contesting.com>
Message-ID: <20120728014210.035EC230345@mail.aitel.com.au>
Content-Type: text/plain; charset="us-ascii"
Excellent solution Bert: always guaranteed to work and set a hard-limit on
the misbehaving / overshooting rig's max power output; and ensure that the
Po can NEVER go above what it is set for to drive the linear amp.
Leigh
VK5KLT
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Bert Almemo
Sent: Saturday, 28 July 2012 11:12 AM
To: 'Joe Subich, W4TV'; amps@contesting.com
Subject: Re: [Amps] Overdrive protection
There is a simple solution. I use a 9V battery and a pot to set an
appropriate/fixed ALC voltage to my 746 for the drive power I want. Works
like a charm and no overshoots.
73 Bert, VE3NR
### Nothing new here Leigh ! I heard about the 9v battery and 50 k pot idea
back in the mid 70’s
It works damn good too, even these days with newer xcvrs. IE’ adjust the pot
for say 86.5 watts....and
scream all you want, with mic gain fully CW..and u will get exactly 86.5 watts
po.
## u don’t have to wait to develop alc inside the xcvr, to throttle the po
to 86.5 watts on a 200 w xcvr,
simply cuz the 9 v battery + 50 k pot has already developed the alc voltage
for you. IE: zero time constant !
### array solutions uses the 9 V battery + pot in their power master
wattmeters to shut down the xcvr, in
an alarm condition, like high swr, etc. that works better than opening off the
key line and hot switching stuff.
In that case, pot is adjusted such that the XCVR puts out ZERO watts.
.### The alc in the Drake L4B is front panel adjustable. BUT its made for the
drake t4XC TX..and nothing else !
If u want to use a L4B amp with a drake TR-7 xcvr,OR say a Drake L7 amp with
a drake T4XC TX.. then you
HAVE to change one cap in the amp in each case.
## The ALC time constant in the amp HAS to match with the alc time constant
in the xcvr.... AND the alc HAS
to be adjustable. Hence the right timing cap has to be used for any given
xcvr- amp combo. The alc was used
back in the 70’s simply cuz all tube radios back then had NO power output
control.. ( and a mic gain is NOT a po
control). Now that all worked cuz a drake tx put out 120 w..and the amp
needed 100w.
## It doesn’t work, when trying to use a swan 700CX to drive a SB-200.
## For CW..my yaesu 1000-D has a front panel DRIVE control. Just set it so
ALC is either zero. or barely moves with
XXX watts out. 80 watts out and zero alc is cleaner than 80 watts out and
globs of alc showing
## on my yaesu MK-V... there is a drive control for TX... for each band, in
the menu. Same deal for cw. turn the tx gain
DOWN, such that alc is either zero, or very little for XXX watts out.
Factory default is set WAY too high for each band.
## On ssb mode, either use the 9v battery + 50 k pot trick ( and feed the NEG
0-9 vdc into the xcvr alc jack) and OR
use an outboard audio nolise gate-downward expander- compressor-
limiter-distortion cancelled clipper set up.
With that, its easy to get 1500w pep out ..AND loads of talk power...AND have
ZERO alc showing on the xcvr !!
## I can set the po control to 200w pep out on the MK-V..and tweak the
outboard audio gear such that pep out
is exactly 80 w pep. Of course, zero alc.
## for u folks with a yaesu MK-V... there is three power levels in the menu.
10-75-200 watts. Each has its own pot
for calibration. I found that each pot will indeed allow me to tweak from
3-200 watts. I took the “75” watt option
in the menu..then tweaked its calibration pot so its now 100w max. By doing
this, the po control now becomes 2-100watts,
which is a lot better than using the 2-200 watt menu option. I have also
seen folks take the “10” watt menu option...
and tweak its pot for 45 watts max.. or say 30 watts max, etc.
## for best imd, use Class A... or if using class AB...find the po that
produces min IMD..which may well be like 60-80% of
max power. Then if its still too much po... then add some padding. IE:
min imd is say 130 watts on a 200w xcvr.
But we only need 65w to drive the 8877...so use a 3db pad and run the radio
at 130w pep out.
Later...... Jim VE7RF
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