Date: Thu, 16 Aug 2012 14:11:02 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
Subject: Re: [Amps] K3 spike problem
On 8/16/2012 5:59 AM, Joe Subich, W4TV wrote:
> In addition, if Charlie is using external ALC (he does not say) and
> has the K3's power control set too high, he will see the spike due
> to ALC time constants*in the amplifier*.
It should be noted that when high quality rigs like the K3 are used to
drive a power amp, amplifier output power should be set by using just
enough drive power, NOT by setting drive power higher and using ALC to
reduce it. For at least 30 years, Ten Tec has recommended AGAINST using
ALC with their rigs and power amps in this manner. The only good reason
for using ALC between a power amp and a transceiver is to protect the
amp in case of a failure in the antenna system.
### Unless the amp has a front panel adjustable ALC control knob,
it won’t work anyway. Also, the alc time constant in the amp HAS
to match the alc time constant in the XCVR..and NONE of em do..
so don’t waste your time.
In an excellent applications note on the topic, Tom, W8JI, noted that
many rigs produce a spike of their full power output when their drive
level is set well below full power. For those rigs Tom recommends a
passive attenuator between the rig and the amplifier. The K3 is NOT one
of those rigs -- indeed, it is quite well behaved. It's also one of the
cleanest rigs on the air.
### IOW, if the amp requires 50 watts of drive, then use a 3 db pad between
the xcvr and amp, then run the xcvr at 100 w output. IE: the pad is used to
control the power output, and not any control in the amp. If 25w drive
required,
then a 6db pad is required. Any pad is a PITA. The 3db pad has to be rated
for at least 50 w CCS. The 6 db pad has to be rated for at least 75 watts
CCS.
Then you also lose 3-6db on RX... which is the last thing you want.
## a simpler approach to adjustable po is to just put –9vdc across a 50 k
pot...then feed 0-9 vdc (neg) into the alc buss on the xcvr. Crank po
control on xcvr to max... then adjust pot for desired output..from 0-max.
No alc time constants to mess with, since you have already developed the
alc voltage with your –9vdc source. And no spike problem.
## The K3 is NOT a clean xcvr on ssb. It has typ lousy imd... like –29 db pep
for IMD3. That’s on a par with sweep tubes from the 70’s. It only does 100 w
pep out, and I require a 200-250w pep put xcvr..so no K3 on my desk.
Ten tec needs to go back to the drawing board, and
build a competitive xcvr.... not one that’s suited for cw only + grid driven
tetrodes.
Jim VE7RF
73, Jim K9YC
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