[AMRadio] Drake L-4B on AM

Gary Schafer garyschafer at comcast.net
Thu Mar 6 15:06:16 EST 2008

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of A.R.S. - WA5AM
> Sent: Thursday, March 06, 2008 10:43 AM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Drake L-4B on AM
> On Thu, Mar 6, 2008 at 9:37 AM, kenw2dtc <kenw2dtc at comcast.net> wrote:
> > "BTW, I tuned the amp for exactly 1500 watts output, then reduced the
> drive
> >  to exactly 25 watts without re-tuning the amplifier"
> >
> >  Actually the best way is to tune for 1500 watts out and reduce the
> drive
> >  until the output is 375 watts under carrier conditions.  Also if you
> have
> >  one of those male voices that that will peak more in one direction than
> the
> >  other (if the audio is phased correctly) you must also de-rate the
> output
> >  power even more.  Naturally, a scope and an audio generator is the
> preferred
> >  way to tune up a linear under AM conditions.
> >   73,
> >  Ken W2DTC
> You're right Ken.  I think I used 25 watts as a good figure, however
> 544 watts of carrier should modulate to over 2100 watts with a pure
> sine wave.  I may try it again later and reduce the drive level to the
> point where the carrier output reaches 375 to see the difference.  I
> suspect if anything the eff. will drop even more...
> Thanks.
> Brian / wa5am

With an amp properly tuned for AM operation the carrier efficiency should be
(theoretically) exactly 1/2 of what it is at PEP or full carrier out. Also
drive power should be right around 25% of what it takes for full carrier.

An amp that puts out 1500 watts with 100 watts of drive should put out
around 375 watts with 25 watts of drive, assuming the amp is linear. Not all
amplifiers are perfectly linear so this can vary one way or the other a
little. This particular amp would have a gain of 15. The gain of 15 figure
should hold close no matter how much drive is applied. The output should
always be 15 times the drive power if the amp remains linear.

The efficiency will vary with drive power. All these parameters are assuming
that once the amp is tuned up for maximum output that the tuning is not
touched as drive power is varied.

If the efficiency of this same amp is 60% at full power out (1500 watts)
then the efficiency at 375 watts out should be right close to 30%.
As modulation is applied the efficiency of the peak envelope power will be

Gary  K4FMX

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