[AMRadio] Amplifier to use with my DX-60


Jim candela jcandela at prodigy.net
Fri Mar 17 14:45:24 EST 2006


Ken,

    The answer to your question is YES, But. The but comes from the fact
that linear amplifiers need to be monitored with a scope looking for proper
linearity, more so than plate modulation. I use the trapezoid pattern, and I
always tune for max as you describe, and then when cutting back the drive to
about 25%, I modulate while looking at the trapezoid. Almost always I have
to adjust the linear loading control for a linear ramp as I approach 100%
modulation. So with a linear you need the scope, and trapezoid pattern
selection to get things correct, and then set the audio level. With plate
modulation you only set the audio level.

So what percentage of this group uses a scope on AM?

So what percentage of the scope users utilize the Trapezoid pattern?

Remember for the Trapezoid you use scope in X:Y mode where the audio at
scope suitable level (about 10 volts peak) goes into the horizontal channel,
and the RF into the vertical channel.

Regards,
Jim

I have been called a square before, but never a trapezoid!


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of kenw2dtc
Sent: Thursday, March 16, 2006 7:25 PM
To: Discussion of AM Radio
Subject: Re: [AMRadio] Amplifier to use with my DX-60


Jim,

I guess my question is really:  Can anyone actually demonstrate a real AM
linear that can output 500 watts carrier and show 2000 watts PEP into a
dummy load with a nice looking audio sine wave on the scope, from the RF
pickup, while running 1500 watts DC input ?  If so, please send me an email
at kenw2dtc at comcast.net


The reason I ask is:  A.  None of my amps has ever been that good and B.
Unfortunately, a number of people running AM linears do not have scopes and
if you tell them that a pair of whatever tubes can achieve a certain carrier
power, they will put the carrier to that level via dipping and peaking the
pi-net until their wattmeter reaches the specified power and talk.  In my
experience, by the time you heavily load the output to achieve the perfect
audio sine wave, your tuning controls are quite different than what is was
when you peaked everything and the efficiency is no longer as good as what
the calculations show.  On top of that most male voices have peaks that
should have an additional de-rating of output power.

So when someone asks about what kind of carrier power a certain pair of
tubes can put out in AM linear service, I really like to give a conservative
number, not the math numbers.

73,
Ken W2DTC



______________________________________
Ken Barber  Middletown, NJ
Radio Website: http://w2dtc.com
Family Photos:  http://kenw2dtc.home.comcast.net
______________________________________


----- Original Message -----
From: "Jim candela" <jcandela at prodigy.net>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Thursday, March 16, 2006 7:22 PM
Subject: RE: [AMRadio] Amplifier to use with my DX-60


>
> Don,
>
>   You said: "So you could run maximum 1500 watts DC input to those tubes,
> with 500 watts
> carrier output, and 1000 watts dissipated by the tubes".
>
> Reply from Jim WD5JKO:
>
>   What is stated above is correct if the tubes can dissipate 1000 watts of
> heat (like two 3-500Z's flat out blushing), and the drive is adjusted to
> about 1/4th that for maximum PEP output. In this case the efficiency is
> assummed to be about 33%, so 1500 X .333 = 500 watts. The efficiency may
> vary some, and is usually lower then that, so the 33% assumption is only a
> first guess. It could be 25%, and that really upsets the situation making
> the linear more of a heater, and less of a transmitter.
>
>   One good thing about the 1500 watt PEP output power rule we all complain
> about is that efficiency doesn't matter as much now. You can legally run a
> 15 KW PEP input amplifier with 10% efficiency to provide 1500 watts PEP
> output. It wouldn't make a lot of sense, or be practical, but it's now
> legal. Back in the 1000 watts DC input days things were a lot different,
> and
> the SB-220's input would likely exceed 1000 watts DC before the AM carrier
> got to 375 like we often do today with these amps. The point here is that
> for low efficiency linears, or grid modulated amplifiers,  we can
> sometimes
> run more power today on AM then before the rules changed from 1000 watts
> DC
> input to 1500 watts PEP output. Of course this isn't the case with plate
> modulation where an output PEP at or maybe more than 3KW PEP WAS possible,
> and completely legal.
>
> Regards,
> Jim
> WD5JKO
>
>
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net]On Behalf Of kenw2dtc
> Sent: Thursday, March 16, 2006 4:21 PM
> To: Discussion of AM Radio
> Subject: Re: [AMRadio] Amplifier to use with my DX-60
>
>
> Don says:
>
> "So you could run maximum 1500 watts DC input to those tubes, with 500
> watts
> carrier output, and 1000 watts dissipated by the tubes"
>
> *******I need a little help with the math here Don.  You are suggesting
> that
> with a class B linear in AM mode, one can get 2000 watts PEP with 1500
> watts
> input ?
>
> 73,
> Ken W2DTC
>
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