Jack,
Others have commented on their thoughts concerning carrier control. One
suggestion I have not heard however is to reduce the amount of carrier control
to no more than 3 db. This might be the ticket to gaining linear amp efficiency
without the nagging extreme carrier pump common to stock carrier controlled
rigs.
When using linear amplification on AM, it is always nice to tune for maximum
linear output or to a level that is 4-5 times the exciter carrier maximum. If
your exciter cannot do that then you could compensate by modulating the driver
to 100 - 120% positive as you set the linear controls. In your case this would
be easier with the TS-430 since it will surely drive a SB-220 to the maximum
(carrier only) whereas the T60 will need to be modulated to get there. Either
way will work, but you need a scope to look at the peaks if you are pulsing, or
modulating the drive. A peak responding watt meter might be useful as well.
I am hearing more an more AM signals using linear amplifiers, and about 2 out
of 3 are not set-up correct, and they are being overdriven, or maybe the linear
output loading is set for a lower PEP power impedance match. I am going to
state here as my opinion that with linear AM you should monitor the RF output
with a scope, and use the Trapezoid pattern when optimizing the linear
controls, drive level, and audio level. The envelope pattern is nice too, but
it is very hard to interpret RF non-linearity whereas it stands out big time
with the Trapezoid. I am set up to do both, and i switch back and forth
regularly.
If I have not bored you thus far, I have been corresponding with another ham
about using his TS-430 with a SB-200. I am pasting the conversation below as
it was written:
BTW, my math may be a little off, so be kind! :-)
Jim
WD5JKO
----- Original Message ----
From: Jack Schmidling <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Sent: Wednesday, October 11, 2006 12:08:26 PM
Subject: [eamarg] Intro
Not sure how to connect this with Europe but there seems to be more AM
activity on the internet than in the ether.
Been a ham since 1955, dropped out in the 60's, dropped back in, in the
80's with a rice box, out again till a few weeks ago.
AM intrigues me (I understand it) and I found a button on my TS430 that
says AM. I have been playing with the button for a few days and think I
am smitten.
I am looking for a real AM rig but in the meantime, I will see what I
can do with this thing and the SB200.
Have had a few qso's but a lot more non responses to trying to break in
on qso's. What's even worse, is no response from very loud CQ's. That's
pretty depressing. I answered a CQ last night 3 times and gave up to
find something else to do.
Got lots of questions but for openers....
There seems to be an area around 3885 where AMers hang out but I am not
clear on the regs regarding where AM is allowed. Could someone clear
this up?
Are there any AM nets on 75?
Jack NR9Q Marengo, IL
----- Original Message ----
From: Jack Schmidling <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Sent: Wednesday, October 11, 2006 11:45:18 PM
Subject: Re: [eamarg] Re: Intro
Thanks for the info. Now I have a more serious question.
I am trying to set up my TS430 and SB200 with a scope but the whole
procedure has me a bit befuddled.
My vintage ARRL handbook describes the process and I understand what is
going on but I do not see how one quantifies anything as it all seems to
depend on audio level.
I understand the trapezoid but am clueless how one uses this when
speaking into a mike.
I hooked an audio generator to a headphone and hold the mike up to it
and can make a textbook picture of 100% mod or anything I want just by
diddling with the generator output or the mike input.
If I hook the generator also to the Horiz and can get any shape
trapezoid I want again by diddling.
I just do not understand how I can do anything in the way of adjusting
the rig in the real world under normal operating condx.
What am I missing?
js
From: Jim Candela <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Sent: Thursday, October 12, 2006 8:22:11 AM
Subject: Re: [eamarg] Re: Intro
Jack,
When using a transceiver on AM, it kind of goes like this:
Tune for maximum output, then back off to about to 20-25% power for AM, and
advance audio till you see the power kick up slightly on audio peaks. With a
scope you usually look at the modulated envelope using sampled RF for the
vertical input, and time sweep on the horizontal of somewhere between 1-2 ms /
division. You can try to trigger on voice peaks to get a syllable on the
display when you say 'HELLO'. Back off audio if the negative peaks brighten up
in the middle at zero volts, or if the ALC is kicking in. Most 100 watt rigs
are good for 20-25 watts AM this way. At 20watts there is more room for
positive modulation peaks beyond 100% (a good thing).
The male voice is likely asymmetrical, and therefore on AM it is important to
have the side with higher peak level being positive modulation, and the other
side being negative modulation. One way to change this if wrong is to invert
the speech waveform, and can often be done at the microphone by swapping the
wires. Sometimes this is not possible however.
Some transceivers have a low level audio input for external modulation. This
usually drives the balanced modulator directly. This is another way to process
your audio, and then bypass the normal restrictive internal speech amp in the
rig. If this input goes straight to the balanced modulator, then it can be an
output as well. This allows you to connect your scope in X:Y mode for trapezoid
waveform. Keep in mind that for AM you use the 'modulated envelope' to look at
audio linearity and level, and the 'trapezoid' to look at RF linearity and
level.
With 50 ohm RF systems, sampling the RF can be done with a simple 10X scope
probe up to several hundred watts.
When you connect an amplifier things get more complicated. A dummy load is
required, as is the antenna must look like 1:1 swr so after tuning up on the
dummy load, you switch to the antenna, and the tuning settings don't need to
change. Long prolonged tune ups on the air are not a good thing. A tuner with
a SWR analyzer will work with the antenna need.
A SB-200 has 320 watts of plate dissipation with two 572B triodes. For linear
AM derate that 320 to 160, and then about 80% of that or about 125 watts for
AM. Don't push beyond that or the tubes and power supply will protest. Tune as
before to maximum into the dummy load to say 600 watts, and then back off to
150 watts by reducing the exciter drive, and then increase the linear LOADING
control (less capacitance) till the output drops slightly to about 125 watts
carrier. Then set the audio level as above.
With a linear on AM the trapezoid shows curvature at maximum PEP output, and
increasing the loading control slightly often straightens out that curve. The
cost is lower efficiency, less output, but the signal will be cleaner.
I hope this helps!
Regards,
Jim
WD5JKO
----- Original Message ----
From: Jack Schmidling <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Sent: Saturday, October 14, 2006 8:30:52 AM
Subject: Re: [eamarg] Re: Intro
Jim Candela wrote:
> A SB-200 has 320 watts of plate dissipation with two 572B triodes.
> For linear AM derate that 320 to 160, and then about 80% of that or
> about 125 watts for AM. Don't push beyond that or the tubes and power
> supply will protest.....
Thanks for the info. I removed the screen over the tubes and cracked
the lid enough to keep an eye on the plates. At 200 ma they glow after
a few minutes, at 100ma, it takes longer and the glow is very light.
I am not sure just how to interpret this. To what extent can or should
the plates redden in "proper" use?
js
----- Forwarded Message ----
From: Jim Candela <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Sent: Saturday, October 14, 2006 9:36:33 AM
Subject: Re: [eamarg] Intro - Linear Tuning
Jack,
I kind of went through that fast. A rule of thumb for AM linear
amplification is to divide the rated plate dissipation of your tubes used by
two and that is the maximum carrier output available. I expand on that and then
take 80% of that. So with a SB-200 you have 2x160 watts with the 572B rating,
or 320 watts / 2 = 160 watts X .80 = 128 watts. I suggested 125 in my last post.
For linear amplifiers, the amps at full output will get 60-70% efficiency for
class AB2 or Class B operation. True class B is biased at cutoff, and that
produces distortion around the zero point, so we bias for some idle current,
and are more Class AB2 than B. When we tune a linear to max output and back
down the drive to 25% of maximum for AM modulation, the efficiency drops in
half to 30-35% (Some linears will be ~ 25%).
So consider AM where we have 150 watts carrier (no modulation). We need the
headroom to modulate to 4X that value since with AM at 100% + modulation the
PEP output is 4X the carrier, or in this case 600 watts PEP. So lets say at 600
watts an SB-220 is 60% efficient. That means the DC input is 1000 watts, and
each tube dumps 200 watts of heat. The 572B however is only rated for 160 watts
and that isn't continuous duty either. So we tune quick, get it done. When we
back off the drive to 1/4th maximum power on AM things are different. First is
that the efficiency drops in half to about 33% and the carrier output drops to
150, and the DC input drops to 450 watts. So with a DC input of 450, and and RF
output of 150, the efficiency is 33% ( 150/450 X 100 =33). Each 572B must
therefore dissipate 150 watts. They will glow red at 150 watts. The power
supply will heat up too at continuous 450 watt draw from it.
If we drop the drive some more to cut the power, both the tubes, and the power
supply will be happier.
I mentioned in my last post about advancing the loading control slightly on the
linear after cutting the power back (drive) to 1/4th. On some amps this helps
RF linearity as seen with a Trapezoid pattern. Usually we do not want to mess
with the linear controls after you tune for maximum.
Regards,
Jim
WD5JKO
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