[AMRadio] TS430 as VFO on a KNIGHT T60

Jim Candela jcandela at prodigy.net
Sun Oct 15 17:15:41 EDT 2006


  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! :-)

----- Original Message ----
From: Jack Schmidling <jack at schmidling.com>
To: eamarg at yahoogroups.com
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 <jack at schmidling.com>
To: eamarg at yahoogroups.com
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?


From: Jim Candela <jcandela at prodigy.net>
To: eamarg at yahoogroups.com
Sent: Thursday, October 12, 2006 8:22:11 AM
Subject: Re: [eamarg] Re: Intro


    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!


----- Original Message ----
From: Jack Schmidling <jack at schmidling.com>
To: eamarg at yahoogroups.com
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?


----- Forwarded Message ----
From: Jim Candela <jcandela at prodigy.net>
To: eamarg at yahoogroups.com
Sent: Saturday, October 14, 2006 9:36:33 AM
Subject: Re: [eamarg] Intro - Linear Tuning


   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.



More information about the AMRadio mailing list

This page last updated 15 Dec 2017.