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Author Topic: class C linear amplifier  (Read 78715 times)
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kb3ouk
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« Reply #75 on: January 15, 2013, 08:23:58 AM »

Carl, I posted about the 304TL amp and the consensus was not do build it.  But I see now what the designer was thinking.

Tom,  I am worried that on a GG Tride amp, the drive power will have to go way up.  If it acts like this Tbolt, its gonna need more then the 100 watt rig most have.

I cant wait to see your results.
C

And that's the problem I know I'll run into, driving the 4 572Bs in the hunter to 150 watts takes about 15 to 25 watts, but that's why I'm considering building a driver that consists of a 1625 modulated by a pair of 6CD6s (sweep tubes that are basically an economy version of an 807), I have most of the parts on hand, just need the mod iron (I'm thinking a TCS modulation transformer would be perfect). That thing with 600v on the plate should do about 40 watts, which should be more than enough drive. With GG amps, it's easy to have too little drive power. I also have to do some work on the input circuit, the current input circuit is showing a unusually high SWR, so a lot of the drive is reflecting back to the FT-901.
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« Reply #76 on: January 15, 2013, 09:24:31 AM »

Quote
Carl, I posted about the 304TL amp and the consensus was not do build it.  But I see now what the designer was thinking.

Which amp was this Clark?

Quote
Tom,  I am worried that on a GG Tride amp, the drive power will have to go way up.  If it acts like this Tbolt, its gonna need more then the 100 watt rig most have.

Im thing more along the line of one of the vintage 100-150W AM rigs or a SB-200 style IPA Roll Eyes

Carl
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« Reply #77 on: January 15, 2013, 12:03:08 PM »

Im thinking more along the line of one of the vintage 100-150W AM rigs or a SB-200 style IPA Roll Eyes
Carl

Yes!  There's nothing wrong with an oversize IPA (intermediate power amplifier) that just loafs along.  The IMD results can be excellent, rather than pushing a single amp to its saturation limit.

For a coupla years I ran a pair of 3-500Z's with 1500 v and loaded very heavily (less C2) to put out about 50 watts to drive my main linear amp.  Eimac shows the 3-500Z's at 1500V to be upwards of  -45 dB 3rd order IMD.  This is stellar and an easy way to start with a beautifully clean signal to drive whatever we wish.

Nothing  more depressing than spending the time to build up a clean final amp and having a trash exciter to muck up the works.  Remember the rule - the final signal output will NEVER be any better than the worse signal in the chain. If our final amp is -45db 3rd and one of the earlier amplifiers is -25db 3rd, then we will get something less than -25db 3rd out of the final amp.  Quite the motivation to work on the whole chain as a unit and use very conservative power demands on each stage.

I am finding that just cutting the power in half, from near saturation to half, can change a signal from abomidable to very nice.  It may mean an extra stage to get where we want to go, but WTF.

Shelby, sounds like you have the right idea with your new plans for the 40w exciter. Work to make it as clean as possible as a stand-alone unit and it will then complement your system when added.

T
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« Reply #78 on: January 15, 2013, 12:26:48 PM »

I have a friend that has a large amp, Its not on the air and is in storage.

The amp has Alpha 77 buttons, meters and controls.  It has two 3-500's Driving a single 3cx3000a7.  The power supply is common but I think the 3-500s are running at lower voltage somehow. The transformer is massive. Must be 300 lbs easy.  At first glance we thought, What the world would anyone go trouble all this trouble to build 3-500's driving a single 3cx3000a7 for! 

Tom, I think I now know what the builder had in mind....


Carl,  The amp I was thinking of was the 304TL or 304TH in GG (or how about a 450TL or TH) You could drive it with the 100 to 150 watt Ham transmitter.  I made a thread about it but everyone started talking about higher power/ lower drive amps which was I was not interested in.. So the thread just died.  I like running the low power rigs here and I thought it would be really neat to have a period correct 304 Amp that would take full drive from any of the typical ham transmitters, Even if the power output would only be 4 times.

I know a man that has a GG Class C, 450TH amplifier that is Driven by a DX100B.  That is then modulated by two 250TH's.  Its one of the loudest radios I have ever heard.  Massive audio.  Makes no sense, as the feed through drive power is not modulated but I have heard the thing and its very loud.
   

C
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« Reply #79 on: January 15, 2013, 03:18:29 PM »

This thread has been very interesting. Anyway, while sipping a coffee this morning I played around with my AL-82 as it was being driven by my Flex 5K on AM. The AL-82 has two series connected 7.5v 10 watt zener diodes in the center-tap return lead of the filament transformer to set the bias for the pair of 3-500’s in the RF deck. On the front panel there is a switch labeled SSB/CW that shorts across one of the zener diodes when in SSB to produce around -7.5 vdc of bias. When it’s switched to CW it puts both zeners in series to produce around -15 vdc of bias. My observation is that there’s quite a bit of variation among these diodes when purchased from Ameritron and they admit that you can see quite a difference in resting current depending upon the charactersitics of the diodes you end up installing. My two zeners are certainly very different from what I can see by the idling current as I switch it between one or both zeners in series.

At any rate, I recorded a few parameters as the AL-82 idled and then recorded the same parameters when it was driven to 100 watts and then 200 watts of carrier power. The plate voltage with no current being drawn is 3700 vdc and in this test it dropped down to between 3600 vdc and 3500 vdc depending on the amount of current being drawn as the tubes were driven. As expected, and as others have noted, it took more power to drive the tubes when they were moved closer to cut-off.

I did not see any difference while watching the scope between either the SSB or the CW bias setting while modulating to 125% positive peaks. I ran it on 10 meter AM and made 2 contacts there and later tried it on 40 meters where I spoke with 2 stations for roughly 45 minutes. Everyone reported that my signal was perfectly clean. I did observe that the 3-500 plates were orange at the bottom and bright red at the top after a few minutes while running 150 watts of carrier with the bias set for SSB. With the bias set for CW after a few minutes of operating at 150 watts of carrier the plates were bright red at the bottom and dull red at the top. That certainly leads me to believe they are running significantly cooler while biased with –15 vdc.


* AL-82 bias 7.5 or 15.jpg (36.46 KB, 1038x277 - viewed 1122 times.)
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« Reply #80 on: January 15, 2013, 03:41:05 PM »

With the chimneys you should easily be able to run 350-400W carrier. Are those Eimac or Chinese graphite tubes?

Time to change that CW zener to a higher voltage or a series string of reverse connected 1N5408 and select increasing voltage steps.

Carl
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« Reply #81 on: January 15, 2013, 04:00:31 PM »

Thank you for running the tests and posting the data.   It seems you cut the plate current down by 100MA at idle and 25ma or so at 200 watts.  I wonder if you retuned the amp after switching to CW?   I noticed that if I did retune, the current was even lower and with CW mode.. 

I see you are at 75MA idle in CW mode.  On my Amp, There is no idle as the drive provides the bias so I think that would be removed also.  That makes sense as I was showing a 100 MA reduction across the board. 

That is a neat feature of the ameritron amp.  I am used to the old SB220 where the plate voltage is cut way down in the CW mode.

C


This thread has been very interesting. Anyway, while sipping a coffee this morning I played around with my AL-82 as it was being driven by my Flex 5K on AM. The AL-82 has two series connected 7.5v 10 watt zener diodes in the center-tap return lead of the filament transformer to set the bias for the pair of 3-500’s in the RF deck. On the front panel there is a switch labeled SSB/CW that shorts across one of the zener diodes when in SSB to produce around -7.5 vdc of bias. When it’s switched to CW it puts both zeners in series to produce around -15 vdc of bias. My observation is that there’s quite a bit of variation among these diodes when purchased from Ameritron and they admit that you can see quite a difference in resting current depending upon the charactersitics of the diodes you end up installing. My two zeners are certainly very different from what I can see by the idling current as I switch it between one or both zeners in series.

At any rate, I recorded a few parameters as the AL-82 idled and then recorded the same parameters when it was driven to 100 watts and then 200 watts of carrier power. The plate voltage with no current being drawn is 3700 vdc and in this test it dropped down to between 3600 vdc and 3500 vdc depending on the amount of current being drawn as the tubes were driven. As expected, and as others have noted, it took more power to drive the tubes when they were moved closer to cut-off.

I did not see any difference while watching the scope between either the SSB or the CW bias setting while modulating to 125% positive peaks. I ran it on 10 meter AM and made 2 contacts there and later tried it on 40 meters where I spoke with 2 stations for roughly 45 minutes. Everyone reported that my signal was perfectly clean. I did observe that the 3-500 plates were orange at the bottom and bright red at the top after a few minutes while running 150 watts of carrier with the bias set for SSB. With the bias set for CW after a few minutes of operating at 150 watts of carrier the plates were bright red at the bottom and dull red at the top. That certainly leads me to believe they are running significantly cooler while biased with –15 vdc.

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« Reply #82 on: January 15, 2013, 05:16:23 PM »

The NCL-2000 reduces input power from 2000W on SSB to 1000 on CW by reducing HV and screen voltage but bias remains the same. The idle current drops from 225ma to barely noticable and I think its time to give it a try in that position.

Carl
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« Reply #83 on: January 15, 2013, 08:58:00 PM »

This thread has been very interesting.....  That certainly leads me to believe they are running significantly cooler while biased with –15 vdc.

Another happy camper... Grin

That's cool to hear, Rob. (pun intended)

Yes, as Carl suggested, try some more diodes for additional bias and see where the diminishing return is... a compromise between drive, heat and IMD.    Add diodes or zeners to the existing zeners - doesn't matter which, except for finer selection.

T

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« Reply #84 on: January 16, 2013, 10:25:20 AM »

Well, I haywired some diodes to a Clipperton L last evening and made several contacts on 3875 and all with clean signal reports. It took a few tries to get the proper balance of increased drive and mike gain from the TS-950SD. The end result was a 400W carrier and absolutely no plate color from 4 brand new Shuguan 572B's...amazing!  After about 90 minutes everything was still quite cool. Keying waveform on CW at 1300W out looked clean on the SA so the amp was not into Class C and most likely well into B; idle current was miniscule and Pout was up about 150W for about a 7% improvement.
 
I'll look at the curves, efficiency plots, and SA more closely on my own amp, the customer wants his back and I'll do a more permanent change to the LK-500ZC.

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« Reply #85 on: January 16, 2013, 12:59:01 PM »

Well, I haywired some diodes to a Clipperton L last evening and made several contacts on 3875 and all with clean signal reports. It took a few tries to get the proper balance of increased drive and mike gain from the TS-950SD. The end result was a 400W carrier and absolutely no plate color from 4 brand new Shuguan 572B's...amazing!  After about 90 minutes everything was still quite cool. Keying waveform on CW at 1300W out looked clean on the SA so the amp was not into Class C and most likely well into B; idle current was miniscule and Pout was up about 150W for about a 7% improvement.


Cooler is the key.  Looks like you gots it working right.  400w carrier with no color sounds like something out of a Gotham vertical-type ad.... HA!


Just a general word about using riceboxes for drivers when increasing the bias in the final linear amp...  I got an email from a friend who brought up a good point.... Many of these Yaesus, Icoms and Kenwoods might be stressed out trying to keep up with the average amplifier that is biased heavily. I agree and want to emphacise again that we must check the overall drive, IMD and heat levels before deciding if it will work for us.  The harder we push the ricebox to full power, the more the IMD degrades.  The first priority is a clean signal, so we try to run our exciter as lightly as possible..   If our exciter is rated for 100w pep, running it at 75w pep can make a big difference in cleanliness. It's a matter of relativity and degree.

Yep, there's no free lunches and will always be a trade off. What else is new?  :-)

This learning exercise will give us more options.  In my case, I use an IPA, so I
have plenty of excess drive available.  Also, some guys use Rangers and
other drivers that are good for more power output than a ricebox.  (Or a grid-driven amp like Clark uses)  This
gives them the option to bias it up and run the final amp cooler at the same power - or the
same temp at more power.

It's just another technique to have in the arsenal.

That's why I keep stressing to find the sweet spot between IMD, heat and
drive requirements. In the end, it may not be the best decision to increase bias. Might be better to keep it as is - in some cases. It's not for every situation.

T
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« Reply #86 on: January 16, 2013, 01:02:52 PM »

Clark - I did re-tune when switching the bias setting but did not actually see any difference in any of the parameters when I was done.

Carl - These are Chinese RF-Parts 3-500 tubes. They seem to have quite a bit more gain than Amperex or Eimac tubes that I have run in this amp. Agreed that the amp would not flinch at all with a +300 watt carrier. The Flex that I drive it with will produce a 20 watt AM signal with 125% positive peaks very cleanly. If I limit the positive peaks to 100% it will easily do 25-30 watts cleanly but I like the idea of slightly more audio and slightly less carrier rather than the inverse. As I move the tubes closer to cut-off the amount of power required to drive the amp increases so I would imagine the amount of drive available (with a clean AM signal) would be the biggest limitation.

Carl and Tom - Access to the zener diodes (they are stud mounts on the chassis divider) is really easy with the AL-82 so adding an additional series zener or a diode string to move the tube closer to cut-off would not be difficult. The next time I have the cover off I'll add what is necessary to drop the idling current closer to cut-off in the CW position. I like the idea of the 3-500's not glowing like orange popsicles while I'm babbling away.

Rob W1AEX


* stud mount 7.5 vdc bias zeners.jpg (30.2 KB, 518x364 - viewed 1392 times.)
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« Reply #87 on: January 16, 2013, 01:43:50 PM »

am I right in thinking that hb'ers who don't have the big bucks for huge transformers, etc., might do best by building a small self-contained 20-50 W carrier plate-modulated xmtr and a "linear" ?

Peter
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« Reply #88 on: January 16, 2013, 02:18:50 PM »

Peter,

That's a great way to go if you want to keep things very simple. An excellent example is what Ken W2DTC does using this method. I have not heard him using it lately, but he has a huge rock-crushing signal with beautiful audio with his tiny rig running into a linear. Purists will point out that this is not an efficient way to operate, but the reality is that it works just fine. Ken detailed his rock crushing mini-rig at the link below.

Rob W1AEX

http://w2dtc.com/w2dtc-homebrew-hi-fi-am-driver-transmitter-page.htm
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« Reply #89 on: January 16, 2013, 03:11:24 PM »

Clark,

The SB220 I did up for you, which did well over 550 watts and would hold it under modulation (did not start to downward modulate) used 40 to 50 6A10s in place of the zener diode.  That translates to roughly 22 to 34 volts of bias.  I also threw a LARGE (can't remember the value) uF cap across the output.  This kept the bias stable under the varying drive conditions.

I may try putting together a regulated pass transistor scheme on my SB220 to test different bias levels. 

I also have a low voltage class A modulator.  Designed to modulate about 10 watt carrier.  Any reason someone can see not to use this as a bias adjust or "assist" to slide the class of bias as we modulate?  IE, basically do grid AND plate modulation?


--Shane
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« Reply #90 on: January 16, 2013, 03:44:17 PM »

As others post up data, I like this more and more. 

Carl, Those are great results,  I am eager to see what your efficiency numbers are. I calculated mine but I think I am doing something wrong as I am showing 80% which I dont think is possible.  I think my plate current meter is off a touch because of the added plate voltage on my Tbolt.   In AB1/AB2, The calc is 55%.

Shane,  I would think there has to be a better way then to use 50! diodes.  I know my current SB220 that I rebuilt has the Harbach board with the string of zeners rather than the stud mount.   I built the board, but I dont remember nearly that many diodes.

I think the EBS-1 removes all need for the 50 diodes as it can be modified to be adjustable and costs just $39.

For reference with the harbach board,  My SB 220 has 100 MA of idle in CW and 150 MA in SSB mode.  Maybe its less than the old factory setup, what does yours run?

Rob, Another Stud mount zener would be cake to install on that amp.

Tom, Very good point about the rice box driving harder and IMD.  Drive power goes WAY up as you go into C or B.   Its more then double on the ranger. Now, the ranger does not care and runs better when loaded fully.  However, I see your point about solid state rigs.  My FT450 WILL overtemp at anything more then 25 watts if you are long winded and are very active.

C

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« Reply #91 on: January 16, 2013, 06:09:25 PM »

Quote
Cooler is the key.  Looks like you gots it working right.  400w carrier with no color sounds like something out of a Gotham vertical-type ad.... HA!

I was shocked as I slowly brought up the drive Tom and the anodes remained dark. I just got the tubes direct from China and Im beginning to wonder if they increased the dissipation after all the complaints about their 125W rating. If they are now 160W as with Cetrons then the 640W Pd is a lot more. Plus the fan on the back helps. A 10000uF 50V lytic did the stabilizing, its what I had handy but at least 4700uF should be used; it depends upon the number of diodes to some degree.

The TS-950SD was used since its rated at 150W CW and 35W AM versus 100/25W for the TS-940. I didnt go over 25 Bird Watts on the input so it runs cool. 

Another issue about using ricers is that a QSY say from 3705 to 3885 requires the amp be retuned and even a small change in input VSWR requires a careful retune rather than just tweaking the Tune cap. I may try it a bit more tonight as the customer is coming tomorrow AM. With a Ranger, etc it wouldnt be as fussy....I suspect. Gotta get the Ranger and AF-54/AF-67 on the shelf here in shape to try low power drive. Id rather have a TS-830M that has AM built in Cool

Quote
Shane,  I would think there has to be a better way then to use 50! diodes.  I know my current SB220 that I rebuilt has the Harbach board with the string of zeners rather than the stud mount.   I built the board, but I dont remember nearly that many diodes.

Those are wimpy 1N4007's Clark, not zeners. I always replace them with just enough 1N5408's to get 200-220ma idle in SSB. Ive had several stock Harbach boards blow up after just an hour or so of OTA testing before getting it to the customers.

Carl

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« Reply #92 on: January 19, 2013, 01:18:08 PM »

Clark,

No, it doesn't have that many diodes.  It doesn't, because it wants to keep the amplifier in class B.  You want it barely in class C, so when you apply drive, you get it to slide to class B again.

If you want to approximate it, throw your zener back in, and use the Harbach board in series.  And while your at it, remove those little diodes they put in, and use real 6A10s.  I just did a rework of an amplifier for someone that had a single 3-500Z in it.  It's now got an AM-FM / SSB / Getter switch in it.  In SSB, it's using a Henry diode from the 1kd.  In AM/FM, it throws 15 more diodes in series with the zener.  In "getter" mode, it's grounded, and throws a nice reddish glow.

Another method: a regulated power supply for the bias voltage, instead of the zener. 

Carl,

For the SB220 and other 3-500 amps that switch HV taps, I ensure that the HV tap runs about 100 mils TOTAL ZSAC, for amps that run app 2800 to 3300 V loaded.  When switched to CW, it's nearly cut off (I can't detect any current).  I've never had a complaint, even from the SSBers 6 to 8 kc away from me banging my Smeter at 50-60 over (I'm 3500 ft above them, line of sight, about 100 miles away).  This greatly increases efficiency of the amplifier, and lets you run it safely on the SSB tap, nearly giving a double in PEP output.


--Shane
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« Reply #93 on: January 19, 2013, 01:21:33 PM »

My SB220 has been runing biased down with the harbach board for many, many years now. Never a single problem.  No need to alter it or mod it.
 
Good idea on the switch!  That would be one way to go!

C
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« Reply #94 on: January 19, 2013, 05:38:28 PM »

The switch is easily put in without messing with the cosmetics.  Replace the meter sensitivity pot with one with a push / pull switch on it, and there you are.

Also works for making a standby switch, if one ever wanted one.

The zener and Harbach still won't give you as much bias as you need, but will definitely help keep the amp cooler on AM.  50 diodes is nearly 32 volts, give or take.  Also, throw a few thousand uF cap across the bias board output.


--Shane
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« Reply #95 on: January 19, 2013, 07:52:03 PM »

Thats too many diodes to mess around with. In the LK-500ZC I'll just add another 50W zener.

I cant wait to get another LK-800 in for a rebuild, wonder how much carrier a trio of 3CPX800's can handle Grin

Now, OTOH, how safe is it to raise the grid flange on a YC-156 and switch between GG and some level of bias?

Carl
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« Reply #96 on: January 19, 2013, 08:31:09 PM »

Thats too many diodes to mess around with. In the LK-500ZC I'll just add another 50W zener.

I cant wait to get another LK-800 in for a rebuild, wonder how much carrier a trio of 3CPX800's can handle Grin

Now, OTOH, how safe is it to raise the grid flange on a YC-156 and switch between GG and some level of bias?

Carl

I know of one Commander HF series (the old, originals) export 3 holer in Nor Cal that does 8Kw on peaks.  He runs it at 1500 carrier.  It is NOT a stock power supply.  I believe it's one of the Dahl transformers for a 8 or 10 3-500Z amp.  It was for sale on  the amps reflector a few years ago, not sure if he sold it or not.

As far as raising the YC tube's grid flange, is it necessary?  If you're going to do it, I'd look at a Zsupp if you don't have one:  Most people with the 156 don't use one, but....  I'm a big fan of a grounded grid being just that.

I use a string of cathode diodes for the 156.  Works FB. 

--Shane
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« Reply #97 on: January 19, 2013, 09:29:28 PM »

Duh....I had a serious brain fart, never mind Grin Shocked
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« Reply #98 on: January 19, 2013, 10:20:53 PM »

8 or 10 3-500Z amp.
Shocked

Wonder how much carrier that would do?
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« Reply #99 on: January 20, 2013, 08:31:27 PM »

8 or 10 3-500Z amp.
Shocked

Wonder how much carrier that would do?

If you care to try to do the math, the last 11 meter idiot told me it worked like this:

His 3CX3000 had the CAPABILITY of keying 14 thousand watts.  That was balls to the wall, as MUCH as it would key.  It would only "hold" (ie, backwards modulate) to 1500, maybe 2500 watts. 

According to him, this means he was getting at LEAST twice the CARRIER power, PLUS the "holding watts" on top, ie, 28,000 plus the 2500 watts.

So, according to this math, which is the math of the amp builder in Tennessee, the 10 3-500Z box would key 12 thousand, hold about 3000, meaning his 10 tube Z box did 27,000 watts PEP, from a 12 thousand watt carrier.

I'll wait for you to pick yourselves up off the floor.  Yeah, I literally laughed at this idiot on the phone.


The last 10 tube amp I saw with Z tubes in it would do an honest 2000 watts carrier, 100 percent modulated.  That was in another life, in a character I played on TV, though Smiley


--Shane
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AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
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