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Has anyone ever hooked up a transformer this way?




 
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K1JJ
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« Reply #25 on: January 29, 2023, 12:38:53 PM »

Hi Mike,

Good decision on using a GG 3-500Z as the driver for the pair of 4X1s in GG linear.  You should not have any issues with getting good IMD results from the 3-500Z itself.

A few suggestions:

Be sure to have more than 1650V available for the 3-500Z in case you need it.  You do not want the tube drawing much grid current for best cleanliness. The grid current demand rises as you try to pull more power out of the tube.  More HV will reduce the grid current requirement and make it cleaner.  I am aware of the -45 dB 3rd order datasheet claim at 1500V, but not sure I believe it. I did not not see a noticable change in IMD when I ran my single one as a driver.  I ended up using 2200V for less grid current as discussed above and it worked out better.  IE, at 2200V, I just had to tickle the 3-500Z to get good power out as a driver for my pair of 4X1s in linear service.

For the 4X1s, use regulated screen voltage with minimum grid current in GG.  (I think that is already your plan)  The 4X1s will be the bottleneck for clean IMD so do everything you can to make them clean.  Expect to have low efficiency and more cooling because of heavily loading the plates and higher idle, thus two tubes for more dissipation to hit your final output requirements.

Now the Flex 5000...   Did you run a two-tone IMD test with it yet to prove out the favorable IMD expection?   If you expect -35 dB 3rd out of the 4X1s, you will probably need at least -38 to 39 dB 3rd IMD out of the Flex 5000 at its required driving power.  Being a solid state final, I don't know if this can be done, considering most solid state ham RF power amplifiers are around -30 db 3rd or so.  Is the Flex 5000 an exception?

As you know, the final result cannot be better than the dirtiest link in the chain.  Now is the time for design mistakes to be made, not later...

And of course, your overall safety net is still using adaptive pre-distortion later on if this does not work out.  

T

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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There's nothing like an old dog.
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« Reply #26 on: January 29, 2023, 01:24:44 PM »

Hi Mike,

Good decision on using a GG 3-500Z as the driver for the pair of 4X1s in GG linear.  You should not have any issues with getting good IMD results from the 3-500Z itself.

A few suggestions:

Be sure to have more than 1650V available for the 3-500Z in case you need it.  You do not want the tube drawing much grid current for best cleanliness. The grid current demand rises as you try to pull more power out of the tube.  More HV will reduce the grid current requirement and make it cleaner.  I am aware of the -45 dB 3rd order datasheet claim at 1500V, but not sure I believe it. I did not not see a noticable change in IMD when I ran my single one as a driver.  I ended up using 2200V for less grid current as discussed above and it worked out better.  IE, at 2200V, I just had to tickle the 3-500Z to get good power out as a driver for my pair of 4X1s in linear service.

For the 4X1s, use regulated screen voltage with minimum grid current in GG.  (I think that is already your plan)  The 4X1s will be the bottleneck for clean IMD so do everything you can to make them clean.  Expect to have low efficiency and more cooling because of heavily loading the plates and higher idle, thus two tubes for more dissipation to hit your final output requirements.

Now the Flex 5000...   Did you run a two-tone IMD test with it yet to prove out the favorable IMD expection?   If you expect -35 dB 3rd out of the 4X1s, you will probably need at least -38 to 39 dB 3rd IMD out of the Flex 5000 at its required driving power.  Being a solid state final, I don't know if this can be done, considering most solid state ham RF power amplifiers are around -30 db 3rd or so.  Is the Flex 5000 an exception?

As you know, the final result cannot be better than the dirtiest link in the chain.  Now is the time for design mistakes to be made, not later...

And of course, your overall safety net is still using adaptive pre-distortion later on if this does not work out.  

T



Thanks, Tom.
I see what you mean. I had forgotten grid current in my mental calculations:
I want to keep that down to about 120mA or so.
Might as well hook hook up to the top tap, though I will need a 10 amp variac to back it off and I was trying to save space (old problem 😬🤪)

I also appreciate the bump in the  FLEX- ratings.
Just downloaded some QST tests from 2008. 3rd order was measured at -34dB in those lab tests. Figured that was at 100 watts. Need to get more detail. Since I only need about 50 watts max, hoped the FLEX was better at half power, so my -38dB was a “hope so” rather than a fact….😳😬
Actually expect to only need 10 -20 watts from FLEX. Need low level lab tests…

This predistortion thing seems essential. Can it be implemented in a FLEX5000a?
I have older tranceivers that used 12BY7- 2x6146 outputs. Figured that they. Were worse than new rigs, though.
I have KenwoodTwins and a Yaesu FT101ZD and even a KWM2, though all of these rigs need service since they have been dark for 30+ years.

How do I get Predistortion panacaea going in my FLEX?

73, Mike
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KA3EKH
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« Reply #27 on: January 30, 2023, 11:12:55 AM »

Just curious, how are you all determining IMD? I look at things like carrier suppression, 2nd and third harmonics and overall bandwidth but I hear you all throwing IMD numbers around and wonder just how you are determining them? Are you looking at spurious noise around the carrier as a byproduct of modulation? something like occupied bandwidth? Back a thousand years ago when I did annual proofs for AM transmitters we did measurements of THD at different frequencies that was used to determine frequency response but no broadcaster has done that for years now, we only look to confirm that are overall bandwidth fits within the NRSC window.
Sorry for being a bone head but just have to say I have no idea what you are talking about with thees IMD numbers.
Please enlighten.

Ray F/KA3EKH



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K9MB
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« Reply #28 on: January 31, 2023, 01:09:52 PM »

Just curious, how are you all determining IMD? I look at things like carrier suppression, 2nd and third harmonics and overall bandwidth but I hear you all throwing IMD numbers around and wonder just how you are determining them? Are you looking at spurious noise around the carrier as a byproduct of modulation? something like occupied bandwidth? Back a thousand years ago when I did annual proofs for AM transmitters we did measurements of THD at different frequencies that was used to determine frequency response but no broadcaster has done that for years now, we only look to confirm that are overall bandwidth fits within the NRSC window.
Sorry for being a bone head but just have to say I have no idea what you are talking about with thees IMD numbers.
Please enlighten.

Ray F/KA3EKH





Hi Ray,
Tom K1JJ is the expert on this. I believe that he uses a spectral analyzer display on an SDR receiver for the purpose, but he is the person to answer.
He may put a two tone test in and observe the near and farther away noise products visible in the narrow band on each side of a signal.
The lower the noise floor in modulation, the lower the IMD. 3rd order products are very close to the frequency an 5th, 7th, 9th order products further away.
Having a very low 3rd order IMD means you have fewer enemies putside of your transmit channel. If you have poor 3rd order IMD, you might sound great to your friends, but guys 5-10kHz away comsider you a LID…😬😉.
73, Mike
As for being a bonehead- we “boneheads” can only be cured by asking those questions and getting counsel from those who know by experience. Tom is a thinker and he always asks for help and it is the reason that he is a great builder and tester of radio equipment and antennas.
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KA3EKH
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« Reply #29 on: January 31, 2023, 02:25:41 PM »

Maybe the real question is in these days of the SDR radio is that the new standard? Think maybe I am asking someone to say SDR and DSP are King, the old way of keeping everything well within design specifications for linear operation and spectrum analyzers is over?
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« Reply #30 on: January 31, 2023, 05:49:16 PM »

Maybe the real question is in these days of the SDR radio is that the new standard? Think maybe I am asking someone to say SDR and DSP are King, the old way of keeping everything well within design specifications for linear operation and spectrum analyzers is over?

No, Ray, I do not think that is true at all.  Of course we all want to have as clean a signal as possible, such that we do not cause QRM to the folks in QSO on adjacent channels with unnecessary splatter.  What is being discussed is no different than maintaining a clean house by monitoring our modulation, with either an oscilloscope or modulation monitor, keeping our modulation percentage and bandwidth under control.

Whether we are using plate-modulated AM, or a lower power AM exciter with a linear amplifier, the signal is only as good as the weakest link.  One of the important things Tom has demonstrated, and is sharing with the masses, is a technique to make the ANALOG linear amplifier as clean as possible.  This involves using the tubes that provide the most linear response within the operating parameters involved, including drive level, bias, gain, output power, as well as plate voltage and current.  Similar parameters are involved with solid-state linear amplifiers.  None of this is new territory, but Tom has taken it to the limit of practicality by spending a great deal of time fine-tuning the parameters for absolute LOWEST IMD possible.  He has even gone to the degree of cascading several linear amplifiers, of increasing power level, to achieve the cleanest signal, as opposed to pushing one or two stages to get the desired output power, albeit more distortion along the way. 

I think a key takeaway here is the fact that the more power one runs, the cleaner the entire system must be, in order to have the same level of spurious energy just outside the needed bandwidth of the transmission. 

The SDR comes into play primarily because the panoramic and waterfall displays provide an excellent, calibrated energy measurement in the frequency domain, obviating the need for the typical ham to purchase an expensive spectrum analyzer.  If one builds an old-school exciter and linear amplifier, or a high-level plate modulated rig, the SDR in this case is simply performing the function of a laboratory instrument to analyze and tune the quality of the resulting transmitted signal.  By the way, the SDR also makes an excellent receiver, able to "hear" much better than some of the best boat-anchor receivers.  The SDR also demonstrates an advantage of receiving full AM signals compared to SSB reception.  The tools at your fingertips, with the mouse, allow you to adjust the passband of each individual sideband, avoiding QRM that affects only one side of the received signal.  It also makes reception under selective fading conditions much more readable.

Mention has been made of the Pure Signal predistortion methods used to clean up artifacts and limitations in the various components of the transmitted signal path.  This is relatively new, as compared to just making the linear amplifiers as clean as possible, but in no way is it required in order to have an AM transmitted signal that is above reporach.
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan

My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
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« Reply #31 on: January 31, 2023, 08:46:43 PM »

Excellent answer, Rick.
I worked with a Polorad Spectrum analyzer for 20 years and it was $12,000.00 used.
I have a nice HP now, and in my youth, I bought an old 455khz Panadapter and it was a wonder to me.

It is astonishing how fast The new SDR receivers have gotten very cheap and for narrow spectrum in a ham band, they are great.

The same goes for storage scopes. I had a Hitachi that set my boss back 5grand in the 90s, but it is pitiful compared to a $400 new Chinese made scope for both speed and for capture time.

I find predistortion both fascinating and a bit disheartening.
I want the cleanest signal I can get, but it makes me think of those commercials for air freshener spray. A woman walks ina room with a can and the place is,full,of dirty clothes and whatever,and,she walks,over and sprays her jeans ans says, “now I can wear them a couple more days before I need to wash them….😳😬🥺😝😝😝
Just think- a LID with a CB linear can sound as good as a skilled RF engineer who takes great pride in running his equipment properly and monitoring it constantly….
Another simile might be liposuction for LIDs. 😉

Not you Tom. K1JJ is thinking a 3rd order IMD of -60 to -80dB. Tom makes the point rightly- The more power you run- the cleaner it has to be to not cause others to notice and feel disgust. That- is what motivates my own efforts.
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w8khk
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« Reply #32 on: January 31, 2023, 11:18:45 PM »


I find predistortion both fascinating and a bit disheartening.
I want the cleanest signal I can get, but it makes me think of those commercials for air freshener spray. A woman walks ina room with a can and the place is,full,of dirty clothes and whatever,and,she walks,over and sprays her jeans ans says, “now I can wear them a couple more days before I need to wash them….😳😬🥺😝😝😝

Mike, you make some very good points.  Brings to mind one of Tom's transmitters that was designed NOT to be a shack heater - he called it "Summer Breeze".

Iff'n I ever set up a rig with predistortion, it will likely be called "Febreze"  Some things are better washed and rinsed than hidden with overspray!
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan

My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
K9MB
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« Reply #33 on: February 01, 2023, 12:39:57 AM »


I find predistortion both fascinating and a bit disheartening.
I want the cleanest signal I can get, but it makes me think of those commercials for air freshener spray. A woman walks ina room with a can and the place is,full,of dirty clothes and whatever,and,she walks,over and sprays her jeans ans says, “now I can wear them a couple more days before I need to wash them….😳😬🥺😝😝😝

Mike, you make some very good points.  Brings to mind one of Tom's transmitters that was designed NOT to be a shack heater - he called it "Summer Breeze".

Iff'n I ever set up a rig with predistortion, it will likely be called "Febreze"  Some things are better washed and rinsed than hidden with overspray!

😂😂😂🤣🤣
Rick, you have wonderful taste in humor, but no sense of smell…😉

Funny story about Fabrese, the toxic stuff that is featured on my commercial story.
I bought some little 2 gallon trash bags to put in our dually when we went in our fifth wheel to South Texas two years ago , so trash did not build up in the floor.
I did not read the label and when I opened the box, it was like getting anhydrous ammonia sprayed in your face…😬. Yep- they featured the fresh Fabrese abomination…
 I washed my hands 8 times because my wife loathes these cheap perfumed products… I threw them in the corner of my garage amd forgot about them until this Summer. I wondered if they had faded any, but they were like the energizer bunny or an old Twinkie- still putrid, so I put them in the trash.

I am definitely going to remember your name for a predistortion rig if I get one to push my mid- 30s IMD up 30 points and may plagiarize the name  it myself, if I can stand the smell…😉.
More likely, though, I will offer to pay a copyright fee.
73, MB
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Patrick J. / KD5OEI
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« Reply #34 on: February 03, 2023, 10:07:52 AM »

rawfire.torche.com is a site on which I was kindly allowed some space many years ago, glad to see Chuck's site is still up. So many have disappeared.

My current site (below) has the article with the simulation result. Good to have things in more than one place!
https://bunkerofdoom.com/lit/4x8/index.html

Patrick, I had not made the connection between you and Bunker of Doom.
Wow! Cannot tell you how many times that site has provided very useful and rare knowledge. Many thanks for your contribution to saving this stuff for the future.
73, Mike

Mike, thank you for the kind compliment. It's just a site of wierd old stuff! I hate to see unusual circuit ideas lost over time, as well as some magnificent ones. "Everything old is new again".
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K1JJ
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« Reply #35 on: February 03, 2023, 12:19:07 PM »

FB on all the comments.  I was away from the computer for a few days and missed all the dialogue.  Mike and Rick pretty well explained it all.  

There is not a lot of "practical" and simple IMD amplifier info available to the average ham. I mean the formulas and complex testing equipment and procedures usually scares away the average guy who passed a multiple choice test to get a ticket. He wants to keep it simple and cheap. Pay the money and blend in.

I've tried to keep transmitter testing simple and in plain language grouped into two main areas:  1) A simple objective two-tone test for 3rd/5th order IMD (and harmonic dstortion) to give a general baseline for changes and improvements.   2) a touchy-feely test involving a common SDR spectrum analyzer using actual microphone voice audio inputs to get a look at how the overall dynamic bandwidth looks in the real world.  It's amazing what an "ssss" "ch"  "yallo" and normal voice speech can tell you about your signal, once you develop an eye for it.  For example, I can tune in any AM or ssb signal using my SDR spectrum analyzer and tell you about his voice power band ranges, sideband suppression, bandwidth using a mask, ratio of blower/background noise to peak signal, carrier suppression, and approximate IMD by freezing a frame and counting the grid dB squares.  Easy to do once time is spent observing good and bad signals.  The voice tests are relative, though the two-tone tests can be quite precise.

It's something akin to setting up our audio on AM.  There is some technical expertise and practical voice tests required, though they do not have to be overcomplicated.

The first objective to all this testing and improving is to be a good neighbor.  Being a good neighbor means being able to operate close to another QSO and simply blend in with the crowd. If you run more power than the average ham, then you will need to work harder than the average ham who just buys a rig and plugs it in.    The second objective is to have a generally clean, pleasant to the ear - and good sounding signal with little background noise (hum and blower noise, etc).  We want a sharp roll off of our sidebands (after the desired audio power band)  with cleanliness that we can actually hear.  

Yes, I think adaptive predistortion IS the new standard. I can instantly see who is running it by the sharp skirts. It is quite dramatic.  If the overall active amateur radio population were increasing like in the distant past, the crowded bands would encourage it.  But because there is getting to be plenty of room to operate, we will probably see no changes in rules in the future.  I mean wideband AM is no problem at this point.   The driving force will simply be manufacturers wanting to sell more radios by telling us to switch to the ultimate mode....  "Yaesu Thin Body" "Like a good neighbor..."    or whatever.... :-)

One last point:  Once a rig is "tricked out" and running FB, it must be checked periodically. How many times I have not used a rig for a year and then fired it up with problems?  Be sure to check the rig's spectrum every time you operate... or better yet, have an outgoing and incoming look at the spectrum.  We want to be able to see our own signal as well as help out others who may have developed an issue.  Rigs love to break.... :-)

T


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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
K9MB
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« Reply #36 on: February 03, 2023, 12:21:50 PM »

rawfire.torche.com is a site on which I was kindly allowed some space many years ago, glad to see Chuck's site is still up. So many have disappeared.

My current site (below) has the article with the simulation result. Good to have things in more than one place!
https://bunkerofdoom.com/lit/4x8/index.html

Patrick, I had not made the connection between you and Bunker of Doom.
Wow! Cannot tell you how many times that site has provided very useful and rare knowledge. Many thanks for your contribution to saving this stuff for the future.
73, Mike

Mike, thank you for the kind compliment. It's just a site of wierd old stuff! I hate to see unusual circuit ideas lost over time, as well as some magnificent ones. "Everything old is new again".

Patrick, I live the sight because just when I think a thing is not obtainable,
I look there and you have it waiting.
Just like all us ole geezers, this art snd knowledge is becoming more and more scarce.
Luckily, there are younger guys who like to explore the old arts and parts.
Weird?- Nah, I like to think of it as rare and esoteric. 😉😁 73, Mike
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K9MB
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« Reply #37 on: February 03, 2023, 12:45:09 PM »

FB on all the comments.  I was away from the computer for a few days and missed all the dialogue.  Mike and Rick pretty well explained it all.  

There is not a lot of "practical" and simple IMD amplifier info available to the average ham. I mean the formulas and complex testing equipment and procedures usually scares away the average guy who passed a multiple choice test to get a ticket. He wants to keep it simple and cheap. Pay the money and blend in.

I've tried to keep transmitter testing simple and in plain language grouped into two main areas:  1) A simple objective two-tone test for 3rd/5th order IMD (and harmonic dstortion) to give a general baseline for changes and improvements.   2) a touchy-feely test involving a common SDR spectrum analyzer using actual microphone voice audio inputs to get a look at how the overall dynamic bandwidth looks in the real world.  It's amazing what an "ssss" "ch"  "yallo" and normal voice speech can tell you about your signal, once you develop an eye for it.  For example, I can tune in any AM or ssb signal using my SDR spectrum analyzer and tell you about his voice power band ranges, sideband suppression, bandwidth using a mask, ratio of blower/background noise to peak signal, carrier suppression, and approximate IMD by freezing a frame and counting the grid dB squares.  Easy to do once time is spent observing good and bad signals.  The voice tests are relative, though the two-tone tests can be quite precise.

It's something akin to setting up our audio on AM.  There is some technical expertise and practical voice tests required, though they do not have to be overcomplicated.

The first objective to all this testing and improving is to be a good neighbor.  Being a good neighbor means being able to operate close to another QSO and simply blend in with the crowd. If you run more power than the average ham, then you will need to work harder than the average ham who just buys a rig and plugs it in.    The second objective is to have a generally clean, pleasant to the ear - and good sounding signal with little background noise (hum and blower noise, etc).  We want a sharp roll off of our sidebands with cleanliness that we can actually hear.  

Yes, I think adaptive predistortion IS the new standard. I can instantly see who is running it by the sharp skirts. It is quite dramatic.  If the oveall amateur radio population was building like in the distant past, the crowded bands would encourage it.  But because there is getting to be plenty of room to operate, we will probably see no changes in rules in the future.  I mean wideband AM is no problem at this point.   The driving force will simply be manufacturers wanting to sell more radios by telling us to switch to the ultimate mode....  "Yaesu Thin Body" or whatever.... :-)



T




Tom, Sorry stepped on your toes because I hit the button s few seconds after you did.
I agree with everything you said and I credit you for moving me from getting the highest output and setting mic gain so the plate meter hovers at about half the tuneup current to actually thinking about the region that my amp will occupy in the transfer curve of the amp. My dumb old school way did prevent me from splattering and causing disgust on all directions, but your ideas on improvement in IMD by better matching to the cathode and operating my big tetrodes with  with fully stabilized screen supplies and monitoring the spectrum at least +-10kHz for Intermodulation distortion that puts fuzz on the sideband(s).
We have all observed a guy with a bucketmouth in your face signal and everybody giving him 40 over and (good audio) reports. The guys 5khz away see this LID differently, however and many are too polite or timid to point it out. When one hears truly great audio and a vertical wall around the channel, it becomes very important to be that guy.
I am taking my time andcspplyingbthevdtuffbyiu have shared from hundreds of hours of testing and experiment so I have equipment that is capable of excellence.
Unless you just want to talk only- the pride of making your own stuff is the real joy if this art.
I also agree in principle to your philosophy of KISS, but my brain always wants to know more, so I love to run numbers snd hope to get closer to a good design before I begin to understand how to run it.
I admit it- I am nutz and afflicted with brain worms and questions.
😉😂
73, Mike
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K1JJ
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« Reply #38 on: February 03, 2023, 01:35:57 PM »

Mike,

It's always nice to see someone make good use of information and improve their lot with it... :-)


I'd like to do a shout-out for Don, K4KYV....

The other night I was tuning around and came across Don's AM signal on 3875.  Don runs a big, old school, plate modulated transmitter.  I would consider it in the big rig class. He has been known for clean and easy to copy audio for decades.  So I gave his signal a close going-over on my SDR spectrum analyzer.  

I think he may have the cleanest analog big rig signal on the band.  His audio power bandwidth was about +- 4.5 KHz max. His skirts after that dropped like a rock, like down 70 dB from full carrier.  It actually looked like an adapted predistortion rig.

He is proof that old school analog plate modulated rigs CAN be very clean and sharp with the right amount of work.  We tend to get lulled into thinking that a big plate modulated rig needs some slack, but he obviously knows different.

I sent him an email to give him a pat on the back and find out how he was limiting the highs at 4- 4.5 KHz so well. He said (on that particular evening) he was using a 3.5 KHz audio transformer low pass filter. The filter appears to roll off sharply after 4 KHz.   He also has a VERY clean rig IMD overall to show skirts that drop into the noise rapidly after 4 KHz. It really does resemble an SDR adaptive predistortion rig.  He matched the band activity to his narrower filter.

I could hear the 4 KHz limitation to my ear, but it was very slight and Don still gets very favorable audio reports - some saying he has the best audio on AM for overall communication and listening.   This is really doing something at 4 KHz cutoff and using generally pre-1940 tubes and parts.

Congrats to Don for a nice job building, testing and optimizing that classic rig.


Anyone looking to add a clean AM signal to their mental library of viewing experience, tune in Don next time you hear him on.

T


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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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« Reply #39 on: February 03, 2023, 09:04:25 PM »

Mike,

It's always nice to see someone make good use of information and improve their lot with it... :-)


I'd like to do a shout-out for Don, K4KYV....

The other night I was tuning around and came across Don's AM signal on 3875.  Don runs a big, old school, plate modulated transmitter.  I would consider it in the big rig class. He has been known for clean and easy to copy audio for decades.  So I gave his signal a close going-over on my SDR spectrum analyzer.  

I think he may have the cleanest analog big rig signal on the band.  His audio power bandwidth was about +- 4.5 KHz max. His skirts after that dropped like a rock, like down 70 dB from full carrier.  It actually looked like an adapted predistortion rig.

He is proof that old school analog plate modulated rigs CAN be very clean and sharp with the right amount of work.  We tend to get lulled into thinking that a big plate modulated rig needs some slack, but he obviously knows different.

I sent him an email to give him a pat on the back and find out how he was limiting the highs at 4- 4.5 KHz so well. He said (on that particular evening) he was using a 3.5 KHz audio transformer low pass filter. The filter appears to roll off sharply after 4 KHz.   He also has a VERY clean rig IMD overall to show skirts that drop into the noise rapidly after 4 KHz. It really does resemble an SDR adaptive predistortion rig.  He matched the band activity to his narrower filter.

I could hear the 4 KHz limitation to my ear, but it was very slight and Don still gets very favorable audio reports - some saying he has the best audio on AM for overall communication and listening.   This is really doing something at 4 KHz cutoff and using generally pre-1940 tubes and parts.

Congrats to Don for a nice job building, testing and optimizing that classic rig.


Anyone looking to add a clean AM signal to their mental library of viewing experience, tune in Don next time you hear him on.

T




Tom,
It is funny that you talk about Don Chester. When I started out as a short wave listener in 1962, K4KYV had the strongest signal on 75 meters. I can still hear him- “K4KYV in Woodlawn Tennessee”.
He was always talking to an Old Timer from Paducah KY. Don was an inspiration to me to want to get on 75 meter phone. His audio was clear and crisp and powerful and natural. He very often expressed contempt for SSB and considered most of them LIDs. I remember one time, he modified his transmitter to a kind of control carrier setup to prove that AM could sound good even if the carrier varied. He put it back to his regular setup after making his point.
He was always more like a Dutch Uncle than Mr Rogers and never suffered LIDs or fools. The fact is that he was right and his respect for the Art was first to him.
He always was an AM guy and OWL was his religion.
I have read and reread his articles on AMfone archives and his Q&A for a great deal of wisdom and knowledge of the classic AM Art. Seems like he ran a pair of 4xKs back in the 60s.
He was not well loved by a lot of appliance operators which he lectured on putting out splatter, but I always had a deep respect for his knowledge and the quality of his AM and still do. Don has said many times that classic iron and tubes should be cared for and preserved because they cannot ever be replaced. I have been influenced by that reverence for the classic equipment from the golden age of AM radio. Don’s legacy is one that I hope will be appreciated.
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« Reply #40 on: February 23, 2023, 05:43:45 PM »

* I have no experience with the defibrillator caps. Seems like a repeat of the photo flash caps.  I've  used photoflash caps here for years. I'll bet someone here from industry knows if defibrillator caps will hold up.   10,000 cycles seems a lot, but what if you T/R key the HV supply like I do?   A soft step-start will probably help lifespan.

BTW, with 6KV @ 32 uF, I'm really surprised there are not a lot of defibrillator electrocutions reported in ER rooms or at nursing homes using them.  I mean, even a common ham 2KV @ 20 uF power supply can take you out....  They must be current limited and well insulated.
The defibrillator cap I'm using in my 4x4-125A amp is 56 uF@4.2KV. The amp is nearly 20 years old (the beginning of the build can be seen in my profile pic) and I don't use it very much,  but it's still going strong  Cool
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« Reply #41 on: February 24, 2023, 04:41:29 AM »

If using the large stack of diodes, which I recommend, to set bias.....  Also throw a large value electrolytic across them.

Helps with dynamic regulation oodles and gobs.


--Shane
WP2ASS / ex KD6VXI
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« Reply #42 on: February 24, 2023, 01:55:29 PM »

If using the large stack of diodes, which I recommend, to set bias.....  Also throw a large value electrolytic across them.
Helps with dynamic regulation oodles and gobs.

  Hi Shane, I see what you are saying, and I hear you.

The fact that it makes a difference suggests that the dynamic regulation of the series diode based bias is poor, where a large swing in cathode current could bring the diode forward bias from around 0.6v at idle, to .9v or so at full boogie. This is for each diode! That is a 50% swing!

There are many circuits floating around with a small zener diode, a power transistor, and a few passive parts. Some versions of them might be better. I like the adjustable Zener diode, the TL-431 as the basis for such a circuit; the zener knee is text book perfect whereas those big stud mounted zeners, around 5V have a poor zener avalanche knee.

Back to the series diode method, it is very simple, and rugged. The big bypass trick you mention is a good idea, but there is a downside. When you key the amplifier, that bypass capacitor must charge up from zero volts, and for an instant, the bias is zero. As a result the cathode current will surge, perhaps as long as a second, all depending on the circuit conditions.
A remedy for this, is an external bias source to trickle charge the capacitor, and could have a series diode to make sure when keyed up, there is no tube bias feeding some other circuit in reverse.

Jim,
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« Reply #43 on: February 25, 2023, 01:33:44 AM »

Thanks for the tips, Shane and Jim.

Just a couple of questions:

1- Shane, what value electrolytic cap would you recommend?
2- By dynamic regulation, do you mean that it is supposed to integrate the voltage drop variations (0.9-0.6)*(number of diodes) to some average DC value?
As an example- let’s say I want +7.5 volts on the cathode:
10 rectifiers can give anything from +6 to +9 volts, depending on the current and the temperature of the diodes.

Below is a graph showing both current and temperature variations.
Given this variation, does the cap need to be 100uF, 1000uF, what?
Seems like an ugly way to generate bias…

Jim, Your point about the delay caused by charging this large cap through a string of “high” resistance diodes sounds scary for say- a 4- 1000a with screen voltage applied….
If I understand your suggestion, you suggest putting a positive DC voltage on series with a diode and resistor to feed the cathode to support a minimum sustained bias in the large cap. The diode would back bias during operation then…

All this makes this method look ugly, though ugly may be acceptable. Comments?

I find the TL431 idea interesting… using a precision regulator to stabilize the bias. It obviously needs a low impedance pass transistor or FET. Anybody have a suggested circuit.
73, Mike


* 5850FAD7-8CD1-4A5D-9A85-B5E559968D9C.jpeg (272.47 KB, 978x1471 - viewed 65 times.)
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« Reply #44 on: February 26, 2023, 11:09:14 AM »



I link to the Triode board PDF download. Also see the image of the TL-431 circuit.

They use a TL-431 with a PNP power transistor to set the PA cathode bias anywhere between 3-27V. They have a Mod to change that range to 27-45v by adding a fixed zener diode.

The PDF discusses the IMD impact of bias variation with SSB transmission. With AM linear operation, the cathode current remains relatively constant, so this is of less concern on AM.

http://www.f1uvn.com/f1cxx/gs35b/triode-manual.pdf

Jim
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* Triode Board TL431 Bias.jpg (29.12 KB, 499x305 - viewed 77 times.)
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« Reply #45 on: February 27, 2023, 12:42:36 AM »



I link to the Triode board PDF download. Also see the image of the TL-431 circuit.

They use a TL-431 with a PNP power transistor to set the PA cathode bias anywhere between 3-27V. They have a Mod to change that range to 27-45v by adding a fixed zener diode.

The PDF discusses the IMD impact of bias variation with SSB transmission. With AM linear operation, the cathode current remains relatively constant, so this is of less concern on AM.

http://www.f1uvn.com/f1cxx/gs35b/triode-manual.pdf

Jim
Wd5JKO



Hi Jim,
Thanks- that is a great resource. I will study the manual.
I wonder how far one might go to increase the range of the citcuit?
For this 3-500z, the 3-27 volt range will suffice for my purposes, but my plans for a 4x1Kx2 amp will need to go up to about 120 volts or more, since I will be putting a regulated 400-450volt screen voltage on the tubes.
For now, the original range looks fine.
73, Mike
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« Reply #46 on: February 27, 2023, 04:44:32 AM »

I've been using the TL431 circuit for well over a decade as well.  Great circuit.

Make sure you bypass the heck out of the input and output of the variable supply.

To increase voltage you just add a series zener.  The only downside to the TL431 is you only get xxx amount of volts of swing.  You can increase the bias level 100 volts with a 100 volt zener, but you will still only have 27 or so volts of adjustable bias.

Works, works well, just remember that one caveat

--Shane
WP2ASS / ex KD6VXI
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« Reply #47 on: February 27, 2023, 01:17:31 PM »

I've been using the TL431 circuit for well over a decade as well.  Great circuit.

Make sure you bypass the heck out of the input and output of the variable supply.

To increase voltage you just add a series zener.  The only downside to the TL431 is you only get xxx amount of volts of swing.  You can increase the bias level 100 volts with a 100 volt zener, but you will still only have 27 or so volts of adjustable bias.

Works, works well, just remember that one caveat

--Shane
WP2ASS / ex KD6VXI

Hi Shane,
Thank you. Builds confidence to know that the circuit has worked well for so long.
It seems to me that the small range is not s problem as long as one may place a reliable two pole rotary switch in the circuit to allow the switching of several different Zeners in as needed. Not necessary for the 3-500z, but nice for 2x 4k amp.
A make before break switch would prevent anything floating or one might just use some small dpdt relays with proper sequencing to assure make before break occurs.
How about that idea?
Of course, the TIP147 would not do for over 100volts, but a 250volt pnp like a MJH11021 would work…
Are there any P-Channel FETs that might be adapted? The offset would be higher, but have not looked at that… probably the PNP BPJT would be easier as long as it handles voltage.
How about varisters? Need to consider higher values for 100+ volts- right? Would need another wafer on the switch it that was added, though one does not need a universal circuit, just custom designs for each tube type profile…
Ideas?? 73, Mike
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« Reply #48 on: February 28, 2023, 03:51:05 AM »

I made sure I put a 100k resistor across the whole shebang. Then you don't have a floating cathode even when switching between bias ranges.

I don't have the part number here (actual parts in California in storage) but I do know that they make devices rated to hundreds of volts.  I used an LR8 and pass transistor to make voltage regulators for screen supplies and grid using them.

Many ways to skin this cat.

--Shane
WP2ASS / ex KD6VXI
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« Reply #49 on: February 28, 2023, 01:58:44 PM »

I made sure I put a 100k resistor across the whole shebang. Then you don't have a floating cathode even when switching between bias ranges.

I don't have the part number here (actual parts in California in storage) but I do know that they make devices rated to hundreds of volts.  I used an LR8 and pass transistor to make voltage regulators for screen supplies and grid using them.

Many ways to skin this cat.

--Shane

WP2ASS / ex KD6VXI

Thanks Shane.
I like the LR8 for screen regulator. I found this circuit attached below.
I would use a 900volt FET in place of the 400volt one they have in there.
How does it look to you?
A good heat sink essential, too. 73, Mike


* C8873DC1-7E9F-4E2A-95EC-5A53B4FD25CE.jpeg (297.58 KB, 2299x1280 - viewed 64 times.)
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