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Author Topic: Series Modulated 813 Rig  (Read 39311 times)
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w8khk
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« Reply #50 on: December 17, 2013, 10:30:52 PM »

Tom, I was typing a reply when you and Jim corresponded.  I agree, the heavy metal is not required for the exciter, but you mentioned that it would be nice to be able to crank it up to 125 watts or so.  I think it would be rather complicated to make the series modulated transmitter work properly for that low to high range of power, while keeping the audio section linear.  I also believe you would need some negative, not positive, bias on the 3-500Z to achieve the voltage split you described between final and modulator.

When looking at placing the series modulator above the final, I see more challenges than it may be worth.  I wanted to try this because the two finals I have restored do not lend themselves to isolation with a large series modulator on the negative supply side. 

With the series modulator on top, it functions a s a cathode follower, and would require massive swing on the control grid to attain decent modulation percentage.  Also, coupling audio to the input would be a challenge to isolate, considering the high voltage at the input.  This is easy to do at RF, but not for full range audio response.   So I may just shelve the idea and use conventional modulation transformers in those rigs.  It is, however, interesting to consider building a bigger rig as you were describing, floating the RF section at HV.

The smaller scale solution looks like it has merit and will fill the bill for your requirements.  GL in the implementation and testing.
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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
K1JJ
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« Reply #51 on: December 18, 2013, 11:26:27 PM »

Rick,

Yes, maybe the cathode approach is the way to do it.

Well, I committed to the project. It's going to be quad 6LB6s series modulating a pair of 807s.  Quad 6LB6s are good for 160 watts dissipation total. I will have a fan on them too.

I like the idea that I can do this with a 1200 volt supply. Reasonably low voltage compared to big rigs.

Today I found a nice chassis and laid out the parts. It will use air variables, so is reasonably compact.

Gonna  take some pictures once the tubes and sockets arrive and get installed.

Here's the circuit again. Planing to add some audio negative feedback from the modulator plates back to the 12AU7 preamp.   The 807s will be driven by the solid state DDS VFO and use a toroid input to eliminate tuning hassles.

http://www.amwindow.org/tech/htm/wb9eckseriesmod.htm


I've never built a 6LB6 / 807 series modulated rig before, so it should be fun getting it working.  Again, this may turn into a pdm rig later on - it wud be easy to add on.

T

The goodies are shipped:


* 6LF6s.jpg (65.78 KB, 800x652 - viewed 507 times.)

* 6LF6 sockets.jpg (69 KB, 525x340 - viewed 503 times.)

* 807 finals.jpg (362.53 KB, 1425x1600 - viewed 480 times.)
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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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w8khk
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« Reply #52 on: December 18, 2013, 11:54:54 PM »

Tom, that seems like a conservative and practical approach. 

In addition to the audio feedback to the driver or preamp stage, you might want to consider playing with some DC feedback as well.  This could function similar to an electronic voltage regulator, with two benefits.   First, it would make the DC voltage applied to the final RF stage relatively constant as you do your tuning and loading.  Second, it could help keep the class A series modulator at the center of the linear portion of the curve, thus minimizing distortion.  Basically, you would use one pot to adjust the operating point of the parallel modulators, and another pot to adjust the desired amount of feedback.   My thinking is that one of the most significant challenges in class A series modulation is keeping the modulator as linear as possible, and the key is the correct bias.  This will vary if the voltage across the modulator tubes is allowed to vary indiscriminately with changes in final plate current.

Another feedback option is audio derived from rectified RF.  Again looking at the circuit for the HP 606A RF generator, it may provide some ideas to make the series modulator extremely clean and stable.  I am not suggesting cloning the circuit, but have a look at how they designed a tube differential amplifier to mix the feedback with the incoming audio signal. 

If I were to build a rig such as this, I might breadboard a section of the modulator, and load it with a fixed resistor equal to the modulating impedance of the final amplifier, and play around with operating points, voltage drops, feedback, etc.  This will enable you to determine just how much "linear" swing you can get, positive and negative from the quiescent operating point, with your proposed modulator.  You could even do it initially with one tube, a test oscillator and input coupling transformer, and a variac on the HV supply.  Then extrapolate the results to estimate how four tubes in parallel might perform.  Minimal test setup and possibly lots of data to come closer to the final design before the full build.
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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
WD5JKO
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« Reply #53 on: December 19, 2013, 07:06:09 AM »

In addition to the audio feedback to the driver or preamp stage, you might want to consider playing with some DC feedback as well.  This could function similar to an electronic voltage regulator, with two benefits.   First, it would make the DC voltage applied to the final RF stage relatively constant as you do your tuning and loading.  Second, it could help keep the class A series modulator at the center of the linear portion of the curve, thus minimizing distortion.

  Here is another series modulator discussion with DC feedback:

http://amfone.net/Amforum/index.php?topic=27856.0

  This differs in that the modulator is on top, and solid sate.

Jim
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K1JJ
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« Reply #54 on: December 23, 2013, 04:49:25 PM »

For a continuation of this thread discussing the final configuration  -  quad 6LF6s PDMing quad 6146s:


http://amfone.net/Amforum/index.php?topic=35443.msg272874#msg272874
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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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