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Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig




 
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Author Topic: Looking for ideas for a ~25 watt plate modulated homebrew Pissweaker rig  (Read 4660 times)
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K1JJ
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« Reply #25 on: September 03, 2020, 11:46:04 PM »

I believe he's talking about the difference in putting the modulator tube between the plate and B+ supply, vs the cathode and B-.

--Shane
KD6VXI

I often thought of doing that with a big PDM rig.  There would need to be a fiber optic isolation to feed the audio into the grid/cathode of the modulator. And the B+ would connect to the CT of the modulator filament xfmr.  Or is there some other way?

Maybe in the end placement at the bottom between the cathode and B- is easiest.  But then the final is hot. Pick your poison.

T
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« Reply #26 on: September 03, 2020, 11:58:23 PM »

Hi Rick,

You have me more interested in series modulation. I like the idea that there are very few guys doing it, so it will be a novel project and take some experimentation to get it running right.  Not counting a few tube PDM rigs, I don't know of any except for W2DTC with his 3CX-3000A7 rig.

You said:
"If you place the series tube above the final, the only thing you need to isolate is the tube socket, and use a filament transformer with very good secondary insulation with high breakdown voltage."

What is meant by placing the series modulator "above the final?" Do you mean in the same place as a conventional mod xfmr?  If so the audio drive circuit would need to be insulated with fiber optics or equivalent to keep the HV off it, right?  Maybe you meant something else.

** Rick:  Could you make a hand-drawn schematic of your best ideas for a 30 watt series modulated tube rig?  I think between the two of us we could come up with a workable design that could later be tried by others once the bugs are worked out.  


Hi Tom,

Yes, the series tube is placed between the HV  positive terminal and the plate circuit of the final amplifier.  Whereas the Class-B modulation transformer adds to and subtracts from the Plate supply (adding power) the series tube just reduces the plate voltage via its resistance and the power is lost in plate dissipation.  It is the same situation whether you place the series dropping device in the positive supply, or in the negative supply between the cathode of the final and B- supply.

The big difference is how you provide grid control on the series tube.  Yes, it is simpler in the cathode side, and that might very well be the easier solution, but then the RF stage needs to be isolated, perhaps adding more complexity.

There is no need for opto-coupling to the grid of the series tube, even if it is placed on the positive side of the final amplifier.  Since no grid current is needed, it is simply necessary to provide a DC bias voltage on the grid to determine the operating point at carrier with no modulation, then add the AC component at the grid for whatever degree of modulation you determine appropriate.  The series tube then works as a cathode follower and provides the modulated plate voltage in step with the grid voltage variations.

Nothing exotic is needed in the driver stage(s).  Just another tube, resistors, and capacitors.  It works very much like the series regulated supply with a pass tube found in many of the older instruments, such as voltmeters generators, and oscilloscopes.  I will share some examples soon.

I will try to get time between other responsibilities in the next couple days to draw some diagrams and provide references of similar circuits,  with explanations.  If we take the design and validation a step at a time with some breadboard testing, I think it will be rather clear and easy for anyone to come up with a design specific to their desired voltage and power level with readily-available tubes.  Sweep tubes may be considered overkill for the 30 watt power range you are considering, but the available emission to perform well at lower plate voltages makes them shine as final amplifiers in this application.  

You did not mention whether you wish to go modern, or retro with regard to the tubes chosen.  I usually try to run what I have rather than go out and obtain something else, and you would probably prefer to do the same.
73..........
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« Reply #27 on: September 04, 2020, 10:39:20 AM »

OK, Rick, sounds like a plan.

One thing I still don't understand...  If the series modulation tube is put in the B+ line, then its cathode, grid and plate structures are tied into the HV potential. How do you isolate the audio drive signal from ground? Do you float the audio driver itself or use large coupling caps to ground to avoid the HV DC or what?   It seems that somewhere along the line before the microphone that HV isolation is needed.  That's why I was considering the fiber optics.

I'm not sure about what tubes to use yet since there has been some discussion that some are better than others for linear curves, emission and best IMD.  I would think that the series tube should be at least three times bigger in dissipation than the RF final.  My 6AQ5 series modulated rig used just one tube in the modulator and it used to cook at 5 watts. I really needed two or a bigger tube in that circuit.

A single 6LF6 RF sweepie,   -     and single 4D32 series modulator still come to mind as a viable pair.   Lots of emission, RF capability and audio headroom.  Both tubes wud take huge HV like 1500V.  Even a single 807 in the final might be a better match for the 4D32 modulator dissipation-wise. I don't know yet.

T
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« Reply #28 on: September 04, 2020, 02:11:42 PM »

OK, Rick, sounds like a plan.

One thing I still don't understand...  If the series modulation tube is put in the B+ line, then its cathode, grid and plate structures are tied into the HV potential. How do you isolate the audio drive signal from ground? Do you float the audio driver itself or use large coupling caps to ground to avoid the HV DC or what?   It seems that somewhere along the line before the microphone that HV isolation is needed.  That's why I was considering the fiber optics.

Hi Tom,

I did some prep work, drew a simple modulator schematic, and researched some similar power supply circuits that do basically what we want to do.  

The driver will simply be a triode, tetrode, or pentode, with the plate resistor tied to the HV B+, and the plate tied to the grid of the series modulator tube, which works as a cathode follower.  The series cathode follower having a DC voltage gain of about .9, the driver plate must swing the AC voltage on the grid of the series modulator a bit more than the final amplifier sees in the modulation swing.  All we need here is a tube that can withstand a large plate voltage, but very little dissipation, such that it can develop the drive voltage across the plate resistor.

I will probably need to do several posts to get the sequence of steps to design your modulator down on the thread.  It would be so much easier with a whiteboard and a zoom meeting, but this will work.

The design needs to work backwards, qualifying and characterizing the series modulator tube, then the driver, so that we will know just how much bias and audio are required at the input to obtain your desired performance characteristics.


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« Reply #29 on: September 04, 2020, 02:19:30 PM »


A single 6LF6 RF sweepie,   -     and single 4D32 series modulator still come to mind as a viable pair.   Lots of emission, RF capability and audio headroom.  Both tubes wud take huge HV like 1500V.  Even a single 807 in the final might be a better match for the 4D32 modulator dissipation-wise. I don't know yet.

We can use one or more parallel tubes in the series modulator to distribute the heat and dissipation.  We might prefer a sweep tube for the RF final, such that we can obtain our desired 25 or 30 watts with less un-modulated plate voltage on the final, thus allowing the series modulator to use the reserve voltage to generate higher positive peaks.  Or we could just be happy with plus and - 100 percent peaks, and run the plate supply at just over double the voltage to the final.  This operating point will be adjusted with a pot, and can be varied as desired.  You might use one or two 4D32s for the mod tube, with the screen tied to the plate with a resistor.  In my tests, I am starting with an RCA 810 or two for the modulator, and an RCA 8000 for the final. Possibly a 4-65 for the grid driver tube, lots of voltage, very little current.  My goal is to get the 3CX3000F3 working as the modulator for a pair of 304-TLs or something similar, with 2000 volts at 500 mA on the final, probably using a 4-250 as the driver tube, just to handle the voltage swing on the grid of the series modulator.  Plate supply will be 5000 volts at 1 AMP CCS from a Collins 5KW FM transmitter.
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« Reply #30 on: September 04, 2020, 02:35:04 PM »

I am sharing snips of portions of the power supply sections of Hewlett Packard 606 and 608 RF generators.   Our series modulator will work in a similar fashion to the series pass tube(s) in these regulators.  

The plate of the series regulator is tied directly to the output of the rectifier/filter.  The cathode of the series tube feeds the power supply loads, and in our case, the Final B+ is derived here.

There is a control tube that sets the grid voltage on the series tube.  It is simply a DC voltage amplifier, with the output taken from the plate resistor, directly coupled to the grid of the series tube.  The source voltage for this stage is the high voltage.   By varying the grid voltage on the control tube, we can take the series tube from cutoff to almost saturation, assuming we tie the cathode of the driver tube to a somewhat negative voltage source.

In the regulator circuit, the error voltage is taken from a voltage divider and pot to the control grid of the control tube.  As the output voltage rises, the control tube grid voltage rises, causing an increase in plate current, and a reduction of plate voltage.  This causes the voltage on the grid of the series tube to decrease, thus decreasing the output voltage and maintaining regulation.

Instead of this feedback loop, we can control the grid bias on the control tube with a pot, using some fixed voltages at each end, thus giving us a control to set DC plate voltage on the final, and easily adjust output power level.

Notice C44 in the 606 schematic, or C41 in the 608 schematic.  It is there to couple the output ripple directly to the grid of the control tube.  This ripple is coupled across the 1 meg DC bias coupling resistors, R78 or R69, respectively.  This allows the regulator to clean up the ripple in the output due to the phase inversion of the control tube.

We can insert our audio in this manner to override the DC set point, thus controlling the AC component of our series modulator.  The issue here is to obtain linearity in both stages as closely as possible.  The cathode follower should be relatively clean throughout its entire range up to but not including saturation.  We will have to play with different driver tubes to see which ones provide a linear transfer over the voltage range needed at the grid of the series modulator tube.

Next I will present the simplified modulator circuit, and explain the design process.  Most of it will entail pine-board bread-boarding, Frankenstein style!



* HP606Regulator.JPG (62.96 KB, 802x434 - viewed 54 times.)

* HP608PowerSupply.JPG (57.35 KB, 903x363 - viewed 57 times.)
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« Reply #31 on: September 04, 2020, 02:46:48 PM »

I drew a simple two-stage series modulator, not to be constructed as drawn, but simply to describe the design process.

Notice the series tube will receive around 700 volts (or more) on the plate, and will be set to provide around 300 volts on the cathode, at 100 mA, such that a sample power input to the final will be around 30 watts, with a modulating impedance of 3000 ohms.  To start the design testing, we take a 3000 ohm resistor at around 50 watts to simulate the final power supply load.

We add a meter and a scope so that we can observe the output of the series tube.  A bench supply with a variac may be used to start with 300 to 400 volts on the modulator plate, increasing the voltage as we define the control voltage needed on  the grid.

A separate supply can be used to vary the output tube's grid voltage, such that we can look at the modulation peak and valley drive requirements with various plate voltages, DC operating points, and try different tubes or pairs of tubes.  We can either graph the AC requirements for modulation by stepping the input voltage (all this being done without the driver tube), or we can just determine the end points (max and min voltage out), close to saturation and cutoff, respectively.

Once this is completed, we can think about what driver tube can handle the voltage swing needed on the series tube grid, hopefully getting out of the 5 to 10 watt pissweakerland up to the desired 25 to 30 watts.  This should be very easy with a sweeper tube for the RF final and one or two 4D32 tubes (rated at 50 W dissipation each) for the series tube.  If that fails, you may always go to one of your spare 8877/3CX1500 tubes.    I know you have a nice stack of those!

If you use a tetrode or pentode for the series tube, just use a resistor from HV to the screen, similar to the way you would provide self-modulation of the screen in a plate modulated tetrode stage.  That is the way most of the series regulators work, and the value of this resistor can make a big difference in the voltage swing needed on the grid of the series modulator.


* SeriesModulatorSample.JPG (63.46 KB, 1098x487 - viewed 77 times.)
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« Reply #32 on: September 04, 2020, 03:04:49 PM »

Now that we know what DC operating point we need on the series tube to arrive at the desired static plate voltage, we can start designing the control tube as a DC amplifier, then add the AC input in the form of modulation.  
This stage looks very similar to the clamp tube circuit used on the AM transmitters with 807s or 6146s, and functions in much the same manner.  It requires very little plate current to swing the voltage on the plate resistor, but it must be good for a very large plate voltage potential.  Here maybe another sweep tube, or other high voltage triode or tetrode.  A tetrode or pentode or beam-power tube will require much less audio swing at the input to provide the desired modulation percentage.  Even though this may be a larger tube than a typical receiving tube. it will not run hot at the very low plate dissipation.

Case in point, back in 1965, I needed to build a power supply for my new Heath SB-101, and I was not able to buy the typical HP-13 supply from Heath.  I designed around a very large TV power transformer that provided almost 1000 volts in a bridge configuration, and 500 volts from half the bridge.  I designed two series regulators around that transformer, one for the 300 volts, using a pair of 6L6 pass tubes in parallel, controlled by a 6AU6 with an 0A2 reference tube.  I provided a series regulator for the 800  volts to the SB-101 using a pair of 6336 dual triodes as series tubes.  These are carbon plate dual triodes on steroids, with about double the dissipation and current rating of the 6AS7/6080 tube.  I was able to provide a solid, stiff source of 800 volts, adjustable, using a single 6146 as the control tube with a 100 Kohm plate resistor to the 1200 volts.  I tied another 100K resistor from the 1200 volt supply to the 6146 screen, and added a screen bypass resistor.  This supply was run for over 20 years with no tube or other part replacements.

For the control tube in the 30 watt rig, perhaps a sweep tube from a very small TV would work, as they can handle enormous amounts of plate voltage due to the way the TV flyback circuit operates.  Very little plate current, but lots of plate voltage will be found here.  A tetrode or pentode instead of a triode will result in significantly reduced input modulator voltage requirements, due to the higher gain of the screen-grid tube.  But more important than gain, is the linearity of this class-A stage, and reduced drive requirements (just voltage) should not be a priority over linearity over the entire modulation swing.

We can discuss this in more detail as needed.  As I finish up the final version of my MAX Audio Processor QC work, I will then have more time to spend on my series mod rig.  The final rev PC boards for the MAX arrived a couple days ago and I need to get to work on that almost full time for a while.  GL on the PW project, and stay tuned!

By the way, looking at some other threads, specifically one on Cathode Modulation, I discovered that Steve, WA1QIX, stated that he has successfully homebrewed several series modulated rigs, mostly before he focused on the PDM and Class-E designs.  He might be able to chime in and offer some great suggestions to short circuit some of the research and development time testing tubes and determining the best ones for modulation linearity.

Edit:  Have a look at reply 21 on this thread:
http://amfone.net/Amforum/index.php?topic=35443.0
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« Reply #33 on: September 04, 2020, 06:10:01 PM »



Here is another, series modulation with some power FET's, and DC feedback with adjustment pot to set the final RF stage B+.

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

Jim
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« Reply #34 on: September 04, 2020, 07:26:35 PM »



Here is another, series modulation with some power FET's, and DC feedback with adjustment pot to set the final RF stage B+.

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

Jim
Wd5JKO

Jim, thanks for sharing that info and thread!  I forgot that was available.  I may consider using some of his solid state design in a hybrid to drive my hollow-state series modulator.  It would be nice to just go with all vacuum tubes, but it might not be as practical as the hybrid approach.  The devices are still available.  I continue to be amazed at the higher and higher voltages the FET devices can withstand!
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« Reply #35 on: September 04, 2020, 08:23:41 PM »

Wow - appreciate all the good info, Rick!

I'm very interested in your circuit below with the class C final at ground and the series modulator on top.  Maybe a 6LF6 sweep tube in triode configuration as the series modulator would work well. Let me digest this stuff some more and see what looks best.  I might even consider a somewhat bigger rig, dunno yet.  Triodes  in the modulator (as you show) wud be simplest. A tetrode RF final wud modulate best.

http://amfone.net/Amforum/index.php?action=dlattach;topic=46165.0;attach=65922;image

Later  -

T
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« Reply #36 on: September 05, 2020, 02:25:40 AM »

Maybe a 6LF6 sweep tube in triode configuration as the series modulator would work well. Let me digest this stuff some more and see what looks best.  I might even consider a somewhat bigger rig, dunno yet.  Triodes  in the modulator (as you show) wud be simplest. A tetrode RF final wud modulate best.

I believe a single or pair of 4D32s in the series modulator would be as simple as a triode, just one more resistor for the screen.  With a pair you could probably run more than 50 watts input to the final.  Kinda hard to find another tube with the emission, plate dissipation, and voltage tolerance of this Raytheon Wonder!  It's no wonder you need almost 8 amps filament current for a pair.

Agreed on sweep tubes for the RF, as they excel in producing lots of emission and power at lower voltages, improving your ability to get larger positive peaks.

As for the driver tube, tetrode or pentode would give more gain and require less audio input voltage, but possibly at the expense of linearity.  It is EASY to get modulation VOLTAGE, as there is really no load here, no real power required, just higher level.

If you proceed with testing various series modulator tubes for static output into a simulated 3000 ohm load, I would be very interested in your findings as to the source plate voltage you choose to test with, and the grid vs output voltages from standing carrier to peak and valley.  We do not need all the steps taken, just the three, and the rest of the linearity evaluation can come with the audio input scope output evaluation.  

I believe the series modulator cathode follower will be very easy to get decent linearity, but the driver tube may require some juggling and some study of the tube handbook recommended values for audio amplifier designs.  I doubt they will go anywhere into the voltage ranges we are interested in, but we should be able to extrapolate beyond their values based upon mu or transconductance.  I have found simulation to be very inaccurate at these extreme voltages compared to typical audio amplifiers, but various wave-forms will tell the story very quickly as we vary the bias (operating point) and the audio level input.

Charting some of this data in tabular or spreadsheet format may prove valuable for future series-modulation addicts.

Based upon Jim's (JKO) recent post on the thread, I would certainly consider MOSFETs as an alternative to the driver tube, but I would probably retain the series tube to allow extension to voltages well above PissWeakLand.

To reiterate, my preference to series over screen modulation is the ability to run a standard modulated class C final with ease of tuning and clean modulation, moving the inefficient dissipation issues to the series tube which entails no critical adjustment to ensure a clean signal.  I gladly trade the power wasted in dissipation for the lack of response-limiting and FRAGILE modulation iron, and it is a welcome breath of fresh HOT air in January, especially in your neck of the woods!  My Motto:  Series modulation is 100 percent efficient in heating the winter shack, but you gotta be an Old Buzzard for it to be effective!  Class E and PDM rule in the summertime.

GL and 73!
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« Reply #37 on: September 06, 2020, 12:00:36 AM »

Hi Rick,

I have decided on a tube lineup and some details for the new series modulated rig.  I wanted to keep you up to date so we can focus.  I was talking with Frank/GFZ tonight and came up with the following ideas:

A single 4D32 RF final in class C - series modulated by FOUR 6LF6 sweep tubes in parallel is the current idea. The modulator would be at ground reference, below the RF final, in the cathode lead, just like conventional tube PDM rigs.   I would tap an adjustable 1-2 KV HV from my Baby Blue 3-500Z amplifier to keep the new rig small. I could use a plate to screen dropping resistor on the final to eliminate the screen supply.

I won't get into the calculations now, but the rig should put out about 35 watts carrier with big, clean, positive peaks.  Frank suggested a 15 ohm, 5 watt cathode resistor for each 6LF6 tube to balance the load and provide a small amount of NFB.

He suggested a resistor across the modulator plate to cathode to act as an NPL.  The 6LF6 control grids will be grounded and the screens driven by a single-ended 11N90 class A MOSFET driver.  The 6LF6 sweep tubes are robust 40 watt tubes. The 4D32 will be loafing as a 50 watt RF tube.  The modulators should be OK heat-wise with a slight breeze.

I will need a HV insulated fil xfmr for the 4D32 floating cathode circuit.

The overall idea is to replace my FT-1000D with a homebrew AM driver. This AM exciter can drive my 4-1000A linear amplifier no problem or act as a stand alone PW 35 watt rig.  Once optimized, the audio should be superb and the RF final will tune easily like any other class C plate modulated rig.

The math shows it should work OK. I will post that later once I am sure this is the right path.

On the subject of an NPL.... my series modulated 6AQ5 rig didn't have a formal NPL but it limited well at the negative peaks like it had one. I could pour on the audio to 200% easily. I think it was the string of diodes I used to set the carrier in the cathode of the modulator.  That may have acted as an unmodulated signal just like the feedthru power of a GG linear amp, but I am not sure.  

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


More later.


** Here's the 4D32 and 6LF6 tube data sheets:

https://frank.pocnet.net/sheets/201/4/4D32.pdf
https://frank.pocnet.net/sheets/084/6/6LF6.pdf



T
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« Reply #38 on: September 06, 2020, 04:26:28 AM »

Tom, that topology should prove very successful.  The 4D32 will have lots of headroom, and we know the screen will self-modulate well with a series resistor.  If you were running only 800 to 900 volts from your supply, then a sweep-tube final would have made it easier to get nice big positive peaks.  But with the potentials derived from the new baby, the need for the sweep-tube final evaporates.  I do like the 4D32 for RF, and it will take everything you throw at it if you need to clear the aether!

You may use grid-leak bias to keep it simple, or also include a fixed bias supply that floats with the 4D32 cathode.  For the heater, 3.75 amps at 6.3 volts isolated is easy.

I like the three 6LF6s with the combined dissipation of 120 watts, and they are small too.  

With transformer-coupled plate modulation, the modulator output adds to and subtracts from the plate voltage, so negative over-modulation can readily occur.  Because the series modulator is in series with the supply and does not add any voltage to the load, the lowest it can go is zero, at modulator cutoff.  The resistor across the modulator should be a very effective way to guarantee that there is a slight amount of supply voltage to the final, even at modulator cutoff, ensuring no zero-crossing.  Simple, elegant solution!

I have never played with the 6LF6, but you and Frank have lots of experience there.  The balancing resistors and screen drive should provide a robust solution, and the variable supply will enable lots of positive peaks, while running the 4D32 in loafing mode, compared to a 32V.

As it comes together, please share the design of the FET driver, as well as the bias set point circuit for carrier level control, and the audio path and input requirements.  This one is sure to be imitated by many.

Floating the 4D32 at the voltages you propose is straightforward, and the input/output RF networks require no special attention.   On the other hand, the rig I propose with the 3CX3000F3 and the P-P 304-TLs is a bit more challenging when floating the RF, especially since the RF deck is already restored from the one my dad built in 1949.  So I still plan to pursue the modulator-on-top architecture, running the 3CX as a cathode follower, and driving it with a 4-250 or 4-400 to handle the high plate voltage.

Just curious what you propose for frequency control?  The ricebox, or a DDS, Huh  Self-contained would be sweet, but I think more important is ease of zero-beating with other folks, so other alternatives may be superior.

Thanks for sharing your plans, it should sound great with the iron stripped out of the signal path!
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« Reply #39 on: September 06, 2020, 11:11:47 AM »

OK Rick -

I changed the 6LF6 series modulator count to FOUR tubes.  Frank suggested that the  operation point may drift around as the tubes heat up. It's probably best to have extra headroom to play with since the 4D32 final is just loafing.  Maybe it will be more of a 40 watt stand-alone PW rig, but I can drop it way down to drive my three bigger linears (or 20 watt PW use) and still have the potential for big audio peaks at lower voltages below 1KV with the high-emission 6LF6s.    My Baby Blue  HV supply transformer taps and the Variac will give a lot of options. Four 6LF6 40 watt class A sweep tubes (160w diss) feeding one 50 watt class C 4D32  RF tube with a wide range of HV variation shud give the platform good utility.

I plan to use the FT-1000D as a VFO as I do with all the rigs in the shack. The 4D32 won't need more than a few watts to drive it.

Yes, grid leak is FB for the 4D32. I also have an insulated fixed grid bias xfmr that I wound -  left over from the old 4D32 PDM rig.

This should be an interesting project.  Thanks for your input to get the boat headed in the right direction. It will come together slowly over time... I will put together a hand drawn schematic as a first step - so we can critique it here soon.

T


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« Reply #40 on: September 06, 2020, 11:41:41 AM »

Tom, I am liking it even more as the design grows.  I would very much like to see what you and Frank come up with for the audio input and carrier control (DC set point) circuits.

Another positive in adding a fourth series modulator - Not just the power level you may run, but also the cleanliness.  Remember you are running class A on the series tube, and it is NOT a cathode follower when placed below the final amplifier.  It will not be perfectly linear throughout it's entire grid-plate swing.  So having extra current capability and dissipation, as well as a lower impedance source by virtue of more parallel paths, you will be able to run at a lower level of audio swing (percentage wise) and stay within the linear portion of the curve when modulating.

I wonder if you plan to do any preliminary modulator testing before building it all up with the RF section?  Easy to tie one, two, three, or even four tubes together, without any chassis, and apply power, audio, bias, and a load.  Figure what your target modulating impedance (plate volts / plate current) will be for your final, and substitute this resistor between plate supply and modulator plates.  Now add your cathode resistors to ground, connect a variable DC supply to the screen grid through a resistor to provide bias (a variac supply is fine here) and inject some audio with a coupling cap from an audio generator and amplifier, if necessary.

Scope the output at the plate through a blocking cap and resistor, and look also at the DC voltage at the plate.  Play with the inputs to see what voltage comes out (peak to peak) and how it varies in performance at different dc set points for input power selection.

This is a very simple procedure that will give you real numbers for your actual tubes, and help to plan actual power levels and audio cleanliness before you even create RF.

I am doing the same thing with my 3CX3000, just testing the series modulator tube, no driver circuits as yet.  My load is a string of either 16 or 20 each 60 watt incandescent lightbulbs, for 1920 volts at 960 watts input, up to 2400 volts at 1200 watts input.  With a fixed DC set point, the bulb resistance is rather stable, and it does not change much with modulation, but with the voltage measurement and known load dissipation the watts are obvious, and the scope tells how clean the output wave form is when passing through the thermionic transconductance.   This is all done with a variac on the 5000 volt 1 amp CCS supply.  The blower on the bench for the 3CX tube is quite noisy!

You might find a string of smaller light bulbs makes a good load for your modulator test.  I think it is worth the time so that modulator and RF issues do not have to be addressed simultaneously in the end.  I think the linearity of modulation is a much higher priority than just the estimation of the raw power output. 

Another consideration is that if you do run into linearity issues when it is all together, you know where you stand with the series tube performance, and you can then address fine-tuning the audio driver circuits, plus, you will already know what bias levels you need to provide for the desired input power ranges.  You might be able simulate all this or calculate it in advance, but with four tubes from the dual quads, and the higher voltages, seeing the triangle and sine waveform and measuring the voltages is probably somewhat more practical an approach.
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« Reply #41 on: September 06, 2020, 11:43:44 AM »

I just dug out my Cheyenne transmitter the other day, basically a DX-60 with a built in VFO. I need to come up with a power supply for it first. Bill, we were talking about this rig a couple weeks ago on 50.4, you had inspired words to say about the screen modulation which I have not done in decades. Also, thinking of modulating my "BC-3.75" mini hybrid mil rig, and thought of suppresser grid modulating a 6AG7 also. How much power did you get out of yours Mike?
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« Reply #42 on: September 06, 2020, 01:08:14 PM »

Tom, I am liking it even more as the design grows.  I would very much like to see what you and Frank come up with for the audio input and carrier control (DC set point) circuits.

Another positive in adding a fourth series modulator - Not just the power level you may run, but also the cleanliness.  Remember you are running class A on the series tube, and it is NOT a cathode follower when placed below the final amplifier.  It will not be perfectly linear throughout it's entire grid-plate swing.  So having extra current capability and dissipation, as well as a lower impedance source by virtue of more parallel paths, you will be able to run at a lower level of audio swing (percentage wise) and stay within the linear portion of the curve when modulating.


Rick,

I'm about to take a slight detour...

Youse gots me concerned about the overall linearity of these four 6LF6 tubes.  Yes, staying on the straight part of the curve is important. I'm thinking it will be even harder to do with four tubes. They are not matched or by any means perfect. They are used and abused from the dual quads PDM days. The whole purpose for this project is perfect audio with heat as the tradeoff - so linearity issues are not acceptable.

It may sound crazy, but a single 3-500Z triode, (a 500w diss tube) in place of the four 6LF6s sounds like a better/cleaner  solution to me. At 1500V it is -46dB 3rd order IMD, the best in the world,  and the curves look straight as an arrow.  The power output of a single class C 4D32 now comes up to an easy 100-125 AM carrier watts out.  The overall efficiency is not much different than running the 3-500Z as a linear. I get out about 100-150 watts with my Baby Blue single 3-500Z or the Summer Breeze 4D32.  

We may need to add to the 3-500Z a resting bias that goes positive when at lower voltages, I dunno..


Check out the beautiful specs and curves:
http://w7brs.com/3500z/3-500z.html

This reminds me of the scene in Animal House where the unwanted pledges (like Flounder) always end up in the same room with the other undesirables.  No matter how hard I try to build a PW rig, I always end building a 100 watter.  "Sit down with the other 100 watt pledges and enjoy yourself, Tom!"

https://www.youtube.com/watch?v=LuFCaIAnETk

From a pure performance point of view, a single 3-500Z class A series driving a single 4D32 class C with 1-2KV would be hard to beat.   I have an extra 3-500Z and chimney/socket in the stash.  I don't think there would be a linearity problem running the rig below 100 watts.   I don't plan to do any pine board pre-testing, rather build it and improve from there.  Choosing the best tube lineup from the start is important.  I can envision  both the 3-500Z and 4D32 loafing along at any power level between 30 to 125 watts with a turn of the Variac - and the ability to do 200%+ audio peaks with the NPL if desired. That's the kind of dream worth building.

T
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« Reply #43 on: September 06, 2020, 01:35:27 PM »

I think we need to take a step back and I should explain how simple the issue is.  I do not think you should abandon the sweep tubes for a 3-500Z.  That is like bringing in a bulldozer to kill an ant.  And you want it quiet, small, no blower noise, etc.

The issue is not the non-linearity of one tube vs another, or old beat tubes vs new.  In parallel, the curves average anyway.



What I am speaking of is using the most linear part of the transfer curve, no matter what the type.  Toward saturation, the curve levels off.  It is a matter of setting the bias and operating point where you will stay on the straight line without your peaks being compromised.  I dare say the 3-500Z is way overkill, and if you were going to do that, just modulate a pair of 813s!

The sweep tubes are really a perfect fit for the application,  as they withstand very high plate voltage, they have outstanding emission for peak plate current, and they will provide higher plate current at lower voltages than most tubes.  I think you have an excellent plan.

I was only trying to provide another method of "getting down the numbers" so that when you design the operating points and the driver circuitry, everything is in the best part of Class A.  I have no doubt the sweep tubes will do a clean job, my only point is whether you do real-time analysis before you build the unit, or fine-tune the parameters after it is assembled.  

I should have realized from history that you prefer to fine-tune after the fact, and run everything in the sweet spot.  I more often breadboard and come closer to the final design before I punch the chassis.  Different strokes.  I would have faith in Frank's suggestions, leverage his ideas and design of the input circuitry with the FET driver, and have fun.  Save the 3-500Z for another project or as  spare.   I hope I did not derail your enthusiasm.  Press on, have fun!
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« Reply #44 on: September 06, 2020, 02:54:18 PM »

OK on all, Rick -

So, it appears the 6LF6s will be fine as long as I find the best part of the straight operating curve thru experimentation.  I can do that no problem.

Two points about using the 3-500Z -  one is that the filament power is reasonably close for both.  72 watts for the 3-500Z  vs: 50.5 watts for the four 6LF6s.  That is the only advantage the 6LF6s have that I can see. I could put a small breeze thru the 3-500Z to keep it cool enuff without much noise as I do running my 3-500Z linear now.

Also, the 3-500Z modulator gives me the option of running 125+ watts carrier out with the single 4D32 whereas with the 6FL6s I am pretty much limited to 45 watts out.   I am using  30% X the mod tube dissipation as the calculation.  IE, 3-500Z, 500 watts X .3 = 150 watts out.       6LF6,  160 watts X .3 = 48 watts.  The 4D32's full potential is being utilized when the 3-500Z is used.

I am assuming that the other performance areas are about equal. I do not have to contend with a screen with the 3-500Z.   So overall, using the 3-500Z is going to waste an additional 22 watts of filament power but give the option of running 100-150 watts AM carrier instead of being limited to 48 watts max. I assume I can reduce the HV and get a range of 30-125 watts carrier, but we will have to see. The tube has a proven linearity design, whereas the 6LF6s have sketchy data, though they are probably OK for the job.

Thoughts?

T
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« Reply #45 on: September 06, 2020, 03:15:27 PM »

Tom -

I don't have a dog in this fight, but as a casual observer of this thread it sounds to me like a good way to go would be to pine-board both modulators along the lines of what Rick has suggested and see which works best...

Just my 2 cents.

Rod
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« Reply #46 on: September 06, 2020, 04:44:00 PM »

Tom,
Been following this discussion, trying to learn as much as possible, very interesting ideas here.
I thought you were designing a 30-40 watt transmitter to drive a linear, that seemed like a cool idea,
For many of us, driving a linear with 100-150 watts means using an attenuator, dummy load,etc. and wasting a lot of power.
I realize with your skill/knowledge level you seem to be able to make any transmitter operate properly at any power level,
...Still it seems it would be useful to have a 30-40 watt transmitter just to drive a linear,etc.
Anyway, whatever you come up with I know it will be interesting, keep-up the good work..
I'm also enjoying learning more about Sweep tubes, interesting...
Donnie  AG5UM
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« Reply #47 on: September 06, 2020, 04:46:54 PM »

You raise many different issues that would need to be evaluated with some calculations or simulation before any obvious answers can be had regarding the switch to the 3-500Z.  It appears your goal of having a pisweaker to run either barefoot or to drive a linear has migrated to having a general purpose AM transmitter with variable power output to the limit of the 4D32.  While this is not a problem, it will take some time to digest and provide meaningful suggestions.

First, the filament power and fan cooling are so small so as not to be a factor.  If size and weight are not a concern, then you pretty much have a clean slate.  But my concern, with the 3-500Z, is whether you will have a "friendly" to operate low power rig with such a series modulator.  It really has more to do with the voltage and power range for which the tube is designed, 

If we go back and look at the series regulated power supply designs, we can compare the basic trade-offs between tubes when looking at series regulators like the 6AS7-6080 vs the 6L6 and similar higher voltage tubes.  The 6AS7 has much lower plate resistance and lots of dissipation, so if a large current is desired from the supply, it can be produced with a relatively low rectifier output, because the series tube drops less voltage for a given output current than a tube with less emission and higher plate resistance.  So to produce a series regulator for a higher current with the 6L6, higher rectifier voltage output is needed, and the dissipation in the series tube goes up with the fixed output current and larger voltage drop across the series tube.

A similar situation occurs if you take an RF final amplifier and switch from 250-THs to 450-THs.  For similar output power, it is necessary to run somewhat higher plate voltage due to the wider electrode spacing. 

I have not made any reference to the tube curves for either the sweep tubes or the big Eimac triode, but I would expect that to achieve the same output current to the 4D32 final with the 3-500Z, you would need to run significantly more HV from the power supply than would be required with the sweep tubes.  Also consider the fact that you have several low-resistance sweep tubes in parallel running at a lower voltage, compared with a single 3-500Z with wide electrode spacing, the higher plate resistance equates to larger voltage drop across the series tube, greatly increasing the overall dissipation for the same RF power output. 

My first inclination is that the 3-500Z is not really a match for this application unless you are going to be running it to obtain maximum power from the 4D32, and you do not care how much voltage or heat you throw from the series tube.  For this application, the linearity of the tube itself is secondary to running the chosen tube at the DC operating point to realize linear audio amplification.  I did not intend to imply that the sweep tubes would not provide satisfactory linearity for audio amplification.

Likely I should not have raised much concern about the linearity of the series modulator tube with regard to the audio signal.  If you look at what happens to the signal between the microphone and the modulator input, you have a great deal of processing or tailoring of your signal.  EQ, compression, filtering, etc.  A slight amount of variation in the waveform in the stage that feeds the series tube will most likely be totally unnoticeable, assuming you are not running it into cutoff or hard saturation with modulation.  And a slight variation in the audio waveform will cause no more IMD in your signal than will your graphic equalizer that tailors you voice to your preferred shape.  Bottom line, with the ability to set the DC output level and power, as well as the modulation level, independently, the resulting sound should surely rival any plate modulated rig, and will probably be more linear than a screen modulated rig even with your used sweep tubes.  I really think there is nothing to worry about with the original plan you and Frank concocted. 

If I was in doubt, I would at least connect one tube, one series resistor, a meter, scope, and a couple variac power supplies and SEE the output waveform, as well as the input and output voltages need at your desired power level (plate current) to know where you stand before building.  Then you could make an educated guess as to how it would behave with multiple tubes in parallel through extrapolation.   I did all this with 6AS7s, 6L6s, and a couple other tubes with clip leads and a couple hours work before building an earlier series mod rig years ago.  I strapped on a few harbor freight DVMs, and did it all from a distance on the workbench, and tabulated the numbers.  I no longer have the data, but it was a very interesting exercise with simple receiving tube types.   I have all the tubes except the sweep tubes, I am getting interested enough to play that game in the next few days if it would help, but unfortunately no 'LF6 tubes live here.  With Frank's experience, I would expect that what he is proposing will be rather easy to find the sweet spot after you assemble it.  Can't wait to see that FET circuit design!

I will continue following this with great interest, but over the next few days I will be spending a great deal of time mentoring students with their "FIRST Robotics" program in south Florida, mostly via zoom meetings.  I also need to spend considerable time on the documentation and final QC for the audio processor project, as we have several folks patiently awaiting results!  The boards desire more solder smoke!
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« Reply #48 on: September 06, 2020, 05:52:24 PM »

What about a 4-125 modulating a 4-65 Smiley


Or a 4-250 modding a 4-125?

Not a huge fan of paralleling tubes.  If you need more than one tube, you need a different tube.

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« Reply #49 on: September 06, 2020, 06:05:31 PM »

Thanks much for the comments and suggestions, guys!

This is exactly how I like to design stuff. Come up with a possible design on paper and then try to improve on it. Keep trying different ideas until there are enuff reasons to stay with the latest one.  No harm trying stuff on paper.

From what I am seeing, there are several reasons to stick with the quad 6LF6s series modulating a 4D32. The sweep tubes are simply better suited for the job.  My first reason for this rig is as a driver for the various linears. It will require 10-30 watts carrier. The sweep tubes are best suited for this. Second, I want a pissweaker rig. I have enuff 100 watt class rigs, but no PW rigs. The sweep tube modulator is best for this purpose. The 3-500Z modulator would be more suited for higher voltage as Rick says. I think keeping it optimized in the 10-40 watt range is the best choice here.  We can design up a matching MOSFET driver suited to this lower range.

And finally, I get the impression there are a lot of guys who might like to build this rig once proven, working FB and there is a schematic and operating procedures for it. Whether as a stand-alone 35-40 watt AM rig or linear driver at 10-20 watts carrier, this will make a great RF source with transparent audio.  Making the modulator a 3-500Z would knock some guys out of the game due to cost and generally larger parts needed..  It would also ruin the pissweaker mystique by making it 125 watt capable.

So, I will draw up a basic schematic for quad 6LF6 series modulators and a single 4D32 final and then get back with Frank and see what he can come up with for a simple MOSFET audio driver.  It looks like we will be grounding the grids of the 6LF6s and directly driving the screens with the MOSFET driver. No audio transformers anywhere. This will make it more stable and will simplify things further.  We might look into audio NFB (neg feedback) later on.

Thanks for the detailed info Rick and comments from the other guys here.

Look for more info on this project soon.

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. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
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