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How much peak power can a class C 6146B put out?




 
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Author Topic: How much peak power can a class C 6146B put out?  (Read 58905 times)
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K1JJ
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« Reply #100 on: April 24, 2014, 03:16:25 PM »

Getting closer.  Bottom all wired and ready to install and do some testing.  

The grid leak and screen dropping resistors all needed different values.  (About 1/5 their old values)

This 4D32 mounting plate gets bolted to a piece of Plexiglas which insulates it from chassis ground. The whole plate floats at 2 KV under full PDM modulation.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #101 on: April 24, 2014, 04:38:56 PM »


  Tom,

   This is getting exciting!

Some random comments. I suspect your drive requirement will go up quite a bit, and that the added circuit capacitance will require you to make some changes. I don't remember if you neutralized the quad of 6146's, but you will likely have to do that to the 4D32's in order to make 40M behave. I wouldn't be surprised if you get 400 watts with only three tubes. One less tube might make the RF drive, and Rf capacitance issue more manageable.

Good Luck...

Jim
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« Reply #102 on: April 24, 2014, 05:58:25 PM »

Thanks for the comments, Jim.

So you think the 4D32s are more apt to take off than the 6146s?   I loaded each grid with a lower swamping resistor this time - 4.7K in series with a .003 cap to ground. I hope this will help.  No neutralization.

Well, I just discovered the space-out of the year. I tried the filaments and nothing happened. The grid meter was negative.  I wired the sockets wrong!  The pins are off by one pin rotation.  So just stripped it down and starting over.  Isn't life grand?

T

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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #103 on: April 24, 2014, 06:34:43 PM »

So you think the 4D32s are more apt to take off than the 6146s?   I loaded each grid with a lower swamping resistor this time - 4.7K in series with a .003 cap to ground. I hope this will help.  No neutralization.

Tom, a case in point. I once took a pair of 7591's in a Central Electronics 20A, and could make > 50 watts out on 160-40M. These were low capacitance tubes, like 0.2pf Cgp like a 6146. Later I tried a pair of EL34's, and I had > 60 watts on 160M, and instability on 80-40m. Removing one tube made 80m work fine, but 40m was still a no go. The circuit wasn't easy to add neutralization, so I skipped that step. The EL34 had over 1 pf Cgp, and that was the deal breaker.

The 4D32 according to Raytheon has 0.4pf Cgp, 30pf Cin, and 16 pf Cout.
The 6146B according to RCA has 0.22 pf Cgp, 13 pf Cin, and 8.5 pf Cout

Those grid swamping resistors are in effect all in parallel....How big is the driver? I suppose that method could work with a big triode too, but even if it did all that Cgp regeneration might skew the modulated envelope linearity a lot. Later on, playing around with the 20A, and a single EL34 in the final, I added a N/C circuit, and it was set correct when I had a nice trapezoid on 20M.

I bet two of the 4D32's will do what the quad of 6146's did. Might start with two, and work your way up to three...then ponder the trip to four...

73,
Jim
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K1JJ
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« Reply #104 on: April 24, 2014, 08:44:50 PM »

Very interesting about your experiences with those tubes, Jim.

The quad 6146Bs worked perfectly on 160 - 40M, so at least there is a good baseline to start with.

I didn't know that the small C g-p  could affect linearity via regen feedback.  I'll run some linearity tests when it's running and see. I could always add neutralization later. Right now I am using a broadband toroidal input to the grids for easy band switching.

Well, it should be a challenge to get working with all four tubes. I would have laid it out in a square, but got painted into a corner with this small box and layout. The tubes will have plenty of air though.

Should have it fired up tmw with some luck and will let ya know how it goes. Yes, 500-600 watts would be very cool for a little rig like this. 

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #105 on: April 24, 2014, 11:57:22 PM »

Well, I fired it up and it actually puts out power.   There was a slight tuning instability, but not bad. It put out 700 watts pep with the modulator bypassed. I MIGHT need to add neutralization, but not sure yet.

I need to reduce the tank impedance to about 1/2 since the 4D32s are drawing more current.  These tubes certainly generate more heat.  My filament voltage drops to about 5.5 volts, so this needs to be corrected too. (6.3V is normal)   The screen dropping resistor needs to be reduced to 1/4. Screen current is very low, thus low power out.  This should be capable of  2KW+ pep output (audio peaks) when optimized.

More tmw.

T

Pics with QUAD 4D32s installed:


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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #106 on: April 25, 2014, 10:48:27 PM »

Did a lot of changes today.

1) Swamped the 4D32 grids each with 2.3K =  600 ohms total input impedance.   Now very stable but harder to drive.  I have an MRF-150 solid state amp dedicated as a driver, so no problem.

2) Replaced the tank coil with a heavy duty silver plated coil.  The old coil was underrated for the new job. I reduced the tank impedance for higher Q for the 4D32s.

3) Padded the screens from 200 pF to 400 Pf for each tube.  (Better stability)

4) Tapped the filament transformer to give a measured 6.2 V.  (Close enuff)

5) Replaced the 15K screen dropping resistor with a 5K, 30 watt unit.

6) Made shorter and more direct connections to the RF mounting plate.


Results:

RF tuning is rock solid with a peak in screen current, dip in plate current and maximum power coincidence.

Easy 500 watts of carrier on 75M.

Audio looks good and stable. No parasitics or other instabilities.


Problem:   (now solved - 6LF6s are OK for the job))

The four 6LF6 PDM modulator tubes cannot handle the peak power needed to fully modulate a 500 watt carrier. The rig will do 1800 watts pep at 1700 volts, but I don't dare raise the voltage higher. So, I have throttled the carrier down to about 350 watts which allows for excellent positive peaks.  In hindsight, four 4D32s in the final really need six 6LF6 modulators OR four 4D32 modulators.

The rig is running very FB now, and I will do some thinking about replacing the 6LF6s with 4D32 modulators.

This is like putting a bigger motor in a car and discovering that the infrastructure can't handle it anymore.

T

Check out the new beefy tank coil. I notice efficiency has gone up somewhat. Lots of air on the finals. I can touch the tube glass after a transmission. The transformers to the left are the floating fixed grid bias supply and the filament transformer - both wound with HV wire on 120V Variac cores - and insulated from chassis for 2KV.


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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #107 on: April 25, 2014, 10:54:31 PM »

How much are you getting out on six meters?
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« Reply #108 on: April 25, 2014, 11:21:27 PM »

How much are you getting out on six meters?

Nothing that I can measure, thank goodness.    Are you suggesting that I neutralize it?


BTW, I'm finding I can't trust my peak reading Bird wattmeter. At 500 watts of carrier, the REA software mod monitor shows 140% - 160% modulation.   The Bird does not show the equivalent audio power peaks. Probably too slow or something.  The scope shows heavy modulation too.  So guess I may be OK using the four 6LF6 modulators after all.  The 6LF6s do show some serious screen PDM current under heavy modulation.


The 4D32 finals show no plate color at 500 watts carrier.  They are very deep into class C.  The 6LF6 modulators show no color in class D switch, as expected.


T

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« Reply #109 on: April 26, 2014, 12:07:19 AM »

I have a Bird 43 wattmeter.  The peak detector is very poor - as are virtually all power meters.  Stu AB2EZ in fact pointed this out to one of the manufacturers and apparently they agreed.  Stu can tell you more, but suffice to say, having designed a number of them, a good peak detector is not necessarily an easy thing to design or implement.
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« Reply #110 on: April 26, 2014, 12:17:51 AM »

How much are you getting out on six meters?

Nothing that I can measure, thank goodness.    Are you suggesting that I neutralize it?





Nah. You're doing great.

Never trust any meters over 30.
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« Reply #111 on: April 26, 2014, 12:33:17 AM »

Quote from: K1JJ

The four 6LF6 PDM modulator tubes cannot handle the peak power needed to fully modulate a 500 watt carrier. The rig will do 1800 watts pep at 1700 volts, but I don't dare raise the voltage higher. So, I have throttled the carrier down to about 350 watts which allows for excellent positive peaks.  In hindsight, four 4D32s in the final really need six 6LF6 modulators OR four 4D32 modulators.


Ok, I have not been following this thread at all, but you have sufficient modulator capacity.  I looked up some ratings on the 6LF6 and from what I could find, the tubes will do more than an amp of peak plate current, 1.4A to be exact (from the rating sheet I read anyway).   So, if this is derated a bit to - say - 1A, at 1700 volts total DC, that's a whopping 6800 watts.  It should be enough ----

But you may not be able to realize this power level (at least not with good linearity) unless there is reasonably significant analog compensation in the modulator (again, I have not been following the thread, so maybe you have analog compensation).   Tubes are poor switches and need help.  Analog compensation increases the drive level of the pulse train as the tube is required to produce more current.  So, longer ON pulses are also driven at a higher amplitude (this is positive modulation).  Shorter ON pulses are driven at a lower amplitude (this is negative modulation).  In fact, at 90% negative and above (in other words, getting close to 100% negative) the tube should be operating pretty close to an analog series modulator because the drive will be very low at this point.

So, real numbers... With 1700V, I would personally run the RF amplifier at around 570VDC.  Maybe that's where you're running it....for 500W out at, say, 75% efficiency that would be 667 watts in, which translates to 1170 mA of plate current.  I think 4  4D32s will do that all OK.  570V will leave a LOT of room for positive peaks.  Not quite 200% positive but a bit more than 175% positive, and that's still pretty good.

If the rig will properly modulate a triangle wave up to the voltage limit of the power supply, then all is well  Cool
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« Reply #112 on: April 26, 2014, 12:48:09 AM »

OK on the 6LF6 tube capabilities, Steve.   Yes, it looks like they will work fine.

Tmw I will measure the finals high voltage and see where we're at.  I set it up by feel and will run some more tests. 600V across the finals seems reasonable to give plenty of headroom.

That Bird peak reading wattmeter is really a fooler. Not only are the peaks wrong, but as the audio frequency is increased, the readings are lower. Whereas the REA shows full modulation at all audio freqs.

Frank and I discussed analog comp a couple months ago and I was able to make some improvements to get the negative peaks close to =95%.  I run the NPL to cover it after that.


If I were to increase the PDM carrier ratio somewhat, I'll bet I could see 600 watts carrier and still see 140% positive modulation.  Those 4D32s have so much emission it's scary. Same wid the 6LF6s.


T

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« Reply #113 on: April 26, 2014, 09:44:31 AM »

Of course the real acid test with respect to linearity is the triangle wave.  What do your triangles look like up to at least 150% positive and down to 95% negative?

Analog compensation is necessary for the positive peaks as well as the negative peaks.  Tubes aren't great switches, so the idea is to keep the voltage drop in line as the amount of current increases.  Driving the tubes harder during positive peaks is just as important as driving them at a lower level as the negative modulation increases.

When I was doing tube PWM design, I actually never hooked anything to the rig except for a triangle wave generator until I could prove linear modulation up to the positive peak capability of the transmitter.  It was truly amazing all the places where the linearity could be compromised including the RF amplifier itself!!  Those non-linearity gremlins sure find a way to sneak in  Wink
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« Reply #114 on: April 26, 2014, 12:27:17 PM »

Steve,

The last triangles I ran after optimization looked perfect up until saturation using the old 6146B finals. (140%)   I will run some tests today using the new 4D32s.

The thang that concerns me is that I am not seeing much more PEAK power using the 4D32 finals compared to the 6146Bs.  The flat-top point seems to occur at the same place.  At flat top, the 6LF6 start drawing big grid current and the 6LF6 plate current starts to dip. I am using regulated screens and grids for the 6LF6s.  It seems the 6LF6 plate voltage is dropping below the screen voltage, thus saturation.   Other than increasing the overall HV, what else can be done?  I have 2200 volts available, but hesitate to ramp it above 1700v.

A friend said the following concerning the 6FL6s in PDM service:

"The 6LF6 can deliver 1.4 amps peak but that is running a low duty cycle. PDM service is very different. Say your saturation voltage is 100 volts. At 1 amp peak current the tube dissipation is 100 watts peak. Say you are running .5 amps peak per tube with 100 volts saturation voltage the dissipation is now 50 watts which is a lot closer to the tube ratings. You should be able to get away with 4 tubes if you blow plenty of air across them.. Time will tell to see if the emission hangs in. One 6DQ5 would not fully modulate two 6146s (equal to 1 4D32) so maybe the 6LF6 has more poop."


So maybe I am running out of headroom with the four 6LF6s, I dunno.   I can run 500 watts carrier now, but the peak power is the same as before.  I will run some more tests today.

T
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« Reply #115 on: April 26, 2014, 03:21:10 PM »

That's interesting about the 6DQ5.  I had a Viking II, out of which I removed the analog modulator, put a bridge around the power supply (with a cap input filter), and used a 6DQ5 as a pulse width modulator.  The 6DQ5 *did* modulate the pair of 6146 tubes without issue.  That was many years ago, so I cannot tell you the parameters under which I ran the RF amplifier.  I might have used a pair of 6DQ5s, but I don't think I did - doesn't ring a bell.  Probably didn't own 2 6DQ5s at the time!

Another rig from back then used a single 4-400 pulse width modulator to modulate an 813.  5kV total supply voltage; 2000V @ 200mA on the final.  The 4-400 is *not* a good pulse width modulator.  It is a low current, high impedance tube, but with coaxing it did the job nicely.

Also designed a transmitter using a single 4PR-1000 to modulate a pair of 450TLs.  The modulator was not particularly bigger than the RF amplifier there either.  That transmitter had a 10kV power supply, and was designed to modulate up to 150% positive.  This was a proof of concept for a broadcast transmitter I was designing way back when (another story for another day).  At 500mA of plate current, that transmitter was absolutely linear up to 150% positive.

Your modulator is pretty hefty, and I would expect it to be able to modulate the power levels you're expecting.

I do know that with tetrode modulators, I ran as low a screen voltage as possible, with as much grid #1 drive as possible and then used analog compensation to make it happen at high levels of positive modulation.

The thing is, with PWM, the tubes are run at peak current and peak dissipation values that are higher than the average values, and by a fair amount at times.  With the analog compensation operating, the grid #1 current on modulation peaks (positive) was considerably higher than the maximum average grid current from the data sheet, but the actual average grid current (and grid dissipation) was not, and the modulator tubes didn't seem to show any ill effects.

Unfortunately, with PWM and tubes, high voltage is your friend with respect to power out, efficiency and modulation capability, which is why I used 10kV on that 450TL rig and 5kV on the 813 rig (and about 2200V on the Viking II, as I recall).  But high voltage is NOT your friend with respect to construction complexity and all of that..

It's a non-trivial project, but you are keeping at it, and eventually will get it !  I can honestly say "I feel your pain" having going through this iteration myself !!!!
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« Reply #116 on: April 26, 2014, 06:09:08 PM »

Interesting.

You used 2200 V  on the Viking rig? That's encouraging, cuz I'd like to hit that level too.    I've been creeping up the HV on my  4D32s and 6LF6s and now up to 2KV.  Maybe I'll go to my Variac limit of 2200V.  It certainly runs well at these levels.  I was just testing at 500 watts out and seeing voice peaks of 140%, so it is working.  With a nudge of the carrier ratio, I am seeing 600w but slightly lower % modulation.

I'm not sure where the breakdown weak link might be...  if the tubes can handle 2200V, I can make the surrounding infrastructure do it too.

It's really a fun rig to play with - being small and compact, all tubes, efficient and a real strapper considering the modest tubes used.
 
T
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« Reply #117 on: April 27, 2014, 12:06:03 PM »

More testing with the triangle wave at 100 Hz, 1Kz...

The triangle and linearity looks very good at 1600V up to 160%. This is at 300 watts carrier out.    But when I ramp it to 2000V and run 500 watts carrier, I cannot get above about 110% positive without the triangle going non linear and the peak gets rounded. There is a power saturation limitation in there somewhere.  

When I push the rig to this non-linear point, I see the 6LF6 grid current start to soar and the 6LF6 plate current start to dip down. Normally they are both constant.   The maximum peak power capability is not much different than when using the old 6146Bs in the final.  

Today I will disconnect the PDM filter and put a simulation power resistor load across the 6LF6s to conquer and divide. We'll see if the peak saturation is related to the PDM filter, RF finals or the 6LF6 PDM stage itself.  

The rig presently runs beautifully at 250 watts at 160% positive as is, but then again, I could have kept it as a 6146B rig and done the same.  So I gots to get this solved even if it means putting in four 4D32 PDM modulators.  I wanna see that peak audio power soar unlimited like an eagle, caw mawn.


More later.

T

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« Reply #118 on: April 27, 2014, 01:06:23 PM »

Ok, well don't change ANYTHING until you know what is actually happening  Cheesy

I would first put a scope on the output of the PWM filter.  Look at that waveform.  Does it round off there?  If so, the problem's in the modulator.  If not, obviously the RF amp is at issue.  How LOW does the waveform at the output of the filter get (this would be the positive peaks) before you see the waveform flattop?

Then, I would look at the PWM waveform at the INPUT of the PWM filter.  You may need to construct a special H.V. probe to do this.  I used a resistive voltage divider (with non-inductive resistors), and a "gimmick" capacitor across the series resistor to make the probe's frequency response flat to at least 50 megacycles or so - all of this looking into a standard 10x scope probe.  It's important that the probe not change the waveform in any way.

So, after all of that business about the probe, what does the waveform look like at the input to the PWM filter?  How LOW does it go when the tube is turned on.  Does the LOW voltage (tube on) value change with loading and/or with pulse width.  If you put the triangle wave into the modulator while observing the output, do you observe a variation in the ON and OFF voltages of the PWM waveform (measured at the input to the filter)?  Ideally, you should not, except for whatever the analog compensation is doing. In the absolutely ideal TUBE pwm world, the ON voltage at the input of the PWM filter might even be a bit lower (tube turning on harder) during positive excursions.  Usually, you cannot get this, but the ON voltage not RISING during hard turn ons is very important to the generation of positive peaks.

These are important tests, and will reveal much.

Regards,  Steve
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« Reply #119 on: April 27, 2014, 03:09:03 PM »

Excellent advice, Steve, tnx.

Good results so far....

Tests showed that the output of the PDM filter was clearly showing PDM modulator flat topping, so the problem is definitely in the modulator as expected.

I increased the 6Lf6 screen voltage from 85V to a higher level - and reduced the 6LF6 grid voltage from +1.5 to ground.  This resulted in more 6LF6 plate current, though hopefully the four  can handle it. (500 ma PDM, meter pinned)

The good news is that the peak power has now risen from about 1500w to about 3000w before flat topping.  I plan to put a bigger 6LF6 plate meter (1A) in there and try again.  


The good sign is I am now getting a clean, straight,  brickwall flat top of the positive peaks. Before it was a squirrelly rounding look.  I feel the modulators are now saturating hard, as they should, at maximum peak power.


Before this modification, I unplugged two of the 4D32s.  Using the limited modulator, I found that I could get out the same RF power as four tubes. So, the final is OK. It just needed a modulator that could keep up with it.

I may add a screen meter to the 6LF6s too.  The problem is that I don't know what currents are reasonable for the PDM grid/screen and plate set of 6LF6s.   The RF 4D32s are drawing 1A  RF carrier current and at the same time the 6LF6s are drawing 500 ma of PDM current.  The grid is at about 5ma now that the screen voltage has been increased. Who knows what the screen current is now.


More tests. Looking better - just don't want to blow the modulators until I meter them better.

** Update:  At 300 watts carrier, using a negative peak limiter, I am able to get the REA mod monitor to flash at 190% voice modulation.  Now THAT is a breakthrough!


T
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« Reply #120 on: April 27, 2014, 03:34:10 PM »

After a long test into the dummyload at 400 watts carrier and 180% voice modulation at 1900V, the 6LF6 modulators show no color and there is very little heat being blown out by the fan. This is at 500 ma of modulator screen current and about 1A of plate current..

The 6LF6 grid draws only 5ma, which seems reasonable. Drawing more grid current doesn't seem to help anything.   The screen current needs to be measured, but I plan to set the screen voltage at the minimum needed to do the job.

So looks like the problem was that more modulator saturation was needed via more screen voltage, thus more plate current.   All other parameters are basically where they were before.

T
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« Reply #121 on: April 27, 2014, 05:09:33 PM »

Check out the 6LF6 tube under "Average Pulse conditions"

http://tubedata.milbert.com/sheets/084/6/6LF6.pdf

I see no screen dissipation  power rating.

In my rig, for the screen I am measuring 500 ma at 97V. (125ma per tube)  This is what I need to get decent saturation performance. (48 screen watts, 12 watts per tube)

They are talking about 275V screen maximum  at 40 to 70 ma.    275V at 40 ma = 11 watts per tube.


My current at  125 ma per tube seems excessive. But the power in watts is only 12 watts.
Is this reasonable or are the 6LF6s going to see a short life due to screen burnout?

The grid and plate currents are well within specs.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #122 on: April 27, 2014, 05:14:50 PM »

Sounds like very good progress indeed!

So, what are your operating parameters and what is PDM current a measure of?

For instance, in PWM transmitters I measure the following:

Total power supply current (that may be your PDM current)
Total power supply voltage (that's probably your 1900 V)
Load current (this is the current as measured at the output of the PWM filter and before any loads)
Load voltage (this is the voltage measured from the output of the PWM filter to the load common point, which may be the H.V. supply or ground, depending on the topology. In solid state, the RF amplifier common point is usually ground, but in tubes it's usually the H.V. supply)

So, what are the operating parameters at carrier (no modulation)?  Be very interested to see the numbers (and hear the rig on the air, too!).

Been kind of off the air for the past number of days, as I have a very academically oriented job interview coming up (in other words, I'm going to be interviewed by much younger, recently graduated computer science majors who are going to ask me about the finer points of polymorphism,  multiple inheritance, virtual classes, and all other theoretical aspects of object oriented programming because that's all they know at the moment) even though I've been doing it for decades!   A friend of mine who is a very brilliant software engineer was recently submarined in a similar manner by a really irrelevant interview question related to generating Fibonacci numbers???!!!??? (and this was relevant to anything?? - NOT)  Suffice to say, I've pulled out my books and reference manuals and have been going over some of the finer points of OO design  Tongue
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« Reply #123 on: April 27, 2014, 07:13:51 PM »

OK Steve -

Well, at this point everything is running FB at my targeted power, so getting into the specific parameters would be a waste of time at this point.  I do plan to finalize everything and post a schematic later on. A few guys have requested one.

All I need to know now is if the 500 ma modulator screen current (125 ma /  11 watts per tube) is excessive and will burn out the 6LF6s. Please re-read my last post and give me your opinion.

Once I get a few more things buttoned down, I think this rig is a wrap... Grin

A good luck at the interview.  Fibb numbers,  1,1, 2,  3 , 5, 8, 13, 21, etc  are simply the sum of the last two digits. Fibb ratios, 0.618, 1.618, etc.  The mystics try to make a big deal out of its predictive value in the markets. BS.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #124 on: April 27, 2014, 09:18:40 PM »

More info for the future builder's archives:

Getting ready to button it up.  Still need to wind a smaller PDM filter later on.

After  optimizing everything, the 6LF6 screens are at 110V, 500 ma = 55 w / 4 = 13.75 w per tube.   Not sure how to reduce this voltage, unless I went to bigger tubes that cud handle it like the 4D32s.  If I go below this  screen voltage I see premature flat topping.  I am told  that 7 watts per tube is the recommended dissipation  number, so I am above it. .
 
That screen current is there even if I reduce the plate voltage and run PW, of course.
  
 
Man, itís really playing now.  At 1600V I get 300 watts carrier out with 200% positive modulation!    At 2200V FULL STRAP, I get 600 w at about 170% positive.   Nice and clean wid the triangle tests, all the way to maximum audio %.   Swept audio response is like a Flex... perfect, no transformers.  (1 Hz to 10 KHz, the PDM filter limit)
 
I will probably run it at  300w-400w most of the time.
 
It's really is a neat way to go for a summertime rig.    
 
T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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