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The "Dual Quads" PDM Tube Transmitter - under construction




 
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Author Topic: The "Dual Quads" PDM Tube Transmitter - under construction  (Read 16068 times)
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« Reply #25 on: January 13, 2014, 09:18:19 PM »

Geez, that was too easy...

I fired up the PDM modulator section and saw nice square pulses at the tube plates.  Fired up the whole rig with 1200 V and it looked  good on the scope and the audio sounded nice in the off-air monitor.   Seeing PDM shark fin audio!

It's running only at 100 watts carrier and 500 w pep for now.  I want to make some more mods for protection and improved metering before ramping it up to 1600V or higher.

I couldn't axe for more from an initial test.  No arcs, no blown fuses or tubes, no instabilities anywhere. No wiring mistakes. Considering past tests, I am amazed.  

To be super critical, I see a very slight waveform distortion, maybe saturation, when I run it up to 150% modulation. Might be the PDM tubes needing some positive bias, or maybe they will come alive when I ramp up to 1600V.  I have about 60 volts on their screens and grid grounded, cathode driven with the MOSFET.

Let it run at 120% modulation for 10 minutes into dummyload.  FB.  Tubes showed no color but felt hot as normal.    No fan air running yet.


More later.

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."
W3GMS
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« Reply #26 on: January 13, 2014, 09:53:55 PM »

Geez, that was too easy...

I fired up the PDM modulator section and saw nice pulses at the tube plates.  Fired up the whole rig with 1200 V and it looked  good on the scope and the audio sounded nice in the off-air monitor.   Seeing PDM shark fin audio!

It's running only at 100 watts carrier and 500 w pep for now.  I want to make some more mods for protection and improved metering before ramping it up to 1600V or higher.

I couldn't axe for more from an initial test.  No arcs, no blown tubes, fuses, no instabilities anywhere. No wiring mistakes. Considering past tests, I am amazed.   

To be super critical, I see a very slight waveform distortion, maybe saturation, when I run it up to 150% modulation. Might be the PDM tubes needing some positive bias, or maybe they will come alive when I ramp up to 1600V.  I have about 60 volts on their screens and grid grounded, cathode driven with the MOSFET.

Let it run at 120% modulation for 10 minutes into dummy load.  FB.  Tubes showed no color but felt hot as normal.    No fan air running yet.


More later.

T

Congrats Tom!  As stated, you could not expect anything more at first power on.  A very fast build indeed.   

150% positive peak modulation will distort on most envelop detectors anyway.  Yea, some guys are using SDR and some have low distortion detectors but most don't.   

Can't wait to hear it on the air.

Joe, GMS
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Simplicity is the Elegance of Design---W3GMS
K1JJ
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« Reply #27 on: January 15, 2014, 11:03:37 PM »

Project Update:

The metering was lacking, so installed plate current, screen current, grid leak voltage and plate voltage on a Plexiglas mount.  Since all meters are floating at high voltage, it required extra precautions.

I increased the HV to 1600V and now seeing about 250 watts of carrier and 1250 watts peak output.  Still not satisfied with that, but it's getting better. Not too bad for a tiny tube rig, I thought.

Still using used tubes - one of the old 6146Bs arced over and made a minor light show, but it was replaced by another soldier and works FB.

Take a look at the nice square and saturated PDM pulse coming out of the high level PDM modulator! Bet that's running near 90% eff.  I needed to increase the 6LF6 screen voltage up to 90V to get it that smooth. The peak power went up a bit too, so I'm thinking that maybe I need some positive bias, not sure yet.

The RF final parameters look good on the meters. The HV meter reads high and needs more R.

When finished, for safety I will probably enclose the whole rig in a Plexiglas cabinet.

Next, I'm trying to come up with a shutdown circuit. Not sure yet. It's tough to do cuz of the floating HV circuitry. For now, I have a 1.5A fuse with a 50K resistor across it in the B+ line.  

So far the rig seems very stable. I even brought it up to 2KV for a moment to see what happens. Stunts like that will be my undoing, I'm sure...  Grin

Still thinking of Steve's suggestion to replace the 6146Bs with sweep tubes. (6LF6s or 6DQ5s) They have 2.5A filaments and could increase the output power.  Even though the 6146s have a higher plate dissipation.

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."
K1JJ
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"Let's go sailing, Tommy!" - Yaz


« Reply #28 on: January 15, 2014, 11:05:58 PM »

More.

Pic #3 shows the DDS VFO driver and the SS PDM generator.  

(Bird dummy load used for testing)

* The original meter on the front panel is now a viewing window for the tubes.


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 #29 on: January 16, 2014, 12:41:41 AM »

Hey, it seems as things are coming along.  Nice work!!

Have you tried the old triangle wave test yet?  That is really the acid test - you'll know how linear everything is.

With dual trace scope, you'll have no problem - one trace is the original input signal, and the other the modulated RF.  With proper adjustment of the scope, the traces will overlay, and if all is well in the transmitter, the outline of the envelope will exactly match the input triangle.

Regards,  Steve
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K1JJ
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« Reply #30 on: January 28, 2014, 10:32:23 PM »

Here is the link to the continuation of this project:


http://amfone.net/Amforum/index.php?topic=35596.0
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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."
WD5JKO
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WD5JKO


« Reply #31 on: January 29, 2014, 09:58:25 AM »


Tom,

  As to the name search for the PDM rig with IGBJTs, consider the Oldsmobile 442. As I recall, 442 stood for"

4 - Four Barrel
4-  Four Speed
2-  Dual Exhaust

  So for the current incarnation PDM rig it could stand for:

4 - Four 6146's
4 - Kill you if you touch them meters
2 - Dual IGBJT's

 Tongue

Jim
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w4bfs
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« Reply #32 on: January 29, 2014, 10:37:23 AM »

thats a goodun Jim .... most folks that climb towers say that they get inspiration while that far offin terra firma ... not a chance that I would ....

so here goes ...

instead of 4x4 hybrid hows about 4x4 high bred (groan)
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Beefus

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