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Author Topic: PW Series Modulated Transmitter  (Read 44929 times)
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ka1tdq
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« Reply #50 on: January 25, 2020, 09:49:26 PM »

The tubes came in and it is making RF and audio.

On first try, I was getting between 15 and 20 watts carrier with low power settings (series pot at the KT-88 cathodes at maximum resistance and close to fully meshed loading capacitor). The problem is that carrier power drops off slowly when transmitting and audio isn't completely clean. Audio pickup is definitely sensitive since I could hear in the headphones low background noise in the room. This was even with the audio gain control near minimum.

I am getting a definite plate dip with the plate cap about half way meshed. I do need to remove some turns from the grid tank inductor since the variable cap is fully unmeshed. Grid current is barely registering on the meter. The only way I could tune the grid tank was by monitoring power output. It drops off if you move off the fully unmeshed position.

I'll give it a rest for the night. First try was pretty good though since nothing blew up, there's close-to-expected RF output and it does modulate.

Jon


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ka1tdq
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« Reply #51 on: January 26, 2020, 09:35:30 AM »

I think I may have found the source of the slowly dropping power problem. The plate choke coupling capacitor is only rated at 1kv and is getting really hot. I knew it was too small but I didn't have any others on hand and I wanted to test the rig.

I have removed turns from the grid tank inductor 3 times so far with maximum power output still being with the cap plates fully unmeshed. I don't want to remove any more turns because I'm starting to think the inductor is now too small for 75 meters. During the build, I had calculated the inductor value for 52uH.

But, I'll fix the plate choke cap first and go from there.

Jon
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« Reply #52 on: January 27, 2020, 05:31:04 PM »

You'll get there.  Smiley

Can you show us your overall schematic, minus the power supply?

You may need to scope the audio chain if you continue to have distortion.

Also, it's important to check the screen grids to make sure the voltages don't vary more than about 15%. If so, you may need to up the cap values.    

Phil
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ka1tdq
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« Reply #53 on: January 27, 2020, 07:57:28 PM »

I'll definitely get there with this rig. Sometimes you build something and it just seems to click.

I'll post a schematic after I do my next iteration. One thing I'm changing is to remove the B+ on/off switch. Things changed mid way through construction and the switch now basically does nothing. I'm putting a green neon lamp in its place.

I'm taking my time with this now though. It has a heartbeat so now I can just putter with it. I sort of blew the budget building it... new tubes, transformer, chassis, parts, etc. The watts per dollar ratio is a little out of whack. I'll order the next round of parts in a couple weeks.

The things we do to talk into a microphone and look at a filament. It's like a caveman and fire thing. We're no different.

Jon
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« Reply #54 on: January 28, 2020, 01:26:09 PM »

.... It's like a caveman and fire thing. We're no different.

Jon

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« Reply #55 on: January 31, 2020, 06:52:30 PM »


..The things we do to talk into a microphone and look at a filament...

Jon

Well, it keeps us out of trouble, most of the time!   Grin


Phil
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« Reply #56 on: March 06, 2020, 10:26:46 PM »

I found the problem for the low grid current indication. I had a 10 ohm resistor in parallel with the Chinese meter. I assumed that a current meter would have a very low resistance, but this one turns out to be 25 ohms. I cut the 10 ohm resistor and everything behaved as it should. I'm getting close to 3mA of grid current with a defined dip being when the plates are fully unmeshed. Still not proper, but I am getting enough grid drive to the final.


I'm still getting the power drop-off problem though during key down. I was daring and let things bake until power dropped to zero. That way I just needed to follow the heat. I found it and burned the back of my finger. It's the plate choke, marked 102k in the picture. Its value is 1mH at 1 amp, but has a ferrite core. I'll replace it with a regular wire wound one.

Jon


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« Reply #57 on: March 29, 2020, 10:43:26 PM »

I replaced the plate choke as well as the two in the oscillator circuit with pi wirewound ones. The RF section now completely behaves. I have unlimited grid drive available with the grid cap tuning about midway for resonance. I have it detuned a little to give me a little less than 4mA of grid current on the 7984 final. Power output is totally stable with the minimum output power being 20 watts carrier. I can get more power by reducing the variable resistance in the cathode of the KT-88's or reducing loading capacitance. Right now the loading cap is fully meshed and the cathode pot is set for maximum resistance.

The audio section is going to need some troubleshooting. When I tune the plate cap for minimum plate current, the audio output seems to be cutoff. When I detune a little off resonance, the power output drops a little but the rig then begins to modulate. However the audio is distorted and average power output drops with modulation (you'd expect it to deflect a little higher with voice peaks).

Also when I crank up the audio drive pot, I get a screeching sound in my audio. The same thing happens when I detune the plate cap too much.

I've tried backing off and driving more grid current but that doesn't seem to have any effect.

Jon


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« Reply #58 on: April 01, 2020, 09:45:07 PM »

Working from home is a dangerous thing. You have time to ponder stuff. I'm going to switch gears and plate modulate this.

I'll use the existing power transformer as the new mod transformer (using Tim's conversion) and I'll get a 600vct transformer for power. I'll use a Ranger's audio section as a template (12AX7 -> 12AU7 -> KT88's).

I'll run 600 volts on the KT-88's and 300 volts on the final. I'll have a negative peak limiter so the whole thing will give me plenty of audio.

The dual KT-88's were sitting there and asking for it.

Jon
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« Reply #59 on: April 04, 2020, 03:21:11 PM »

I have the power transformer converted to a mod transformer with a properly phased negative feedback winding. The transmitter is sufficiently disassembled to rebuild as plate modulated. The power control knob hole position works out well to put another tube socket for the 12AU7.

To determine the phasing of the negative feedback winding I put a 2k ohm resistor between the B+ and final lead to simulate the resistance of the RF stage. I then put 6.3 vac into the leads for the two modulator tubes along with oscope leads for channel 2. I put the channel 1 leads on the 6.3 volt winding of the transformer. I then just had to compare the two waveforms to see when they were out of phase.

I think that's all right... correct me if I'm wrong.

The new power transformer and interstage audio transformer will arrive today with all the other components coming in tomorrow.

Jon


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« Reply #60 on: April 08, 2020, 09:42:31 PM »

Almost done. Just waiting on a few small components for the audio section.

I'm curious how a Hammond power transformer will work as a mod iron. It is unbalanced, so who knows. If it sounds like crap then I might need to add a heising circuit. There just isn't much room to squeeze in any more transformers.

Jon


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« Reply #61 on: April 08, 2020, 10:07:18 PM »

Hi Jon,

Looking good!  That rig will sound great once you optimize it and get the bugs out.  Audio negative feedback around the modulator is a good thing.   That was the biggest audio improvement I did to my 4X1 rig.  Plate modulation will be a challenge but give it a try.

I like reading threads like this that document the whole project as you are building it. 4,000 hits now will probably turn into 40,000 in a few years as it captures the web search engines.

You probably have quite a few rigs built up by now. You've come a long way in the last 5-10 years as shown by your hand-drawn schematics and construction work.  I remember your first 3-500Z linear and other early projects.

Continue to post evolving performance and rig features as you improve this transmitter and get it running right.  It's all a matter of constant rig improvement and refinement once it's running. That can be the hardest part of all.   Keep up the good work, OM!   I'll be following the threads.

BTW, what is that little enameled copper wire coil on the loading cap?  Is that a HV safety choke using a ferrite core? If so, I've never seen that done with ferrite, just air, but it shud work FB.

Tom, K1JJ
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ka1tdq
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« Reply #62 on: April 09, 2020, 06:05:27 AM »

Thanks Tom!

Yeah, I thought about replacing that ferrite choke on the loading cap with an air wound one but it seems to work fine. Since there's no current flowing through it, it just sits there. If the DC blocking plate cap were to fail, then yeah, it would take the brunt. But, it could be good since the wiring is stiffer with a ferrite choke. It would get hot and pussy (pusss-eee) really fast and things would start to fail catastrophically at that point. Probably the fuse would blow pretty quickly.

I actually don't have that many rigs built up now. I could kick myself for tearing apart most of the things I've built, like my triple 3-500 amp. I'd like to have that right now as a nice companion for this transmitter. It's ok though, I was just talking with my local parts supplier (Dennis, W7TFO) who always has a large stock of iron. My rule as of late has been to keep everything I build, no matter how small. The problem was so bad that I actually got on the AM Rally this year with a barefoot Icom 7300 (how embarrassing is that...)

AND, after months of waiting, my HOA has granted me permission to stick my 75 meter dipole up again! I was fine for years but all of a sudden they had spotted me. I tore it down and hid the non-resonant dipole on the 2nd floor eves of my house with a low powered feed point tuner. Then, lo-and-behold I got the permission to put the old one back up again. Yaye...!

I'm rambling and off topic.

I'll post the results of this rig fairly shortly. I'm feeling good about this one!

Jon


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« Reply #63 on: April 11, 2020, 12:24:10 AM »

Finished.

I'm getting 12 watts carrier and modulation peaks show 40 watts. The audio is a little distorted but definitely sensitive. I can hear background noise in the room on my headphones. I'm sampling the RF output and sending the audio right to my headphones.

I'm flat topping somewhere. When I whisper in the microphone everything is perfect. As I speak louder it starts to distort.

I need to putter and find where the distortion is coming from, but this rig will definitely be a keeper. I'll draw a schematic in a little bit, but here's the skinny:

300 volts B+ throughout. KT-88's are connected in triode mode with a 100 ohm resistor connecting the screen from the plate of each tube.



Jon


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« Reply #64 on: April 11, 2020, 09:33:24 AM »

It looks like there's clipping on the output of the 12AU7 driver stage. The oscope picture shows the output (pins 1 & 6) in blue and the input (pins 2 & 7) in yellow. This is with no negative feedback. I connected the 'cold' end of R27 (grid resistor) to ground rather than the negative feedback lead. I wanted to eliminate the mod transformer as part of the problem.

The waveform is a 'Helloooooooooo.' I don't have an audio generator.

The schematic I'm using for the audio section comes from the Ranger. The only things I did differently was to change the audio coupling cap (C52) to .022 and use KT-88's in triode mode for the modulators.

Also, secondary tap of the driver transformer is grounded and I have the positive bias instead in the cathode of the KT-88's.

How do I move the bias point for the tube to give me more headroom?

Jon


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« Reply #65 on: April 11, 2020, 11:43:25 AM »

...I've tried both raising both the grid and cathode resistance (separate times). No change.

One try, I doubled the grid to 300k. I then went to 2.4k on the cathode and then 1.6k.

Like I say, quiet audio is undistorted. As I speak louder or raise the audio gain pot, it starts to distort.

There's plenty of audio power as I'm getting excellent PEP peaks on the wattmeter.

Jon
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« Reply #66 on: April 11, 2020, 01:57:06 PM »

Is your driver section capable of sinking power to the triode connected kt88s?

I've only used the kt88 single ended....  Replacing a 6l6.  Few circuit changes and it came right up. 

Other than that I don't have a lot of experience in those radios.  But I know in tetrode mode power sink is small vs triode mode requiring real power.

In a linear triode circuit you need a low q input network to provide the flywheel effect on the driving signal.

Could the lack thereof be causing your problem?

Again, not familiar with your circuit or the ranger very much.  So I could be off in very far left field.

--Shane
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« Reply #67 on: April 11, 2020, 03:31:22 PM »

That makes sense. I didn't think of the 12AU7 being loaded down.

I could drive the KT-88's as pentodes and that would make driving them easier. I'd need to up the B+ to 600 volts and run the screens at 300 volts. The 7984 would be begging for mercy at that point.

Instead I'll reconfigure for a couple 6L6's and see what that does.

Thanks!

Jon

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« Reply #68 on: April 12, 2020, 09:12:43 AM »

I wrote this yesterday but it must’ve gotten lost on the way...

One thing I see in your scope photo is the result of triodes being inverting amplifiers. The plate trace is inverted relative to the grid. This makes it hard to see what’s really happening with an asymmetrical waveform like your voice. Your scope should have an invert setting on each channel, flip one and then look at it. It’s hard to mentally invert an image and compare it, but when I try I see some rounding, not flat top clipping. Is it possible you’ve misidentified the source?

Also... as an anecdote, I spent a week of hair pulling and circuit mods on a tube amp I built once....and then finally, while changing the circuit values yet again, realized that the input stage’s grid resistor ground end was in fact floating...resulting in high current draw, low plate voltage, limited output, distortion...in the end there was nothing wrong with my original design, just the dummy who assembled it. Check for simple stuff before you give up on it....

Ed
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« Reply #69 on: April 12, 2020, 11:44:54 AM »

I did notice that the output waveform was the inverse of the input, but spaced on hitting the invert button. It would've made it a lot easier to analyze.

One thing I tried but didn't mention was to temporarily put a modified Heising circuit inline with a 20H choke and cap. I wanted to rule out the modified mod transformer being the issue with unbalanced current,  but it didn't make a difference.

I didn't realize it until after I looked up the data sheet but the original 1614 mod tubes were a version of 6L6's. I'll give it a go and see what happens after I put the circuit back to original from all my troubleshooting butchering.

Jon
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« Reply #70 on: April 13, 2020, 08:23:49 AM »

Actually, I think I just figured it out and can maybe use the KT-88's after all.

I took a measure of the DC voltage at the cathodes of the KT-88's and it's only 0.28 volts. The grids are biased at ground since the center tap of the interstage transformer is grounded. There's not much voltage differential between the grid and the cathode for audio to go.

I'm going to bias the cathodes positive with a power supply.  I'll use the existing 12 volt filament transformer for the 7984 filament. I'll parallel in a voltage doubler circuit and inject >24 volts DC onto the cathodes.

Jon
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« Reply #71 on: April 13, 2020, 01:15:23 PM »

Hold it - am I missing something? What happened to the series modulation? I was just following the thread because I also also have tried it and never have been successful, and I figured if any group can make this happen... I mean you see all kinds of schematics and threads about it, but somebody show me a proven circuit. I have made it work with small tubes like 6AQ5s and 50C5's stacked but...   
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« Reply #72 on: April 13, 2020, 01:50:35 PM »

Good point. Mine wasn't working too well as a series modulator. I think to make it work you need to be a guy like Mr. Carlson's Lab on YouTube. He's smart, but he can't pronounce the word, 'solder.'

I'm more of a guerilla with a soldering iron, so plate modulation works well.  The schematic I used was from 1957.

Jon

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« Reply #73 on: April 13, 2020, 02:27:45 PM »

Actually, I think I just figured it out and can maybe use the KT-88's after all.

I took a measure of the DC voltage at the cathodes of the KT-88's and it's only 0.28 volts. The grids are biased at ground since the center tap of the interstage transformer is grounded. There's not much voltage differential between the grid and the cathode for audio to go.

Ahhh....

That Ranger schematic has bias, and the center tap is not DC grounded --it provides the bias connection. The cap grounds it for AC.

Another way to do this without another PSU is to add cathode resistors to the KT88s -- cathode bias.. Data sheets suggest someplace around 40v bias and 75mA per tube.... a couple of 470 ohm resistors will get you kinda close enough to see if its going in the right direction...maybe a pair of 1k 3w units in parallel per tube to keep them from burning up.  That should be a fast test for you!

Ed

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« Reply #74 on: April 13, 2020, 05:05:15 PM »

Hold it - am I missing something? What happened to the series modulation? I was just following the thread because I also also have tried it and never have been successful, and I figured if any group can make this happen... I mean you see all kinds of schematics and threads about it, but somebody show me a proven circuit. I have made it work with small tubes like 6AQ5s and 50C5's stacked but...  

I have successfully cathode modulated three transmitters, a 6DQ5 version, a Globe Champ 90 with dual 807's, and a vacuum Tube Broadcast transmitter so it can be done, but solid stating the modulator is the most efficient method IMHO.

Phil - AC0OB

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