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K1KW 2 X 813 modulated by 2 X 572B's




 
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Author Topic: K1KW 2 X 813 modulated by 2 X 572B's  (Read 3500 times)
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MikeKE0ZUinkcmo
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« Reply #25 on: May 29, 2020, 08:47:49 AM »

Beautiful work, Chuck.   Very impressive!!!
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Chuck...K1KW
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« Reply #26 on: July 04, 2020, 09:31:28 PM »

Update:

The HG-303 Globe transmitter is probably one on the worst rigs released!  After chasing parasitics and other stuff, it turns out that using this rig as a driver was not easy!  I have three of these things and they all exhibit the same instabilities.  No wonder they didn't sell many. 

So....the when I had time over the last few weeks I fixed it.  Too much stuff to mention, but it is an excellent starting platform for a driver, unfortunately, I had to redesign most of it.

Now the 813 rig is working unconditionally stable and in an interesting discovery, it works on 6 Meters!  While checking out performance on 12 Meters I noticed that at minimum capacity on my 813  plate tank cap there was a spike just below 6M!  The final was apparently doubling.  So...thinking that perhaps 6M was in the future for this rig, I decided to try driving it with 6M drive instead of the 24.95MHz.  Modified the HG303 to do 6M output and driving the 813's directly with 50 MHz resulted in more output!  About 450W out at 700W input, vs 200 out when doubling but very tricky tuning.  I did the series resonance trick on the screens and that fixed it.  Now if I want 6M I need to switch the screen bypassing.  Not sure yet but maybe I'll do this.  How about an 2 X 813 rig that does 160 through 6M?

NOTE: I use my spectrum analyser when tuning up new rigs to look for parasitics and harmonic rejection.  Without this I would never have discovered this!  An absolutely invaluable tool!!!


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« Reply #27 on: July 04, 2020, 09:50:56 PM »

160 through 6 - I like that very much. It's great.  Smiley
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« Reply #28 on: July 05, 2020, 10:39:41 AM »

Hi Chuck, very nice job !!  Interesting on the capacitor failure.... something to keep an eye on.

On your 4  304TLs in parallel - I believe it will work on 40 meters no problem.  I had a rig with a pair of 304TLs and that one worked quite well on 40 meters.  Then I added a 3rd 304TL and it still worked on 40 meters.  So, I suspect yours will as well with 4 304TLs.  Watch out for parasitics (but you know all about that sort of thing).  That was the challenge with my rig.

I used a balanced grid input circuit (for neutralization) and a PI section output.
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Chuck...K1KW
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« Reply #29 on: July 07, 2020, 11:32:23 PM »

Yup...running triodes is a challenge in RF service!  Many years ago I built a pair of 833A's in push pull using the B&W 1KW plug in tank coils with the swinging link. (Modulated it with a pair as well)

I used a balanced input with cross neutralization.. It worked very well on 160 through 10M

Sold that rig.  Then.....

Built another rig with a pair of 833A's in parallel instead of push pull,  Pi network output, balanced grid input to get the 180 degree shift for neutralization.  That rig worked well, but I had to put a control on the front panel for the neutralizing capacitor since neutralization didn't hold up across more than two bands! 

Anyway, that's what I'm going to try.  Four 304TH's in parallel, Pi Net output, balanced grid input using the National Radio MB-150, and a front panel control for neutralization with dial indicator to set the neutralization when I change bands.  Nothing wrong with that, more knobs on old buzzard rigs are required !  The filament heater power is crazy for 4 tubes, but this will be a room heater.

I'm not expecting anything more than a high frequency end of 40M BUT I will try for 20M!

This will be a Winter project...

Stay tuned, I've got all the components.  Color me bored with all this coronavirus crap...

Chuck

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« Reply #30 on: August 02, 2020, 01:25:01 PM »

Just tuning around on N1SDR's SDR and ran across Chuck operating his new 813 rig. It was about 1:10 PM today and conditions we not the best. He was talking with Glen WA3GPE  somewhere in 3 land who was a bit stronger than Chuck.


I originally posted a audio sample on here but it sounded horrible not because of Chucks rig, but because I had it set up to record via the web cam microphone. YUK! 

Maybe someone else hear him and can post the audio? He sounded very good to these ears!

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« Reply #31 on: August 05, 2020, 05:52:41 AM »

One design/implementation detail that is missing is anything related to the audio driver or modulator.

If the tubes in the modulator are triode connected (that is the assumption), and the mod transformer has that much inductance, it should be possible, with the proper audio driver, to achieve very good results using a direct coupled audio driver.

I have done this with excellent results - getting almost class E audio performance, at least down to about 30 cycles.  The rig would properly modulate triangle waves, which is what I use for testing.

Because the only phase shift in a proper, direct coupled modulator is in the transformer itself, it is possible to run a reasonably high amount of negative feedback and include the modulation transformer in the loop.

In my particular implementation, I included a high level, 3 diode negative peak limiter (which was of course not in the feedback loop).

Anyway, curious about the audio driver, etc.  Wink
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« Reply #32 on: August 05, 2020, 01:24:47 PM »

Quote
I like the big heat-sink plate caps.
From Tom K1JJ

They are quite old buzzard and I think they were part of the 833A tube socket kit made by Eimac. I think RCA took a stab at it but their socket would not flex on the filament mount as well as Eimac. I think there are plate caps that may radiate heat more efficiently than the Eimac, but they have their own cool factor look which is more modern than the Eimac. What's this note about? To let the younger guys on here know where to source the cool old stuff Grin Grin Grin

I have some them here and one problem I have has is that some will not tighten down without using a mini vice grip. I wonder why that is. Smaller diameter caps on the tube or some other problem?
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