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Author Topic: new project for winter...  (Read 26348 times)
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fg5fc
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« Reply #50 on: January 19, 2015, 11:00:11 AM »

But what does the grid drive got to do with it?
The modulator drives the screen grid, that sits at say 200 volts, I had 1000 volts on the plates of the 4x150's, and it sounds bad unless the grid drive is reduced to a very low value.
With my other screen modulator, the grid drive does not seem to make much difference in how things work.
Low or high grid drive, the screen is still going to be at 200 volts and modulated from there.
There was no problem getting 100% modulation positive or negative  with high grid drive, but lots of distortion.



Maybe WA1QIX,Steve, could explain that!
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N2DTS
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« Reply #51 on: January 19, 2015, 11:00:55 AM »

Some things I will try after work, adding a 100k ohm to ground from the output of the modulator to stabilize the load, and also trying the 4D32 rf deck with 1, 2 and 3 tubes in it.

If I want to change the negative peak limiter, I guess I would need to change the 12 volt zener zd4 to something less then 12 volts.

I also want to see what happens if I stiffen up the bias supply and add the feedback from the KS3K design.

It looks like a simple circuit, but it ends up being a lot of stuff.

Lots of fun playing with screen modulators!
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N2DTS
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« Reply #52 on: January 19, 2015, 11:06:32 AM »

I think going very light on the grid drive with the solid state modulator makes things worse, but above that, it does not seem to matter much over a wide range.
With the solid state modulators, it all comes down to how you load the rf deck really, over load it and you get loads of peak modulation, load it light and you get a lot more carrier power out and less or no peaks.
Its easy to tune up once you get used to it.


But what does the grid drive got to do with it?
The modulator drives the screen grid, that sits at say 200 volts, I had 1000 volts on the plates of the 4x150's, and it sounds bad unless the grid drive is reduced to a very low value.
With my other screen modulator, the grid drive does not seem to make much difference in how things work.
Low or high grid drive, the screen is still going to be at 200 volts and modulated from there.
There was no problem getting 100% modulation positive or negative  with high grid drive, but lots of distortion.



Maybe WA1QIX,Steve, could explain that!
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N2DTS
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« Reply #53 on: January 19, 2015, 08:22:19 PM »

Well!
The first thing I tried after work was adding the KS3K feedback circuit, from the screen output to the 2nd stage cathode with a .003 and a 1 meg resistor.
Vast improvement, the grid drive now makes little difference in the audio quality and the audio sounds very good, d104 or line input.
I also tried a heavy bleed on the - bias supply, no change, more capacitance, no change, a 100K on the output (screen to ground, real no change.
I thought it sounded quite good, so I put a sine wave into the line input and it looks good down to about 80% negative modulation then it looks funky like the negative limiter is not working well/right.
I tried various experiments with the zd4 and the 10 uf cap across it with no improvement.
Otherwise the circuit seems to work quite well, it modulates a pair of 4x150's nicely, with 1300 volts on the plates its easy to get 100 watts out with boat loads of positive modulation.

The audio chain into the line input sounds smooth and has lots of lows, the D104 sounds like a D104, but I like it, just right for a low power rig.
The only flaw is the poor negative peak limiter.

 
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Patrick J. / KD5OEI
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« Reply #54 on: January 19, 2015, 09:21:04 PM »

Schematic?
What's that?

It's a piece of art for sharing with friends!
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Steve - K4HX
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« Reply #55 on: January 19, 2015, 09:34:26 PM »

Yep. Without a schematic, this is just a blog about the project. This is the Technical section.  Cheesy
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KD6VXI
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Making AM GREAT Again!


« Reply #56 on: January 19, 2015, 09:42:07 PM »

Current source vs voltage source?

Neg screen current making current modulator unhappy?

--Shane
KD6VXI
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N2DTS
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« Reply #57 on: January 19, 2015, 09:49:47 PM »

Its the qix screen modulator from page two here or the original thread:
http://amfone.net/Amforum/index.php?topic=26603.0

I just added a d104 input.

I did more tests, the negative peak limiter looks ok at higher frequencies, at 40 Hz it looks funky, but at 400 Hz it looks good....
At 75 watts carrier and 90% modulation, the distortion products were 42 db down with a 400 Hz tone.

I hooked the front panel meter up, and set it to read 0-500 volts on the output, and at 75 watts carrier it reads 140 volts.
75 watts is about the best the rig can do with 1300 volts on the plates of the pair of 4x150's.
I might be able to get more with a different tank coil.

I wonder if the modulator would run my quad 4x150 rf deck at 300 watts out.
Crazy to think a 9 pin tube could modulate a 300 watt rig...
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N2DTS
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« Reply #58 on: January 19, 2015, 11:03:28 PM »

Last update.
Tried the qix/modified DX60 modulator on the quad of 4x150's and it worked well up to about 100 watts carrier, and then I think the current overwhelms the modulator. Audibly (over the air) it sounded like something was flashing over or breaking down.
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fg5fc
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« Reply #59 on: January 20, 2015, 10:06:59 AM »

Last update.
Tried the qix/modified DX60 modulator on the quad of 4x150's and it worked well up to about 100 watts carrier, and then I think the current overwhelms the modulator. Audibly (over the air) it sounded like something was flashing over or breaking down.


Hello Brett
Maybe you could put 2x6de7 in parallel!
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N2DTS
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« Reply #60 on: January 21, 2015, 08:26:09 PM »

I tried the qix/dx60 modulator on the 4D32 deck and with one 4D32 in it I could get about 5 or 10 watts out with it sounding somewhat clean.
I tried everything, grid drive, carrier level setting, loading and tuning, various plate voltages, it did not want to modulate well in the positive direction.
At one point I had the plate glowing very red, only at very low powers could i get it to sound somewhat clean.

I put the pair of 4x150 deck back on and it sounds very good at 75 to about 100 watts out.
That nixes the idea of a screen modulated 4D32 transmitter.

The 4D32 calls for a stiffer modulator I guess.
The solid state modulator works well with the 4D32 or even a mess of them.
The 4X150 seems to be very easy to screen modulate, but in order to get much power out needs a lot of voltage on the plate. 1200 to 2000 volts.

The solid state modulator with its big heat sinks is sort of out for a small standalone transmitter.
Back to square one...
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