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Old 29th Jan 2006, 12:33 am   #41
Oskar.B
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Some more progress today / tonight. I could not resist doing a test assembly of the major parts to get a sneak preview of what the final result will be:

http://gargnas.net:3000/bilder/dassrull/P1290003.JPG
http://gargnas.net:3000/bilder/dassrull/P1290004.JPG

Ed, can you see your meters?!

(Thanks, and thanks again, they are awesome!)

Build exitement level 8(10)! =)

/Oskar
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Old 29th Jan 2006, 5:45 pm   #42
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar, the meters cleaned up nicely. Looking at the tuning cap I remembered that there used to be articles about making your own in the 20's magazines. These could easily have been adapted to suit the higher voltages and spacings required for transmitting use.

Ed
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Old 29th Jan 2006, 8:01 pm   #43
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Ed,

Yes, I took them apart and cleaned the glass carefully with cotton "tops" dipped into window cleaner (not sure what you call them, they are supposed to be used to clean your ears : ) ). I had to use a razorblade to remove the most persistant tape remains.

The tuning cap is indeed homebrew. At least I think so. It might be something sold in a kit form to hams.

brg Oskar
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Old 30th Jan 2006, 11:29 pm   #44
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Just some work done to the VFO this evening.

http://gargnas.net:3000/bilder/dassr...o/P1300004.JPG

The mechanics of it is basically sorted. I am not sure if I really should be using pressed paper for the oscillator coil-form. The alternative is some grey PVC tubing I found in the garage.

Good night ... zzz

/Oskar
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Old 1st Feb 2006, 9:28 pm   #45
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Use something stable with low rf absorbtion for the coil. Polythene or similar. The cardboard won't be physically stable enough to keep the coil form stable and hence the frequency will not be stable.

Robin
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Old 2nd Feb 2006, 10:23 am   #46
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
The cardboard won't be physically stable enough
It can be made so by soaking it in Shellac.
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Old 2nd Feb 2006, 12:09 pm   #47
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

My usual technique is to wind the coil and test it, and then cover it witch laquer. I think it should work. If the coil is not covered in laquer it is also easy to accidentally mess up the turns and move them apart for example.

/Oskar
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Old 2nd Feb 2006, 1:35 pm   #48
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Gents, on the topic of coil formers, Philips had some very good ribbed glass ones. The design aim is to get a stable (mechanically) former that has an Er (permitivitty) near 1 (air). This is why a ribbed former is often used.
Cardboard, well lacquered is a good substitute, with 6 or 8 ribs of varnished hardwood glued on. These were some of the tricks used for home construction in the early days; they appeared to work very well.

HTH

Ed
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Old 2nd Feb 2006, 1:47 pm   #49
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

I am having a language difficulty here. =) Could you explain "ribbed" Ed, how such a coil form looks? And how would the wooden ribbs be applied?

/Oskar
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Old 2nd Feb 2006, 4:05 pm   #50
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Imagine a number of fins all glued together in the middle to form a star. The outside of the ribs are slotted to take the turns and locate them.

The wire is then wound round this. The wire in these cases is often guite large gauge.

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Old 2nd Feb 2006, 4:11 pm   #51
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Aaah, ok now I get it! Thanks!

Unobtainable though. I will make a test coil on the cardboard form to be able to test the VFO. Maybe later I will find more suitable coil form material. Glass or ceramic would be nice.

(If I could only get my Snowmobile together so I can go on building! Track suspension was busted in all sort of ways )

/Oskar
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Old 6th Feb 2006, 7:38 pm   #52
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar,

That former looks massive!

I would doubt you need anything like that size for a VFO working on 80m - I might have some nice ribbed ceramic formers here, but nowhere near that big....


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Old 10th Feb 2006, 8:41 pm   #53
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi all,

Not much done as far as building is concerned I'm afraid. I have made a coil and tested the VFO though. It is working! But I have to arrange a capacitor combination to limit the band coverage. Right now it tunes all over the place!

Oh, that former was intended for 160m. I'm quite curious on that band, so I'll make coils for 80 & 160. When the time comes, 160m seems like a better place to test the transmitter, not so crowded That way I am less likely to make someone angry

The transmitter will not be instantly band-switchable. But could rather be rebuilt in a couple of minutes for different bands.

B.t.W .. OT. I got my Snowmobile together at last, runs like a dream .. ah. =) Have to trow clothes into washer .. they're soaked with two-stroke-fumes .. :P

/Oskar
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Old 20th Mar 2006, 12:06 am   #54
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi all!

Just a quick progress update before I go to bed

Did a test-assembly of the PA on a scrap chassis. No disasters fortunatly .. phew! (the magic smoke is still there!)

Looks like I have some tweaking to do though. I could not get the plate current to get any higher than ~35mA. I have to do some measurements, to see if it's my grid resistor that is to large, or if the grid drive is not enough. Also the grid resistor feels a bit warm (it is a 3W part!), I will have to check this.

Could the low plate current be down that the 807 I am using is weak? I took it from a old variable DC supply*. That unit used two 807's, and they were both marked 70mA.

Anywhay, it looks promising! The 807 gets a really pretty "blue-glow" when the P.T.T is closed! (I do not think the valve is gassy, the glow is just present on the inside of the glass, not inside the valve structure, and this is quite normal (correct me if I'm wrong!).)

*Don't worry, I did not scrap a nice piece of equipment, I found it at the local dump, and it had been run over by a tractor!

Good night!

/Oskar
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Old 20th Mar 2006, 7:07 am   #55
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar,
Quote:
Originally Posted by Oskar.B
I have to do some measurements, to see if it's my grid resistor that is to large, or if the grid drive is not enough. Also the grid resistor feels a bit warm (it is a 3W part!), I will have to check this.
Measuring the grid current should definitely be on the top of your todo list. I suspect lack of grid drive isn't the problem, and if you overdo it, you may fry the g1. Your V2 circuit diagram shows a meter in the grid circuit in the right location. Compare the grid current to what the datasheet, 3.5-4.0 mA. If it is anywhere close to that figure, then that isn't the problem.

Quote:
Originally Posted by Oskar.B
Could the low plate current be down that the 807 I am using is weak?
Yes. In a class C amp, the anode current is chopped up in some comparatively high spikes much higher than the average, spaced out with long periods of nothing happening (zero anode current) in between. If you are looking for 70mA of anode current on the meter, the peak currents are 3 to 5 time that figure, 200 to 350mA(!). A new cathode will easily conduct that amount of current, but if the cathode emission isn't top notch, then low average current and efficiency will result.

In the PSU you got the tube from, it was probably running in pure DC mode. Any lack of excessive emission would go completely unnoticed. So it can be very weak for your application indeed, without having caused any problems in its old job.

Among my collection of eight used 807s, only one of them has emission strong enough to be able to run class C at full ratings.

Quote:
Originally Posted by Oskar.B
Anywhay, it looks promising! The 807 gets a really pretty "blue-glow" when the P.T.T is closed! (I do not think the valve is gassy, the glow is just present on the inside of the glass, not inside the valve structure, and this is quite normal (correct me if I'm wrong!).)
Should be OK. If you had gas (or worse, air) in the bottle, you would probably get a fantastic purple or orange fireworks when you apply RF to the valve. Blue glown on the glass is semi-normal for small transmitter valves.

Best regards

Frank N.
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Old 20th Mar 2006, 8:04 am   #56
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Wait a minute!

Should I not have an RFC between the grid and the grid resistor? Maybe the grid resistor is dampening all of the grid drive ... hmm (could be the RF that is making it warm?).

I'll have to look at the voltage at the grid with the scope to see how it looks. Measuring the grid current could reveal a few things also for sure.

I did not want to connect my very nice meters into this prototype until I had everything measured up.

I should also try to get a couple of NOS 807s'.

/Oskar
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Old 20th Mar 2006, 9:43 am   #57
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

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Should I not have an RFC between the grid and the grid resistor? Maybe the grid resistor is dampening all of the grid drive ... hmm (could be the RF that is making it warm?).
No, you probably don't need that, I explained that one earlier. The 'unshielded' resistor 'only' consumes about half of your grid drive, while stabilizing the PA and providing a constant load for the oscillator to work into. A good 6AG7 should be able to provide plenty of drive for a single 807. You should only add the RFC if you are sure you cannot get sufficient grid current, and that your low output isn't due to any other problem (parasitics oscillations, weak 807, improper wiring/component values, suboptimal physical layout etc.)

Quote:
Originally Posted by Oskar.B
Measuring the grid current could reveal a few things also for sure.

I did not want to connect my very nice meters into this prototype until I had everything measured up.
Use a DVM or something similar, temporarily in place of the mechanical meter. If you have added the 10nF cap from the 'cold' end of the grid resistor to ground like your circuit diagram shows, the RF level ought to be low enough at that point that this will work without problems.

Any chance of a few snapshots of your component layout, both topside and below the chassis?

Quote:
Originally Posted by Oskar.B
I should also try to get a couple of NOS 807s'.
Check the grid current first. In fact check the grid current before you do anything else. If you put the RFC in place before measuring the grid current, you will drastically increase the drive voltage available. If it is something else than lack of drive or a weak 807, which limits the output, you may burn out an otherwise good 807 by putting the RFC in series with the grid resistor.

Did I mention your next step must be measuring the grid current?

Frank N.
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Old 20th Mar 2006, 12:45 pm   #58
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Yes, I will definitely measure the grid current tonight!

Here's some photos:

http://www.gargnas.net:3000/bilder/d...l/P3200024.JPG

http://www.gargnas.net:3000/bilder/d...l/P3200023.JPG

http://www.gargnas.net:3000/bilder/d...l/P3200022.JPG

http://www.gargnas.net:3000/bilder/d...l/P3200016.JPG

http://www.gargnas.net:3000/bilder/d...l/P3200017.JPG

A bit of a lash-up really, I will probably need to tidy it all upp to get some decent results out of it.

The 807 plate RFC is about 2mH, and I'm testing now 160m. The tuning cap for the ouput PI filter is a bit (quite alot actually) oversized, but hey, it makes future expansion possible .

I also have a strange problem. Sometimes when I fiddle about with the VFO frequency, and 807 grid tuning, something starts squeling (sounds like it is coming from the 807 grid tuning cap actually!) and the waveform on the scope goes crazy. I then quickly change frequency, and the problem is gone. Parasitics? I was thinking of adding parasitics supressors to the VFO also.

/Oskar
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Old 20th Mar 2006, 1:13 pm   #59
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Oskar, you have a PM.
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Old 20th Mar 2006, 2:30 pm   #60
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar,

It looks very nice!,

However, that may be the cause of the problem, well, one or two of them anyway......

RF construction is very different to AF construction! single earth points as close to the valve as possible are the norm, and short interconnects are a must!

I could probably detect your VFO/PA radiation in the UK

Still, it is heading in sort of the right direction

Cheers
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