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PostPosted: Jan Sun 16, 2011 4:36 am 
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pixellany wrote:
Well.....do you have a coil with 3 terminals?

You can us an ohmmeter to see which terminal is which---the highest resistance will be across the ends of the coil. Since the tap is nearer one end, the resitance from the tap to one end will be lower than to the other end.

According to your schematic, the tuning cap goes across the whole coil.


The coil I have is a Miller OSC-71 with 6 terminals. Found the data sheet on google but I didn't save the link since I have a paper one for it.


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PostPosted: Jan Sun 16, 2011 3:53 pm 
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I'm afraid I'm getting a little confused. Did you get this coil specifically for this circuit---or is it just something you had?

Does your schematic or other material give you a coil part number---or any other specs?

If you post the specs on the coil you have, we may be able to figure out how to use it.


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PostPosted: Jan Sun 16, 2011 7:35 pm 
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I just had this one available.

Here's the data:

http://www.junkbox.com/electronics/shee ... r_coil.bmp


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PostPosted: Jan Mon 17, 2011 3:09 pm 
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First, you need to visualize your circuit with the coil as a transformer------Looking at your schematic, the primary of the "transformer" is from the center tap to the top end--connected to the plate. The secondary is from the center tap to the bottom--which goes to the grid.

Now look at your coil spec--in the second group where the secondary goes to the anode (plate) of the tube. In all cases, the secondary connection from ground to grid is the same----and for the primary, there are various taps used.

In all cases, your circuit has "ground" and "B+" both at the center-tap. If this is confusing, note that what the coil cares about is the location of the RF ground, not the DC ground.

So, one possible hookup is:
Pin 1 to the grid
Pins 2&4 where your circuit shows the center-tap
Pin 3 to the plate


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PostPosted: Jan Mon 17, 2011 9:50 pm 
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Great, thanks so much for that. Definitely cleared it up. The only other question I have about it then is for my tuning capacitor would I just go from pin 1 to pin 6? or how does that connect in to everything?


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PostPosted: Jan Tue 18, 2011 6:42 am 
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According to your schematic, the tuning cap (and another fixed one (C6) ) go across the whole length of the coil---ie from the plate to the right hand side of C5, where it connects to the coil.


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PostPosted: Jan Fri 21, 2011 9:04 pm 
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pixellany wrote:
According to your schematic, the tuning cap (and another fixed one (C6) ) go across the whole length of the coil---ie from the plate to the right hand side of C5, where it connects to the coil.


So, I understand I connect one wire to the terminal of the tuning capacitor, but where exactly does the other wire go on it? Just to the back of it somewhere?

Thanks


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PostPosted: Jan Sat 22, 2011 1:40 pm 
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Assuming you have the typical variable capacitor, there are two connections---one to each set of interleaved plates. The metal case is one connection, and then there is typically a solder terminal for the other. The fixed (stationary) plates will be connected to this terminal, and the rotating plates will be connected to the case.


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PostPosted: Jan Sat 22, 2011 5:49 pm 
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So this would then lead me to believe I have to isolate the capacitor from the chassis? Makes me wonder why then have the mounting holes be conductive...


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PostPosted: Jan Sat 22, 2011 9:42 pm 
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In many radios---but not all--- the tuning cap is isolated by rubber grommets. This is for vibration isolation and/or electrical isolation.

I need to go look at your circuit again to see what your options are.

Update:
As shown in you circuit, both sides of the tuning cap will be at B+ potential. Therefor, it must be isolated from the chassis--AND: there needs to be some protection to keep the operator from touching B+. Personally, I would not build it this way.

Other than the schematic, do you have any instructions for this project?


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PostPosted: Jan Sat 22, 2011 10:00 pm 
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I don't have any other instructions unfortunately. I will purchase some rubber grommets and I have a knob that will protect the user from B+. Now- and this SHOULD be my last question...

On my schematic it appears that the plate of the 6l6's will go to the top tap of my output coil and the antenna- is that correct?


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PostPosted: Jan Sat 22, 2011 10:07 pm 
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filmmaker516 wrote:
On my schematic it appears that the plate of the 6l6's will go to the top tap of my output coil and the antenna- is that correct?

Wire it exactly as the schematic shows-----the plates do NOT connect directly to the antenna!!


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PostPosted: Jan Sun 23, 2011 1:38 am 
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pixellany wrote:
filmmaker516 wrote:
On my schematic it appears that the plate of the 6l6's will go to the top tap of my output coil and the antenna- is that correct?

Wire it exactly as the schematic shows-----the plates do NOT connect directly to the antenna!!


Will do, appreciate it. I was just thinking, on my oscillator coil (yes back to this again, lol) on which you said pin 2 and 4 are center tap, do I still use the specified pin for ground or one those? Right now I have it wired for B+ to pin 4 and ground pin 2, but I may have misunderstood what you wrote. Does B+ go to both 2 and 4?

I truly can't thank you enough!


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PostPosted: Jan Sun 23, 2011 3:24 pm 
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In your schematic, none of the coil connections go to ground.

My example showed one of several ways to wire your coil so that it would be equivalent to the one in the schematic.

In my example, pins 2&4 are the equivalent of the center-tap in your schematic so--yes--they both go to B+

Going back to the comment about DC on the tuning cap, note that there are MANY ways to wire a circuit like this. What matters is:
1. The coil (transformer, in your case) having the right inductance and turns ratio.

2. The phasing being correct between primary and secondary


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PostPosted: Jan Thu 27, 2011 12:46 am 
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Hello again. I have one more question. I'm so close to being done with the wiring, but I'm stuck once again at a coil. This time, the output coil, how on earth do I wire this thing?

Here's the coil I'm using:
http://www.33audio.com/enter/data/miller2.pdf

It's the #2002 Sub-Miniature Antenna loop

I don't know what the hell goes to which of the three pins on there!

I tried it with B+ going to pin 3 and 6l6 going to pin 1, with the antenna coming off pin 1, but I turn on the TX and get nothing on any channel. Something is wrong...

Steve


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PostPosted: Jan Thu 27, 2011 2:38 pm 
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The Miller coil you refer to is designed for use at the input of a broadcast-band receiver. (These are used to match the antenna impedance to the input stage of the receiver.) In your case, you need a 2-terminal coil to use in the plate circuit of your output stage.

I don't think you have told us what frequency this thing is supposed to operate at. (That will determine what this output coil is supposed to be).

I would think that you could simply put a resistor in place of the coil--and not connect the antenna-- in order to verify that the rest of the circuit is working.

To get information on what your output coil needs to be, try some Googel searches on transmitter design.


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PostPosted: Jan Thu 27, 2011 6:30 pm 
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Had a few developments. This is for upper AM band. Upon testing the voltages on the 6sa7 I discovered there is no reading on the plate, which leads me to believe it is not even oscillating. I'm guessing something is awry with the OSC coil but I'm not sure.


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PostPosted: Jan Thu 27, 2011 9:29 pm 
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Quote:
..no reading on the plate..
Are you measuring AC or DC? Using a scope? voltmeter?

Assuming the circuit is wired correctly, you will get predictable DC voltages, even if it is not oscillating.

I suggest that you post your actual as-built circuit, including the implementation of the various suggestions you have received here.

Specifically for the oscillator, recall that my example for the wiring of the coil was only one of several possibilities---also, the importance of coil polarity that was discussed.


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PostPosted: Jan Fri 28, 2011 10:10 pm 
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I AM indeed getting a voltage reading on my 6SA7 plate, sorry about that. Here's the "as built" schematic, and this is exactly how it is hooked up right now. Again, the problem is that something, I'm guessing the OSC circuit, is not working correctly as I cannot tune it to an AM station and I have no way to find out where it IS tuned right now.

http://dl.dropbox.com/u/15275313/TX-AS%20BUILT.png

Image


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PostPosted: Jan Sat 29, 2011 2:43 pm 
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film*;

Sorry for the delay getting back--I'm in the middle of a big proposal (but this is more fun....:))

First, here is an article on the Hartley oscillator:
http://www.electronics-tutorials.ws/osc ... rtley.html

to relate to your circuit, the transistor base is equivalent to the tube grid, and the transistor collector is equivalent to the plate.

Recall that I said that my suggested hookup of the oscillator coil was only one of several possibilities. You may have to try some of the other combinations shown in the Miller data sheet. Note the the Hartley circuit resonates at a frequency determined by the L of the 2 coils together, and the total C across both (formula in the article)**

You schematic does not show all of the component values.

One thing I am concerned about is the parallel 6L6s. First, why?
More to the point of getting it working, the use of the 2 tubes will likely require changing some component values. For example, this circuit uses cathode bias---the plate current flowas thru R9 and thus produces a + bias on the cathode with respect to the grid. With 2 tubes, R9 would have to be adjusted to get correct bias. R11 might have to be adjusted also.

Also, because of the higher total plate current, you may need to adjust the series resistors in the power supply.

What did you use for the interstage transformer? (The original topic of the thread)

Finally, you say you got a reading on the 6SA7 plate. Please tell us WHAT reading.

If you do not have a scope, then you can perhaps see if it is oscillating by putting it near a radio---this is hit and miss, however---e.g it could easily be oscillating outside the tuning range of the radio.

** you'll note in the article the use of the "dot convention" to show the polarity of the coils. The equivalent notation on the Miller spec sheet is "ST", which I assume means the STart of the winding.


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