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 Post subject: New to this, output transformer question!
PostPosted: Jan Thu 13, 2011 8:30 pm 
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Location: Cleveland, OH 44133
Hi all, I recently got into tube electronics and specifically radios. I'm working on my first major build, a transmitter powered by two 6L6 tubes. I have an audio output transformer, an A3849. On one side it has 6 terminals, of which I am assuming are inputs, and on the other side it has terminals labeled "B" and "P". I'm assuming "P" is plate, so I'm sending that to one of my 6L6's. I had thought that "B" was indicating B+ line, but on my schematic (which does not have the terminals labeled, and I cannot find the man who made it) it has one side of the output txformer going to the 6L6 (like I said I'm assuming that's P) and the other terminal going to a resistor and capacitor and ground.

What do these letters mean and what am I going for? Where do I connect the B+?

I really appreciate any help.


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 Post subject:
PostPosted: Jan Thu 13, 2011 9:34 pm 
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Assuming that it is really an output transformer, then the side with 6 terminals is probably the output (to the speaker). A typical arrangement is something like:
common
4 ohm
8 ohm
12 ohm
etc....

The most important thing to know is the turns ratio. For example, it might be 40:1, which gives an impedance ratio of 1600:1 (This would match--eg--a 6400 ohm plate circuit to a 4 ohm speaker.)

You can check the turns ratio by putting ~ 1 volt AC on the secondary and then measuring the primary voltage.

More simply, tell us the DC resistances, and we can make an educated guess.

Better yet, tell us the brand name that goes with the part number


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 Post subject:
PostPosted: Jan Thu 13, 2011 9:34 pm 
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oops--dupe


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 Post subject:
PostPosted: Jan Thu 13, 2011 9:38 pm 
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Uncle Google found this:
http://antiqueradios.com/forums/viewtop ... 47bccea23d

Is yours a Stancor?


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 Post subject:
PostPosted: Jan Thu 13, 2011 9:45 pm 
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It is indeed a Stancor. I found that article last night, I couldn't find what I was looking for in there. The TXmitter isn't done yet, but I have no experience with these transformers and so I have no idea what I'm doing with it,,, So you think the P and B are input? would that then be Plate and B+, except input? If so, great, that makes it easy. But then not sure about the output, since I'm going to a 6l6 and not a speaker...

Steve


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 Post subject:
PostPosted: Jan Thu 13, 2011 9:50 pm 
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TO a 6L6?----from what?

If you want to drive the input of a power tube like a 6L6, then you would not use an output transformer.

Can you post the schematic?[/u]


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 Post subject:
PostPosted: Jan Thu 13, 2011 9:53 pm 
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Location: Cleveland, OH 44133
Sure:
http://www.vacuumtuberadio.com/AM_Trans ... 20copy.jpg

Same schematic but I changed the tubes and doubled the 6L6's. Now the audio circuit is powered by a single 12AU7, rectifier is a 5Y3 and the 6AQ5 is a 6K6.

So I would prob take the plate from the 6K6 to the P on the transformer?

EDIT: I mean I'm using 2 6L6's. The output of the Transformer goes to the Grid of the 6L6 if I understand this correctly... why wouldn't that work?


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 Post subject:
PostPosted: Jan Fri 14, 2011 2:17 am 
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I think it would help to post the schematic you are actually using.

Regardless, note the ratio on the transformer (They specify something in the range of 1:1 to 4:1-----As I said earlier, a typical output transformer is more like 40:1, so it's probably not going to work here.


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 Post subject:
PostPosted: Jan Sat 15, 2011 3:55 pm 
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I'm a little confused. Are you talking about a receiver output circuit, or a transmitter?


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 Post subject:
PostPosted: Jan Sat 15, 2011 4:11 pm 
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He's building a transmitter.

But he also seems to have abandoned this thread.......


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 Post subject:
PostPosted: Jan Sat 15, 2011 4:37 pm 
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The design is beyond bizzare.

Cathode modulated, and no RF bypassing on the cathode. I'll stop there.

Pete


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 Post subject:
PostPosted: Jan Sat 15, 2011 5:39 pm 
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I didn't abandon it. I just am very confused by the whole thing. For instance, I have no idea how to hook up the variable capacitor. I appreciate the help so far and I understand this design is very odd but ive nevr done something like this before.


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 Post subject:
PostPosted: Jan Sat 15, 2011 8:21 pm 
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You realize that the power input to this rig is WAY more than 100mW, and that means that you will need a station license.

-David


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 Post subject:
PostPosted: Jan Sat 15, 2011 8:27 pm 
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Location: Powell River BC Canada
Can you tell us what type of transmitter it is ,
and what bands it is for.

I am a ham radio operator and our regs
limit our permission to assist others to operate
or test stations in certain bands.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject:
PostPosted: Jan Sat 15, 2011 10:00 pm 
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Location: Cleveland, OH 44133
This is for AM. And I am aware of the FCC regulation issues.


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 Post subject:
PostPosted: Jan Sat 15, 2011 10:05 pm 
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film*;

We want to help......

Please post the schematic that you are actually building to....(eg you mentioned using 2 6L6s + some other changes.)

Did you resolve the original transformer question?


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 Post subject:
PostPosted: Jan Sat 15, 2011 10:42 pm 
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Hi

Stancor A 3849 is an universal output transformer. Primary 1000-10000, secondary ..02 - 21 ohms.

Are resistors marked M around the 6L6 really meg? Other places K is used for 1000.

_________________
Norm


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 Post subject:
PostPosted: Jan Sat 15, 2011 11:04 pm 
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Location: Cleveland, OH 44133
I did in fact resolve my other transformer issue. The only change to the actual schematic is I added a 6L6 in parallel with the other one. The thing that's throwing me for a loop now is the oscillator circuit. My oscillator coil is a Miller OSC-71 which I got NOS so I have the data sheet for it, but the pins on the coil in my schematic aren't labeled so I have no idea how to hook it up to the 6SA7. Also I don't know what the hell I'm doing with the variable capacitor.

I'm really glad I know what I'm doing here... [/sarcasm] :p

Thanks again guys :D


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 Post subject:
PostPosted: Jan Sun 16, 2011 2:24 am 
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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.


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 Post subject:
PostPosted: Jan Sun 16, 2011 3:53 am 
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It looks like they are trying to decouple RF from the screen of the 6L6 back to the B+ line :? Why not to ground? This cap across the screen resistor is common among plate modulation circuits. It keeps RF off the screen and allows some high level audio (voltage) to it, increasing audio power (modulating the screen) It appears that this drawing may have intended high level modulation, but has been modified for cathode modulation instead.......I think you should get a proper modulation transformer and modulate the plate. But thats just my opinion.....and you know what they say about opinions :D


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