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 Post subject: adding an RF stage
PostPosted: Jan Wed 24, 2007 2:59 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1086
Location: Zip : 80751
How would I add an RF amp to this? http://www.mines.uidaho.edu/~glowbugs/6u8receiver.htm

It seems like a good design, but i'm worried about sensitivity, i'm also going to add a power pentode after the triode so i can use a speaker. Any thoughts on this regen?

Thanks for the help
Jeremy


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 Post subject:
PostPosted: Jan Wed 24, 2007 3:11 am 
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Location: Maryland 20709, USA
Hi Jeremy,

The world will thank you for adding an RF stage :D

One problem with regen receivers is that they transmit on the receiving frequency. This can interfere with other receivers in the vicinity. Adding an RF stage isolates the regenerative circuit from the antenna and eliminates this spurious transmission.

The RF stage should also significantly increase the sensitivity of the set, at a cost of some additional complexity when tuning.

And lastly, the RF stage will stabilize the regenerative stage by isolating the antenna impedance from the regen tuned circuits.

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 Post subject:
PostPosted: Jan Wed 24, 2007 3:18 am 
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Location: Powder Springs,Ga. USA
Take a look at this radio.

http://www.qsl.net/wd4nka/TEXTS/REGENf~1.HTM#WHAT

You might get some ideas.

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"Never run out of airspeed, altitude and ideas at the same time"


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 Post subject:
PostPosted: Jan Wed 24, 2007 4:11 pm 
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Joined: May Tue 30, 2006 8:34 pm
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Location: Akershus, Norway
How about this one:
Image

This is a simplified version of the RF amp, at the bottom of this page http://www.jogis-roehrenbude.de/Leserbr ... -Super.htm
I don't think you have to change component values.
Possible tubes (9 pin miniature types) are: 6EH7/EF183, 6DA6/EF89 amongst MANY others.


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 Post subject:
PostPosted: Jan Wed 24, 2007 6:09 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1450
Location: Watsonville, CA, US
You don't need to make it tuned. A grounded grid RF amp is the best I have found. To make one use a triode, 6J5 etc.

Follow along and draw this:

Draw a triode

Ground the grid

Ground the metal tube envelope.

Hang a 2-5K pot off the cathode and ground the other end of the resistive element. Put a 2-300 pf cap on the whiper, attach the antenna to this.

Use a 10K or better blocking resistor on the plate. Run a tiny cap from the plate to the top of your LC tank. I use gimmick caps for this, twist two pieces of hook up wire together that are are not conected to make one. You are looking for less than 5-pf. That is like five twists.

That is all there is to it. You now have isolation fo the detector and an input attenuator. This is very handy for SSB signals, necessary for 80M signals.


Last edited by Scot Armstrong on Jan Wed 24, 2007 9:29 pm, edited 1 time in total.

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 Post subject:
PostPosted: Jan Wed 24, 2007 7:56 pm 
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Location: Zip : 80751
which would you use, i have lots 6EH7's and and lots of 6AU6's. Would performance be about the same with the above schematic?


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 Post subject:
PostPosted: Jan Wed 24, 2007 8:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1450
Location: Watsonville, CA, US
High gain is not needed or wanted here. A triode will do the job nicely. The aim is isolation, staying electronicly quiet and controlling the input. You can try tieing G2 to the plate and calling it a triode. I would just use a 6c4 if you are wanting to use miniature tubes. They are common as dirt.

You can skip the tube and use a 5K pot. The antenna goes on top and the ground on the bottom. The gimmick cap goes on the whiper. The gimmick goes to the top of the tuning coil.

I would not loose any sleep about flooding the neighborhood with re-radiated RF. IF you are using a cap of less than 5pf, and you should be, it is not a big problem. Besides, nobody listens to SW anyway. You are probably the only one for miles who is.

The 47pf bewtween the antenna and the L/C is about 20 to 40X too big. Try a gimmick cap instead. About 1-3 pf is pleanty.


Last edited by Scot Armstrong on Jan Thu 25, 2007 4:30 pm, edited 2 times in total.

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 Post subject:
PostPosted: Jan Wed 24, 2007 11:19 pm 
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Location: Zip : 80751
OK, I give up!!!!!! i cannot post photos!!!, AHHHHHHHH. I've tried everything to find it. I would like someone to double check it to make sure it is right, to make sure i didn't make a mistake, (i probly did). How exactly do i upload it to the temp section, or can some one post it? Its not very clean drawn, my handwritting is terrible.

Thanks for all the help,
Jeremy


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 Post subject:
PostPosted: Jan Wed 24, 2007 11:40 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Watsonville, CA, US
I could not post a schematic of my GG RF amp to the pictures section today.


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 Post subject:
PostPosted: Jan Thu 25, 2007 8:16 am 
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Location: Perrysburg, OH, U.S.A.
Scott,

Don't feel bad. I've tried to post photos to the Temp and Misc Photo sections a few times in the last few weeks and NONE of them has shown up in either section. Until someone proves it's fixed, I won't try again.

John

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 Post subject:
PostPosted: Jan Thu 25, 2007 11:11 am 
Banned

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Location: 18.1000N 65.4763W
Jeremy, email me your photo and I'll post it for the thread.

-Bill


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 Post subject:
PostPosted: Jan Fri 26, 2007 12:12 am 
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Location: 18.1000N 65.4763W
Here's Jeremy's schematic. Unfortunately a bit on the input side was cropped out as I received it, Jer if you send a better copy I'll replace it.

Image

Comment on my part: I just received my 1920s QST CDs and I jumped into the 1924-era looking for some specific articles. Seems in 1924 an Rf stage was all the buzz. Don't hear much afterwards. Where I live I have had limited success with an RF Amp stage. Broadcast overload is the big buggaboo. I have only one local station and can trap it out but for anybody who lives where there are several local BC stations you probably don't want to get into "rf amps" unless they are tuned stages.

As for picture posting to the Broken Gallery there have been numerous threads recently about workarounds. This pic was successfully posted using one of those techniques.

-Bill


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 Post subject:
PostPosted: Jan Fri 26, 2007 3:14 am 
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Location: Zip : 80751
Thanks ElectricBill. I'l have to try to post a pic again, maybe i'll get it right.


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 Post subject:
PostPosted: Jan Fri 26, 2007 5:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1450
Location: Watsonville, CA, US
"Broadcast overload is the big bugaboo."

Are the locals within a mile? IF so you may have a tough situation. I have locals within a couple miles and have no issue. You can of course make a notch filter that blocks the BCB completely. This would be your solution. Aside form that I see some issues.

You are probably picking up your locals in the set's wiring. Putting it in a grounded box will tell you this as will disconnecting the antenna. Make a grounded foil coated cardboard box as a temporary enclosure. Try less cap on the antenna, maybe 100 pf? You don't need a variable here. Your 2K is you input attenuator. Put the gimmick on the plate solder lug of the 6c4 and use shielded wiring to go to the LC. Add an aluminum cover over the LC coil; ground it. Beer can? Reduce the grid leak to 50 pf and 5+ Megs. The throttle voltage should be a voltage divider. 68K to the variable resistor, 20K variable, 10K to ground. This will put the screen voltage in the happy place. Adjust the cathode tap to make this work. I have not had as good a result with Hartley feedback as with ticklers, put them on the ground end of the coil, maybe 3-turns. Keep all wires, particularly RF leads as short as humanly possible. The best set up is no leads, you will need to arrange and build to make this so.

The suppressor grid on the detector goes to the cathode tap, ground it separately.

Make all grounds on the tube socket and ground the socket to the aluminum deck. Tight short grounds are imperative. Build in little tight modules and them connect them with shielded wire.

Use a .01 for audio coupling. Use a 25 mf and a 2K resistor on the first audio amp at the cathode. Put a 500K at the first audio tube G1 as your volume. The power audio set up is no the way I make them. I prefer a hookup that is more isolate from the psu.

Sure wish I could post to the photo album, I could give examples, and I have all this stuff in pretty schematics on my computer. Your basic scheme is very good. this set should be a very hot performer.


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 Post subject:
PostPosted: Jan Sun 28, 2007 8:50 pm 
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Posts: 1086
Location: Zip : 80751
Ok, well i built the original schematic to see if it would worok, and it sort of does. I can make it squeal, so the regen is wroking, but i cannot pick up any stations. I have not added the RF amp, i did build the the AF amp using a 6CL6. I'm wondering if my coil is wrong, i wanted to try it on BCB, so i started from the bottom of the coil put 7 turns on, tapped it, and added another 35 turns, and connected it just like the schematic. I just geussed at the coil turns, so its probly way off, do i need to add more, or take off turns? What else could be wrong?

Jeremy


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 Post subject:
PostPosted: Jan Sun 28, 2007 8:59 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 34326
Location: Sandpoint, IDAHO 83864
I hope I don't get flamed here, but something serious has been eating at my insides for well over two weeks here concerning both homebrew part 15 transmitters and regenerative receivers. I see that the answer to most problems people are having with both can be narrowed down to one word.

Resonance!

If the tuned circuits do not resonate at the desired frequency, you are pi**ing in the wind. Resonance is a basic item concerning radio and has been for well over a century. Those who don't understand it, I suggest looking it up and studying it. Any radio handbook should have details on it.
Curt

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 Post subject:
PostPosted: Jan Sun 28, 2007 11:00 pm 
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Joined: May Tue 30, 2006 8:34 pm
Posts: 169
Location: Akershus, Norway
Well Curt, but sometimes The Truth is hard. Understanding resonance circuits when fiddling with homebrew radios is crucial
But maybe we can give people a "softer landing" by making a post explaining resonance, and make it sticky?
I have a very nice chart here, for use with a ruler, that people can print out. Very easy to understand, and you can actually see whats happening in real time (well, sort of), when you varying a capacitor connected to a coil.


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 Post subject:
PostPosted: Jan Mon 29, 2007 2:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3048
Location: aston, pa, usa
I like Runes idea about a resonance sticky tutorial. I guess it would take moderator approval? What do other homebrewers think?

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 Post subject:
PostPosted: Jan Mon 29, 2007 3:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1086
Location: Zip : 80751
Curt, your right. I need to learn it fully some time. Here is a link to Wikipedia, has, i think, all the calculations.


http://en.wikipedia.org/wiki/LC_circuit

Geuss the next few nght are going to be full of reading,.......... and trying to learn algebra again :cry:

Thanks all for the constructive critisisms,
Jeremy


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 Post subject:
PostPosted: Jan Mon 29, 2007 3:44 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2782
Location: Powder Springs,Ga. USA
There are several calculators on the web. I use

http://www.cvs1.uklinux.net/calculators/index.html

and

http://my.athenet.net/~multiplx/cgi-bin/pinet.main.cgi

extensively when building stuff.

Using the calculators reduces the likelihood of misplaced decimal points a serious problem I have. :)

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Terry, K4TLJ
"Never run out of airspeed, altitude and ideas at the same time"


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