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So, there was a schematic of a basic AA5 type radio online that read everything in an easy-to-read top-down manner -- but it had a lot of errors. Nevertheless, once corrected it would be a good baseline to make a hobbyists based AA5 like radio. But subtract the rectifier and add a full wave bridge, add power isolation and a filament transformer. The original schematic had 6V tubes, I changed it to a 12V system and the type of tubes fit neatly into relay sockets. Also, the rig has a built-in GFCI which is to help keep the tinkerer a bit more safe (I know, that's a relative term especially since there are stored capacitances). I named this homemade contraption the AS4, American Safe Four Hobbyists Tube Radio.
It's a work in progress. I'm working on a Patreon video to share the construction of this set.
Being a Hobbyists radio, the plan is to make it where additional bands and other features can added over time either by myself or others that care to build one.
I'd like to share the schematic here and I'm open to any comments on it and/or spotting of any glaring errors. (I found one earlier today just before this post).
Those of you on the other side of the ocean will find it nice that you can substitute transformers
[Image: http://kingsmills.us/Radio/AS4%20Radio/AS4.jpg]
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My drawings are in TinyCad, along with additions to it's library for vacuum tubes:
http://kingsmills.us/Radio/AS4%20Radio/
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You might want to lower the screen voltage of V1 and V2. Use a screen grid resistor off B+ and feed the voltage to the two grids. This will result in less screen grid current with little effect on output. Maybe extend the life of the tube.
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Thanks, that's information. I'll have to figure out how much to change it by.
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The typical RMA color code for the output transformer's primary would be red would connect to B+ and blue to the plate. If it was a p/p transformer the blue and brown would be for the plates and red for the B+.
Nowdays transformers have different colors with no rhyme or reason.
You got the ant section of the tuning cap ganged with the oscillator trimmer cap.
Lose the 12sa7 and use a 12BA7, much better performance.
Increase the voltage rating of the filters to 250v. With a bridge rectifier you're going to be @ abt 170vdc.
Avc circuit is a little odd as it adds avc voltage to the oscillator circuit? I'd ground the tuning cap and float L1. But to each their own.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 12-24-2020, 11:06 PM by Radioroslyn.)
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City: Kokomo, IN
the 12sk7 screen grid should not be connected to the plate but to b+.
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Good catches. I'm correcting it.
I have a question. Most AA5 drawings have the second anode to the diode on the 12SQ7 to ground. Would tying both anodes together cause noise or something?
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I guess you mean the second detector or AVC diodes, it seems to have been a matter of preference by the manufacturer, or perhaps the engineer, what to do with the second diode, if they chose not to use both of them. I've seen some where both plates were tied together as well as tying the plate to ground, I don't think that one approach won out more then the other. In fact I have some radios where they used a 6H6 double diode, rather then the pair in a 6Q7, and they tied both diodes together, other then for tube stuffing the chassis why even bother?
Regards
Arran
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Yes, that is what I meant. The drawing up above is a link so it should show the latest updates.
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My latest revision of the schematic is based on a realization that the rotor on the variable capacitor is common to both the broadcast portion and the oscillator portion. The previous design would not have worked. It would have shorted the AGC circuit.
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[Image: http://bingamon.com/Radio/AS4%20Radio/Photos/as4-4.jpg]Here is the latest photo of it. Getting closer to completion.
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My latest update on this. I made a PC Board for the parts and tubes. Revised the schematic.
Also found that the capacitor across the Plate and Cathode of the 12L6 was causing oscillations. It was moved to go across the audio transformer instead. Now the audio is crystal clear but I was having issues with the local 500KW AM station that is 3 miles away by air getting into everything. I'm replacing the IF transformers yet again and will employ some additional shielding.
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The rectifier section make me think about the wall wart transformers that are so ubiquitous today.
Everyone I have taken apart have a common mode choke with a safety cap and a bridge rectifying the line AC and filter cap.
It's all ready to go for 170vdc. Just don't get zapped.
!!!!!!!!!!!!!!!!!!!RESISTANCE IS FUTILE!!!!!!!!!!!!!!!!!!!!!!
/\/\/\/\/\/\/\/\/\
[Image: http://philcoradio.com/phorum/images/smi...on_eek.gif] Chris
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I want to thank everyone here that helped with this. I finally reversed the second IF transformer and it aligned properly.
Here are photos of it in it's cabinet.
[Image: https://bingamon.com/Radio/AS4%20Radio/A...0Front.jpg]
And
[Image: https://bingamon.com/Radio/AS4%20Radio/a...20Back.jpg]
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