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 Post subject: Another one of my modified AA5 radios, to run off 8VAC
PostPosted: Feb Tue 14, 2017 5:32 am 
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Joined: Aug Mon 16, 2010 7:01 pm
Posts: 232
Location: USA
Yet another AA5 (an Emerson 708B) modified with some subminis and designed to run off 8VAC. Why 8V you ask? This dates back to my late 1960's model railroad days when I ran HO scale streetlamps off 8V instead of 14VAC, to prolong bulb life and make the lamps look more realistic. Built a 8VAC 5 amp bus for this. Still have it. Anyway:
Image
To get plate voltage I used a 110V to 12V transformer run in reverse 8V to 72V. And I connected the new 72V secondary atop the 8V line to get 80V (unloaded). Then voltage doubled it, though the secondary's resistance lets the resulting B+ sag to around 135V. The nature of the current filter cap recharge is that it looks like pulses with a duty cycle of around 5%. This radio's current draw on the B+ is about 40ma, and this combined with this 5% duty cycle makes for peak currents 20 times larger, here about 0.8A. And with the 72V secondary resistance of about 40 ohms that makes a loss of 32VAC. Subtract 32V off the 80V and that yields 48VAC, and with the voltage doubler (x2.8 ) that produces 134VDC. About what I measured. Secondary resistance is effectively multiplied, making it worse than you'd think.
You don't see this with hot chassis silicon diode rectifier power supplies, as American 120V 15A branch circuits have an effective resistance of about 1⁄3 Ω.
What it looks like: Image I edited the tube placement chart, printed it out and glued it to the cabinet bottom. It's here as an insert in this picture.

More such adventures at my web pages and sites http://pw2.netcom.com/~wa2ise/radios/AA5submini.html and http://pw2.netcom.com/~wa2ise/ and http://www.wa2ise.com/


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 Post subject: Re: Another one of my modified AA5 radios, to run off 8VAC
PostPosted: Feb Thu 16, 2017 3:40 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16774
Location: Warner Robins, GA
Very nice work.


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 Post subject: Re: Another one of my modified AA5 radios, to run off 8VAC
PostPosted: Feb Thu 16, 2017 4:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12006
The purists are gonna be seething but I like it...


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 Post subject: Re: Another one of my modified AA5 radios, to run off 8VAC
PostPosted: Feb Thu 16, 2017 4:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16774
Location: Warner Robins, GA
They might, but if it is one of the mass produced AA5 radios that is as common as TV tubes then I don't see why anyone should get upset.


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 Post subject: Re: Another one of my modified AA5 radios, to run off 8VAC
PostPosted: Feb Sat 18, 2017 10:06 pm 
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Joined: Dec Mon 29, 2014 9:39 pm
Posts: 2255
Location: Los Angeles
I like it. Sure would like to see your 8v power supply :wink:

_________________
John Smith, over and out.
If I did something right, I made a mistake.


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 Post subject: Re: Another one of my modified AA5 radios, to run off 8VAC
PostPosted: Feb Tue 21, 2017 7:24 am 
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Joined: Aug Wed 25, 2010 3:23 pm
Posts: 1063
Location: WA 98407
What a fun project!

Tom


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 Post subject: Re: Another one of my modified AA5 radios, to run off 8VAC
PostPosted: Feb Tue 21, 2017 8:48 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19598
Location: Haledon, NJ, usa
If you like that 5902 output tube.... you'll LOVE making the fantastic 5902 PP stereo amp. There are several threads on ARF here
( http://www.antiqueradios.com/forums/vie ... 2&t=268832)
(viewtopic.php?f=6&t=182783)
...following the build of this 5902 amp by a few of us. Do a search for 5902 or Ipod amp etc.:
Image

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To be a man, Be a non-conformist, Nothing's sacred as the integrity of your own mind.
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