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  #1  
Old 09-17-2007, 03:52 PM
2litre 2litre is offline
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AM/FM mono radio information

Hi All,

I've been reading on and off about about AA5 radios on this site:

http://www.funwithtubes.net/

It seems to me (total novice) to be quite informative, but it deals almost exclusively with mini tube AM radios.

Are there any sites that that explain or break down AM/FM units?

I'd like to know more about Octal tube (early?) and mini-tube (later?) AM/FM radios, thier differences, similarities and relative merits.

R/

Jim
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Old 09-17-2007, 05:55 PM
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mhardy6647 mhardy6647 is offline
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AA5's are cool, but they're not isolated from the AC mains, so use care. Better yet, use an isolating transformer, especially when servicing!

They're fun to collect!



http://www.angelfire.com/electronic/...bes/AA5-1.html
http://antiqueradios.com/chrs/journal/aa5.html
http://www.rwonline.com/pages/s.0053/t.849.html
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Old 09-17-2007, 09:38 PM
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Tom Bavis Tom Bavis is offline
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Almost all FM tube radios used miniature tubes, as they were introduced about the same time as the modern FM band, and the smaller size is better for high frequencies. I have a 1946 radio that usees MOSTLY octal tubes - the RF amp stage being an exception. Zenith and Stromberg Carlson made some of the better early FM radios - many Stromberg schematics on my website: http://www.audiophool.cjb.net/MadeInRoch.html

The 12SA7 / 12BE6 converter tube used in AM radios makes a LOUSY converter at 100 MHz, but some used it anyway. A much better circuit is the dual triode oscillator/mixer, using 12AT7, 12DT8, 17EW8 - tubes designed for high frequency use.
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Old 09-18-2007, 07:14 AM
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mhardy6647 mhardy6647 is offline
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speaking of AM/FM transformerless sets... there's always this guy, which actually has an outboard speaker (not shown) and MPX stereo. It's got aux inputs, too, but I don't think a sane person would use them :-)



With regard to Tom's comment... as I recall, miniature tubes are preferred for (V)HF, so (I think) it's no surprise that you will see few FM radios with octal tubes. (EDIT: oops, Tom already said that, didn't he?!)
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Old 09-18-2007, 11:05 AM
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Tom Bavis Tom Bavis is offline
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Gee... I have that radio, and I DON'T have a schematic... I did use the Aux input with a CD player, assuming that it had an power transformer inside...but that turns out to be for heaters only, the B+ comes right off the AC line. Won't do that again! In fact, I think I'm going to re-cap it before I plug it in again.
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Old 09-18-2007, 06:57 PM
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Chad Hauris Chad Hauris is offline
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Some of the very early post-war AM/FM radios (especially Philco) used a loctal dual triode tube in the FM RF/convertor stage. We worked on one like this which had very good FM performance.

I haven't heard of a Zenith with hot chassis and just filament transformer...I do have one which has a series heater string and a plate autotransformer to boost the plate supply voltage...the chassis is still "hot" though.
The aux input ground connection on these "hot" sets usually goes through a .047 capacitor to the hot chassis so it will work OK on a ceramic phono cartrtidge (which is what it was designed for) but you will get hum and a possible shock hazard hooking it up to powered auxiliary equipment.
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Old 09-18-2007, 11:02 PM
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wa2ise wa2ise is offline
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Quote:
Originally Posted by Tom Bavis View Post
Gee... I have that radio, and I DON'T have a schematic... I did use the Aux input with a CD player, assuming that it had an power transformer inside...but that turns out to be for heaters only, the B+ comes right off the AC line.
Usually such sets had a capacitor that coupled the ground of the aux input jack to the set's internal ground. Usually 0.1uF, sized to pass low frequencies reasonably well in a high impedance circuit, but to limit shocking current if a user gets between the aux jack ground and an external grounded item (and the set's internal ground happens to be connected to the hot side of the powerline).
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Old 09-19-2007, 07:54 AM
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mhardy6647 mhardy6647 is offline
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Quote:
Usually such sets had a capacitor that coupled the ground of the aux input jack to the set's internal ground. Usually 0.1uF, sized to pass low frequencies reasonably well in a high impedance circuit, but to limit shocking current if a user gets between the aux jack ground and an external grounded item (and the set's internal ground happens to be connected to the hot side of the powerline).
Yeah, that inspires a lot of confidence, doesn't it? ;-)
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Old 09-21-2007, 09:32 PM
2litre 2litre is offline
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So most AM/FM radios will have mini tubes. That answers one nagging question of mine. All the larger consoles I've seen have had mini tubes too.

I've got one AA5 that started life out as a Wards Airline AM/phono. I got it with a broken phono and damaged casing. So I cut down the casing and made it just a box radio.

I guess I'll start learning on it, identifying components and learning about recapping as well as making it safer.
Thanks for the links mhardy6647.

Jim
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