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 Post subject: Jackson Bell 25A - How's it work?
PostPosted: Apr Mon 13, 2020 6:04 pm 
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Can anyone explain the function of each tube in this Jackson Bell 25A? Is it really a "Superhet with RF stage" as suggested by RMorg? Or is it a more traditional 5-tube superhet with no RF stage?

Radio:
https://www.radiomuseum.org/r/jackson_be_25a.html

Schematic (lower right corner):
http://www.nostalgiaair.org/Resources/657/M0009657.htm

Thanks!

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 Post subject: Re: Jackson Bell 25A - How's it work?
PostPosted: Apr Mon 13, 2020 6:45 pm 
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The first tube 58 is an RF amplifier. The second tube 24 is an oscillator/mixer. There is no IF amplifier stage. Third tube 58 is the detector. Fourth tube 47 is the audio output. 80 is rectifier.

So I agree with the RMorg description.

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 Post subject: Re: Jackson Bell 25A - How's it work?
PostPosted: Apr Mon 13, 2020 10:00 pm 
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Thanks, Tom. That's what I was thinking, but I wasn't sure.

I have a similar radio with no schematic. I'm trying to draw a schematic so it makes sense. Stay tuned....

In a day or two, I'll probably continue this line of thinking with that schematic, see what you say about that.

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 Post subject: Re: Jackson Bell 25A - How's it work?
PostPosted: Apr Tue 14, 2020 4:48 pm 
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OK, how about this one? RadioMuseum says TRF. This is almost exactly like the one I have in my hands.

https://www.radiomuseum.org/r/empire_elp_550ac.html
http://www.nostalgiaair.org/Resources/216/M0005216.htm

As I look at this, I see the 1st 58 is an untuned RF amp, the 2nd 58 is a tuned RF amp, and the 3rd 58 (or possibly a 57, going by the chassis diagram in the corner) is a detector. It throws me off due to having only a 2-gang tuning cap.

Am I reading that correctly?

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 Post subject: Re: Jackson Bell 25A - How's it work?
PostPosted: Apr Tue 14, 2020 5:18 pm 
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Yes, you're reading that correctly. That particular TRF is not going to be a very hot performer, particularly when it comes to selectivity.

The way to recognize a superheterodyne is to look for the oscillator. In the last Stewart-Warner above, see how both tuned circuits simply take signal from the anode of the previous stage, pass it through a tuned transformer, to the grid of the next stage? That means that neither of those is an oscillator.

Whereas in the first Stewart-Warner you asked about, you see a tuned circuit "down below" on the schematic which does not pass its signal to the grid of the next stage, but rather has a coil on the right passing signal backwards to the grid of the same stage. That's feedback that excites it into oscillation.

By the way, on that last schematic, I see something peculiar. You know how people sometimes get confused seeing "M" on a resistor meaning 1000's of ohms? Well here we actually see "Meg" meaning 1000's. In other words, the confusion about "M" was there already in the 1930s! I'm pretty sure that the cathode resistor on the detector, both resistors in the voltage divider, and the tone control labeled with "Meg" are in fact kilohms. Curiously, the resistor labeled ".25 Megohms" actually is megohms, or 250 K. That was one very confused guy drawing that schematic!

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 Post subject: Re: Jackson Bell 25A - How's it work?
PostPosted: Apr Wed 15, 2020 4:04 am 
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Thank you, Tom, very much appreciated!

I will keep an eye out for the oscillator feedback line you mention. I've been trying (in passing) to spot the key feature of an oscillator circuit, so now I know what to look for. Very good to know!

And yes, I did see the confusion around kOhms and Megohms. I chalked it up to Empire being a small radio reseller. The guy who drew up the schematic was probably a marketing guy who also answered the phones. :D

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