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Old 14th February 2006, 11:04 PM   #1
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Default Output coupling - Choke-coil method?

Hi,

I'm currently reviewing the RCA Receiving Tube Manual, and on page 56 there's a section that tells about two output coupling types: the transformer method (the one we're all familiar with), and the choke-coil method (using a choke and a bypass cap).

I've tried to research more on the choke method but I haven't found any info. Does anyone know about it? Or know any example application of this type?

Thanks for any info!
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Old 15th February 2006, 05:46 AM   #2
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Hi danielle,

I'm not familiar with its use for driving loudspeakers but otherwise it looks like the standard capacitor coupled, choke loaded common cathode stage often used inter-stage. The big clue to the original purpose is the recommended coupling cap size. Into 8 ohms 6 uF begins rolling off below approximately 3300 Hz. My guess this circuit is intended for the 600 to 800 ohm speakers manufacturers were experimenting with prior to the arrival of solid state. Maybe a search on high impedance speakers?
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Old 15th February 2006, 05:57 AM   #3
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Choke load with capacitor coupling is often used in driver stages / preamps. When used to drive a normal low impedance loudspeaker, a transformer is required after the capacitor.
I once saw a very old high impedance loudspeaker with an energised field coil. I guess this sort of device might have been used in the configuration you describe? Maybe a member with knowledge of vintage radio can tell us more.
You will find some good info on output configurations here
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Old 15th February 2006, 09:57 PM   #4
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Danielle,

That circuit is really of academic interest only. At the time (very long ago) there existed loudspeakers with 1 - 2K impedances (not the voice coil type). The choke coupling (it is actually capacitor coupling) topology is mostly found in single-ended (SE) amplifiers, which has a following although they are comparatively expensive and inefficient.

There the choke is used to take the dc component out of the output transformer. But because dc magnetizes it, thus "fixing" part of the usefull magnetic flux, the core must be quite large to have enough "signal flux" left after the dc has taken some out of the picture. Even this is a bit idealistic; one might as well have wound the loudspeaker secondary on the choke because it is in parallel with the output transformer, thus limiting the total available impedance anyway, irrespective of how good an output transformer is used. Nowadays with lots of power semiconductors available, should you be interested in SE, it is preferable to replace the choke by a constant current circuit. Such a design will needed a higher voltage but at least there is then nothing in parallel with the output transformer to limit its low frequency response.

This is perhaps a lot more than you wanted, so to sum up: Realise what the choke was there for in such an application, then move on. You are not likely to find many such chokes on the market. They are no longer necessary. (The RCA Tube Manual is a very good book; I also learnt the basics from there, but the first one was written more than 70 years ago; I do not know exactly when).

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