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Old 7th March 2012, 06:04 PM   #1
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Default Output stage for a Mono FM tube tuner

Since this is a tube question, I hope this is the correct forum. Question regarding capacitor values for the output stage of a mono FM tube tuner. The output of the ratio detector is capacitor coupled, .01uF, to the cathode of a 6BN8 through a voltage divider then capacitor coupled to the output through a .05uF as indicated on the schematic. The actual output coupling capacitor is labeled 224K. I can't seem to find the right conversion. I think it would be either .22uF or 2.2uF. In any case do these values seem appropriate for this type of circuit? Circuit design is from 1961. Thanks
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Old 7th March 2012, 06:46 PM   #2
DF96 is offline DF96  England
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224K is 0.22uF. I don't think this marking scheme was used in 1961 so the cap is probably part of a repair. The value is not critical, provided the load is high enough in resistance.

To say any more we would need to see the circuit.
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Old 7th March 2012, 07:03 PM   #3
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Smile .22uF OK circuit pics to follow

I kinda thought so but wasn't sure. The original value, .05uF was from the original 1961 schematic. I kind of figured the 224K was a replacement. Pics to follow. Thank you.
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Old 7th March 2012, 08:11 PM   #4
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Smile Schematic of tube tuner output stage

OK Here's the diagram of the output stage of my tube tuner. The output of the ratio detector goes to a volume control and also to the front end for an AFC control. Any info on the values of these capacitors would be greatly appreciated. Thanks
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File Type: jpg tuner schmtc 003.jpg (286.0 KB, 92 views)
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Old 7th March 2012, 08:25 PM   #5
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Somewhere there needs to be a 75uS de-emphasis (for USA). Not enough is shown to be certain, but C36 might be involved.

All good fortune,
Chris
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Old 7th March 2012, 08:35 PM   #6
DF96 is offline DF96  England
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Yes, not enough shown to be clear what is happening.
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Old 7th March 2012, 09:15 PM   #7
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I have looked at a Quad FM1 schematic which was also in production in 1961.

I find very similar values to you describe. In the FM1 The Acoustical Manufacturing Company ( Quad ) used .04uf coupling capacitor then 220k in series, to the audio out - with 100k to ground and 1000pf in parallel with that 100k resistor.

Cheers / Chris
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Old 7th March 2012, 09:49 PM   #8
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Default The broken line above the output stage......

The broken line above the ouput stage indicates a sealed package that contains all the de-emphisis and high frequency suppresion, above audio frequencies, circuitry. The manufacturer calls this a P.E.C., "Packaged Electronic Circuit". The manufacturer states "The use of a P.E.C. is to eliminate any variations in performance caused by variations in wiring techniques". I quess all kit builders don't build things the same.
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