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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 19th Sep 2023, 5:34 pm   #1
nigelr2000
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Default Wega Brilliant German Radio volume issue

I have never seen one of these strange radios. Made in 1949 using 1930's side contact valves. Initially it just hummed a bit. After changing a lot of the caps and the EBL1 Output valve I had sound of sorts. When tuned to a signal it performs OK ish at low volume but increase the volume and it all goes weird. Very difficult to explain but here is a video of it
******************************r8dySLThyDop1t9K6

As you increase the volume the signal disappears then comes back and goes really spiky back it off a tad and it comes clearer.
I don't really understand how this bit works. If I feed audio straight into the grid it's fine so it's something in the magic preceding stage.

Any idea or just an explanation how it works
Here it is on Radio museum with a diagram https://www.radiomuseum.org/r/wega_brillant_364w_1.html
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Old 19th Sep 2023, 5:44 pm   #2
G6Tanuki
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Default Re: Wega Brilliant German Radio volume issue

Could it be using a regenerative feedback circuit? In which case there will be a point on the 'volume' control at which it becomes oscillatory and there will be whistles/unpredictable effects.
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Old 19th Sep 2023, 6:00 pm   #3
ms660
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Default Re: Wega Brilliant German Radio volume issue

It's a standard superhet.

The output from the detector is fed via the volume control etc to the grid of the audio amplifier valve (triode of the ECH4) the output from its anode is coupled to the control grid of the audio output valve.

Bias for those two stages is obtained from the back bias voltage developed across the two bias chain resistors (70 Ohm & 30 Ohm in a similar schematic I'm looking at)

Lawrence.
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Old 19th Sep 2023, 7:09 pm   #4
nigelr2000
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Default Re: Wega Brilliant German Radio volume issue

Thanks Lawrence I forgot there's a spare triode there of course it goes through the detector then back to the triode, I was following it the wrong way. Ooops.
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Old 20th Sep 2023, 1:15 pm   #5
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Default Re: Wega Brilliant German Radio volume issue

All sorted now I know how it works, I missed 2 capacitors and because they were leaky the volume control was varying the grid bias on the triode. It was a long day and I am used to reading diagrams left to right. Sounds really good using my Spitfire as a signal source.

Vid of it here ******************************kjGRDaGxAEUxXtgA9
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