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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 22nd Jun 2009, 11:07 am   #1
John12868
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Default "That Cap"

Hi Guys,

Being a bit thick and also sleep deprived at the moment, I'm wondering if some kindly soul could describe/explain what is meant by "That Cap" in terms of valve radio restoration?

Regards

John
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Old 22nd Jun 2009, 11:18 am   #2
ppppenguin
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Default Re: "That Cap"

"That cap" couples the grid of the audio output stage to the anode of the previous stage. The grid is normally at 0V or slightly negative. The anode is at a fairly high positive voltage. The resistor from the grid to 0V is usually large. 1M is common.

Any leakage in "that cap" will raise the grid voltage causing the output valve to pass too much anode current. This will swiftly wreck the valve and possibly the output transformer. May also damage the rectifier and mains transformer and cause general heat damage.
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Old 22nd Jun 2009, 11:20 am   #3
HamishBoxer
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Default Re: "That Cap"

Could be the main smoothing cap or if refers to a Bush Dac90 series the one across the mains input that needs replacement.Other than that the coupling cap on audio output valves.Not really any other thoughts after 45 years in the trade.
Regards David
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Old 22nd Jun 2009, 11:21 am   #4
HamishBoxer
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Default Re: "That Cap"

Jeffrey posted just before me!
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Old 22nd Jun 2009, 11:22 am   #5
paulsherwin
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Default Re: "That Cap"

It's widely referred to as 'that capacitor' because it is a change-on-sight component, the first thing to do when trying to bring an old radio back to life. It is almost always leaky to some extent, and as Jeffrey says it can result in all sorts of expensive problems if left unchanged.

Paul
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Old 22nd Jun 2009, 2:10 pm   #6
Nickthedentist
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Default Re: "That Cap"

... and a replacement, top-quality component only costs about 50p
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Old 22nd Jun 2009, 3:23 pm   #7
Reelman
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Default Re: "That Cap"

If "that" cap leaks then the resultant sound can be distorted, replace it as soon as possible to avoid damage.

On valve battery portables leakage of "that capacitor" can kill all sound altogether.

Perhaps a good time to re-mention that a valve output stage should never be run without a load on the secondary of the loudspeaker transformer - ie never run it with the speaker disconnected. If you do then you may still hear a little sound as the output transformer will buzz in time to the source - just before it expires!

Peter
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