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Old 16th August 2006, 04:39 PM   #1
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Default "Y" caps? current bleed?

I'm building a guitar amp from parts of an old tube radio. the radio's power transformer had a 0.22uF cap between one side of the main secondary and the center tap.
Looking in the schematic, it seem there should have been two caps in that area, one from each side of the secondary to the CT with a value of 82nF.

What are those caps anyway?
The radio (from around 1947) looked original in that area, nothing missing or non original, so why just a single cap? why much bigger value?

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Old 16th August 2006, 06:50 PM   #2
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That is a bit odd. Usually old radios have a cap from one side of the primary (AC line) to the chassis...

This cap was from one side to the CT of the B+ winding? Did it use a tube rectifier or SS?

Pete
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Old 16th August 2006, 08:05 PM   #3
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The amp/radio is/was tube rectified.
I've attached a picture of the power supply section as can be found in the schematic. caps C130 and C131 are spec'ed as 80nF, but as I said in practice only C131 was installed, and it's 0.22uF...
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File Type: gif bx765x-power-supply.gif (12.9 KB, 150 views)
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Old 16th August 2006, 09:02 PM   #4
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Hmmm, never seen that done in old radios. I don't see much purpose for the caps, other than providing some HF noise filtering. I wouldn't expect there to be any noise issues caused by the rectifier itself unless it were a mercury or gas type.

The only reason to put such a cap on only one side of the winding - and this is purely a guess - might be that that side of the winding was wound on top of the primary, so was more susceptable to noise coupling from the primary?

0.22uF across half of the B+ winding seems pretty large. If it were me, I would remove it... you could always add something back in if you experience any line-related noise issues.

Pete
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Old 18th August 2006, 03:03 PM   #5
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Yea, without knowing what it is for, I will omit this cap from my build.
From poking around in some schematics of other amps (guitar amps anyway), it seems some amp designs had those caps there, on each side of course. for example:

Marshall JMP series 1986 and 1987
Marshall 1992 Superbass

So the mystery remains


Btw, what the term CCS means?
I see it on this board again and again...
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Old 18th August 2006, 06:20 PM   #6
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CCS = Constant-Current Source. They often make a good plate load (very high AC impedance). Also useful for cathode circuits of differential circuits.

Pete
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