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Old 13th December 2012, 07:30 PM   #1
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Lightbulb DIY Phono Preamp Inputs & Wiring

Hi all,

So, I'm wrapping up building a Tube Phono-pre (6SL7->6SN7) and I have some questions on wiring the inputs. I've searched and found a few threads but none specifically seem to answer my question.

In summary, I'm going to be using twisted pair with silver-plated teflon wire, one signal, one ground (with only one end grounded). The RCA jacks are completely isolated from the chassis. Here's my questions:

1.) Do I attach the shield of the RCA jack to my twisted-pair ground or just attach the jack shield directly to star ground?
2.) My circuit has a 47-ohm resistor before the grid of the first 6SL7 stage. Which order should I put things to minimize hum:

RCA Jack > 47ohm > twisted-pair > 6SL7 Grid
OR
RCA Jack > twisted-pair > 47ohm > 6SL7 Grid

In other words, resistor near jack or near tube socket? And should I continue to wrap my shield wire around the resistor?
----
Appreciate the help!

Steve
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Old 13th December 2012, 10:04 PM   #2
DF96 is offline DF96  England
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The resistor is presumably a grid stopper, although it seems a bit small for that purpose. It must be connected directly to the grid tag, with as short a lead on that side as possible.

The RCA jack has two contacts, so these must connect to the twisted pair. The screen (which may or may not be actually necessary) should connect to the ground wire of the twisted pair at the valve end. Whether this point coincides with the star ground depends on your grounding scheme.

I understand that silver and teflon are a bad combination for audio due to triboelectricity, yet they seem popular.
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Old 14th December 2012, 04:32 PM   #3
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DF96– Thanks for your reply! Just one question– what are you referring to when you mention the "screen"?

Steve
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Old 14th December 2012, 06:31 PM   #4
DF96 is offline DF96  England
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I have just reread your post. I thought you had a screen/shield for your cable, but now I see you haven't. What you called the shield of the RCA jack I would call the outer of the phono socket. Your mention of wrapping a shield around the resistor also sent me off on the wrong path. OK, now I think I know what you are doing.

No, don't wrap anything around the resistor. Just run the ground wire of the twisted pair near the resistor. The idea is to keep the input loop small.

Sorry for creating confusion.
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Old 30th June 2013, 01:49 AM   #5
italgil is offline italgil  Portugal
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Default which wire u advise? thickness?

Quote:
Originally Posted by DF96 View Post

I understand that silver and teflon are a bad combination for audio due to triboelectricity, yet they seem popular.
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