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Old 12th Aug 2022, 8:39 am   #1
Gabe001
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Default Marconi 209, 3 pickup sockets

I was just wondering if I could pick your brains.

Just working on a Marconi 209 which is fully working, but I'm just wondering why it has 3 pickup sockets instead of 2.

The relevant part of the circuit is shown.

Top and middle go across the volume control as usual, and the third goes to chassis. The lower end of the volume control is connected to the cathode of the MH4 valve so my understanding is that it would have a DC voltage on it equivalent to the bias.

Just wondering what the reason is for such an arrangement

Thanks
Gabriel
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Old 12th Aug 2022, 9:11 am   #2
G6Tanuki
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Default Re: Marconi 209, 3 pickup sockets

It allows the screen / braid on the pickup lead to be earthed without shorting the cathode bias network to earth.
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Old 12th Aug 2022, 10:59 am   #3
Silicon
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Default Re: Marconi 209, 3 pickup sockets

In case some people are not familiar with pickup cables, they often have two insulated conductors for the signal and an external screen / braid which is usually grounded.

The two insulated conductors are effectively wired across the volume control.

As G6Tanuki explained, the volume control is not at ground potential.
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Old 12th Aug 2022, 11:09 am   #4
Gabe001
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Default Re: Marconi 209, 3 pickup sockets

Thank you. I get it.

Would it be safe for me to plug in my phone via pick up across the volume control if I use 0.1uf blocking caps?
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Old 12th Aug 2022, 1:45 pm   #5
ms660
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Default Re: Marconi 209, 3 pickup sockets

Quote:
Originally Posted by G6Tanuki View Post
It allows the screen / braid on the pickup lead to be earthed without shorting the cathode bias network to earth.
Connecting the ground side from the pickup to the receivers ground wouldn't short out the cathode bias so far as I can see ?

Lawrence.
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