Bias Supply Question

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jckid649
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Bias Supply Question

Post by jckid649 » Wed May 30, 2018 1:14 am

Can anybody tell me why the '67 plexi uses a 68k resistor across the first 10uf bias supply filter cap?

Tazin
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Re: Bias Supply Question

Post by Tazin » Wed May 30, 2018 2:37 pm

I think you may have meant the "typical" 56K resistor. If so, then its purpose is to set the bias voltage. If the resistor is used in series with a trimmer pot then it allows a bias voltage range to be established.

jckid649
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Re: Bias Supply Question

Post by jckid649 » Fri Jun 01, 2018 6:55 pm

Actually no. I mean the one I've circled here.

Image

Tazin
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Re: Bias Supply Question

Post by Tazin » Fri Jun 01, 2018 7:35 pm

Actually it is the same resistor I was refering to and it normally is a 56K rather than a 68K that is shown in that layout. Either way, its purpose is to set the bias voltage.

EDIT: If the layout you posted is one of George's then he might have decided on a 68K resistor instead of a 56K because he changed the first resistor before the diode in the bias circuit to 27K instead of the normal 33K when using a Drake 1203-80 PT.
The bias circuit when using a Dagnall T2562 PT has a 27K before the diode and a 56K resistor over the bias filter caps.

jckid649
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Re: Bias Supply Question

Post by jckid649 » Fri Jun 01, 2018 9:45 pm

Ah ok. Thanks. Yes, I got that from the layout that George posted. Just wondering because I've not seen a resistor right there on any other layout.

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