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August 05, 2020, 03:48:15 AM

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Author Topic: Lab Series L3  (Read 951 times)

jman

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Lab Series L3
« on: July 10, 2019, 09:33:46 PM »
Can some one tell me how to set the bias on a lab series L3 Amp.

Jazz P Bass

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Re: Lab Series L3
« Reply #1 on: July 11, 2019, 11:18:09 AM »
I can't seem to put my finger on the L3 schematic.

If it is anything like the L5/L7, then the bias is fixed.
3mv across any output ballast resistor.
http://www.rru.com/~meo/Guitar/Amps/Lab/schem.html

edvard

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Re: Lab Series L3
« Reply #2 on: July 11, 2019, 09:33:15 PM »
Try this: 
https://mega.nz/#!1ph3CSrY!44oxSuZl6oZOZnnXJ12ikMNFncLJ2-AqZqEFzqUrnKg

Interesting schematic, that L3; it uses a CD4007 strung up to use the top half of it's 3 inverters as P-MOSFETs running the tone controls and another for the reverb driver/master out.
I don't see anywhere that counts as adjustable bias, but I could be wrong...

Jazz P Bass

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Re: Lab Series L3
« Reply #3 on: July 12, 2019, 12:51:42 PM »
Nice find on the L3 schematic, PE.

It appears that R9 is a variable resistor for idle current adjustment.

Measuring the voltage drop across R16, 0.22 ohm ballast resistor, will give you your bias voltage.

jman

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Re: Lab Series L3
« Reply #4 on: July 12, 2019, 11:02:11 PM »
Thanks for all the info.