Newbie - Stuck with Biasing....

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Tim F
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Newbie - Stuck with Biasing....

Post by Tim F » Sun Nov 01, 2015 6:37 pm

Hi all,

I've been asking on Pinkfish for the answer to this and was recommended to try asking here. Thanks in advance :)

I'm trying to bias my amps at the moment and need some help. I have Audio Research V140 monoblocks:
http://www.audioresearch.com/ContentsFi ... Adjust.pdf

I need 65ma as I'm using KT88's not the KT90's in the instructions. The multimeter is set to 200m V DC. I can get no lower than 95 after full warm up.

Anything obviously wrong here? I am measuring in the right place and adjusting the right pot. Amp is warmed up for 30 minutes too. This is the multi-meter I'm using (and the setting I'm using too) ...
http://i26.photobucket.com/albums/c144/ ... 254c77.jpg

Speakers are connected to the 4 ohm tap, these are Martin Logan Clarity and I've made sure they're on too before measuring.

Many thanks, Tim

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Nick
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Re: Newbie - Stuck with Biasing....

Post by Nick » Sun Nov 01, 2015 7:09 pm

Well, assuming 425v on the anode (the doc says 32W and 75ma), looking at the kt90 triode curves, that would indicate about -37v on the grid. The same point for a kt88, needs -50v on the grid. So if the amp was designed for kt90, it may not have enough range in its bias setting to work with kt88.
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Tim F
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Re: Newbie - Stuck with Biasing....

Post by Tim F » Sun Nov 01, 2015 7:17 pm

Thanks Nick, they are designed to work with KT90/ KT88/ 6550 with the last one being the least powerful so I'm not sure why that'd be incorrect from Audio Research's perspective.

I kind of follow what you're saying, I may be putting too much current through them and would require a change of resistors etc. to make this work properly? These are Svetlana KT88 Winged C if that helps, and I'm using the 65ma from memory as being the right value (can't remember where from).

Appreciate your time! :)

Tim F
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Re: Newbie - Stuck with Biasing....

Post by Tim F » Tue Nov 03, 2015 7:24 pm

Can this be fixed? Thanks, Tim

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Paul Barker
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Re: Newbie - Stuck with Biasing....

Post by Paul Barker » Tue Nov 03, 2015 9:26 pm

Seems you may need to alter the value of R97 to achieve 65mA. It must be a power resistor. Try the 100k value suggested in the text under a section annotated 2.

While you can't bias it correctly I would not use it.
I can do it right or I can do it now, but I can't do it right now.

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