audio-talk

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

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.

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!

Can this be fixed? Thanks, Tim

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.