ARC Reference 150 Tube Biasing Issue

You don't say if you are using kt120 or kt150 tubes. Are you using matched pairs of tubes? I use matched quads that are hand matched after they are burned in a bit. This gives you much closer matched tubes. This I the same thing Audio Research does to match their tubes.

Installed new tubes for my ARC Ref 150, and the bias voltages look pretty weird. When I set the bias for 65 mv on the power tube, the slave tube shows voltages in the NEGATIVE 65 mv range. Two of the tube pairs do that. The other two pairs show voltage around NEGATIVE 90 mv on the slave tubes. Any one have any ideas what might be causing this?

Where did you buy the tubes?

"When I set the bias for 65 mv on the power tube, the slave tube shows voltages in the NEGATIVE 65 mv range. "

You just use the absolute value. You can't have -65mv. What kind of DVOM do you have?

Scott ... the negative voltage readings on the slave tubes are normal. If it bothers you, flip the probes around and the readings will read positive.

The other problem .. the bias voltage delta between the set and slave tubes relates to a tube matching issue. It has been my experience that even Uncle Kevin at Upscale can't seem to get the bias voltages as tight as ARC.

The problem with ARC is that they charge twice as much as most tube vendors. For example, Upscale charges about $50 per KT-120 as compared to ARC which charges about $100 per KT-120. It's double that for both sources with respect to the KT-150s.

You didn't mention where you bought the tubes. If from ARC, call Kal. If from a tube vendor, call the vendor.

Here's a quick suggestion that may help. If two pair are pretty close (forget the negative voltage reading from the slave tubes) to 65 mV, then you're good to go. Leave those pairs alone.

Let's go back to the other two other pairs. If both slave tubes on the other two sets read 90 mV when the set tubes read 65 mV, try this. Make the two 65 mV set tubes a matched pair. And do the same with the 90 mV tubes.

Hopefully that will get you where you need to be. That said, right now your amp is NOT operating within ARC spec. Spec for the amp is within a range 57 to 73 mV. Your slave tubes are way outside the spec range likely resulting in distortion.

Please report back on how the tube switch works out. If you are still outside the spec range, call the vendor.

BIF

Jim McShane

http://mcshanedesign.net/tubes.htm

He burns the tubes in before hand matching them.

Tell him they are for an Audio Research amp.