ARC Reference 150 Tube Biasing Issue

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.

Scott ... how did you make out with your tube biasing issue. Did you try switching the tubes around??

Thanks to everyone. I appreciate the help. Before I got to your responses, I put the old tubes back in and realized the negative voltage was normal (push-pull amp, maybe?). So I got over that problem - felt a little stupid, frankly. I took Bifwynne's advice and paired up the tubes that were running high voltage, this got things much closer. But I am still running a bit out of spec on one pair of tubes -- the other three pairs are within spec. I am at 63 mv (instead of 65) and 78 mv on the corresponding slave (instead of 72 max). I took a chance and fired up the amp and it sounds fine. I thought I would run it for a few hours and see if things improve. I got the tubes from a guy on A-gon who had some spares, and he claimed they were matched quads -- they certainly were new. But perhaps they're not as close as they should be. They are KT-120's. So I saved a few bucks, but in the long run I may have made a mistake. I was not in the mood to spend almost $ 1K on the KT-150's, in spite of the all the word from you guys stating how good they are. Well I will let you know in a few days after I check it again. Maybe with more fiddling I can get them in spec. Is there any chance of damaging the amp by running it out of spec?

In case anyone is interested, I am running the amp to Rockport Mira's, and this is a marvelous combination.

Thanks to all....Scott

Scott, I doubt you will damage the amp. But it may be making some distortion because the one tube pair is a little out of spec.

Here's another idea ... focus again on the other 3 tube pairs. With the set tubes biased to 65 mV, is there any slave that is between 57mV and 60 mV? If so, switch the 78 mV slave with the other low mV bias slave. Check bias again. Incidentally, spec is between 57mV and 73 mV.

And btw, you should always let the amp sit for at least 5+ minutes between turn-off and turn-on. You could damage the amp if you don't wait a little. The power supply caps need to discharge a bit.

Another thought if the idea above is a non-starter is consider buying just one matched pair set from Kevin Deal at Upscale. IME, Kevin does not match as closely as ARC, but he's also half the price. He may get you closer than where you are now.

Please let me know how you make out.