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

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

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.

Scott, just a thought for the next time. I am not an electronics tech, but many members happen to be. Perhaps one of them will jump in.

It is my understanding that ARC, and perhaps many other manufacturers, recommend using matched tube pairs for their amps. And this is where the experts can chime in ... matching entails more than just buying tubes which happen to bias closely. There are other electrical attributes that are taken into consideration when matching tubes, for example, transconductance, G2, resistance, microphony, and so forth.

I really do hope one of our other experts will jump in now. So my point is that if my thoughts, as expressed in the preceding paragraph are accurate, that should encourage you to buy power tubes from reputable sources, such as ARC, or Upscale Audio, and the many other fine tube vendors out there.

Btw, surf through the Upscale web site. Uncle Kevin talks about his tube testing and matching procedures.

Cheers,

BIF