ARC Reference 150 Tube Biasing Issue

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.

Bifwynne, thanks. I will try the procedure you suggest on Saturday.

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