Audio Research I/50

@mezzanine 

       I have a question for you. If the power tube 6550 bias is very stable. Is that to say, the tube is ok for use and not even near the end of lifespan?

I get the reply from Audio Research regarding I/50,

"Dear James

Summary: The I50 is not auto bias. The tubes should rarely need to be biased on this product but if it needs to be checked, the unit needs to be disconnected, turned upside down ,and the bottom cover removed, then there will be points to connect a voltage meter and read the settings. There is a small pot to turn with a plastic screwdriver that will adjust the bias. We are working on written instructions for this as well as a short video. Those will be available in the near future."

With my vt100 MK iii, we adjust the bias while the amplifier is connected.

Why for I/50,it need disconnected and do adjustment? I do not know.

@runwell

I wish I could answer your question correctly. I’m not a technician just an audiophile. I’m not 100% sure why tube amps drift out of bias. If it is just about the tube or if it also about the amplifier? I believe the bias can change at anytime during the life of the tube. I don’t think there is a period where it preforms at exactly the same level over a long period of time. This change is not necessarily an indication that a tube is nearing the end of its life. This is just my best guess. I am sure someone else on this thread can clarify it better than I can.

Also, thanks for asking Audio Research directly and posting their response. I’m confident now this amplifier is not for me. Why are the trim pots hidden on the bottom of the amp? Turning over the amp, taking off the bottom cover and then we can check the bias? I suppose anyone could do that but who really wants to? I thought Audio Research said that a trained technician is supposed do that? Or is that only when the tubes are replaced? But why? If the tubes need adjustment then why can’t the owner also adjust the new tubes themselves? Makes no sense. The bottom line is that Audio Research should have designed this brand new amplifier with the trim pots on the top of the amp poking through the top plate. Make it easy for the user. Just like 90% of all other tube amps do.

To answer your last question. I assume that after unplugging the amp, turning it over, then opening up the bottom plate that you’ll have to plug it back into an electrical outlet again. It’s the only way to check the bias. Why AR didn’t mention that in their reply to you? Odd. Furthermore, the fact that the bottom plate is opened up and the unit is plugged into a wall outlet is a warning. Especially to new users. Can you say potential lethal voltages? 

it's really  inconvenient but at least there is an option to bias.
a friend of mine, a very experienced technician with tube amps said that there is also a disadvantage in auto bias.
sometimes you cant use other brand or type of tubes when it's auto bias.
for me the i50 is still an option.