Audio Research I/50

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.

There is an advantage to having trim pots in certain places. When you have a product like AR, you don't want anything to look out of place, collect dust that is difficult to clean and certainly not around trim pots. Mr. Clean Jeans with his Q-Tips and micro cloth could move things. As a bulletproof design. I'd hide it too. LOL

OLD design almost always hid trim pot under the bottom plate (if there was a plate) or through a hole in the top that was covered with a metal or wood cover.

Auto bias sucks. I use it but I install trim pots on Macs. I BIAS cool. Unless I'm doing a lot of late night low volume listening. I still bias and listen. If I can keep the SQ and bias cooler I do. Why? pretty simple, the cooler you bias the longer the valves last.

BUT that is just part of the story. It also stops the power valves from breaking up sooner than they would have with the higher bias. Distortion! HF distortion, and LF both suffer when you start to crank it. The higher you go the worse it get, then you run out of gas. 

If you scope bias the actual watts created can double. Cary Six Pacs are a good example. Factory bias is 220-240ma per 6 valves. 50 wpc. Scope and alignment increased power on demand output to 100+ wpc and COOLED the bias to 160ma for 6 (lets say) EL34s.

So what do I get out of the deal being able to bias my own units?

1. A lot longer valve life, you can see why. Lower bias, lower volume, longer life.

2. Less distortion. Cleaner sound, COOLER running. 

3. More horsepower, the amps will play louder and cleaner before distortion sets in or the tubes break up.

Side note: 

I had WE 300 (116a) amps they were my fathers. That is what he left me. His snap-on tools, and Western Electric Amps. 

I learned how to maintain those valve amps. The 300WE valves that were in it were used. I had 4 new, and one used sets of valves. In 15 year of playing those watt sucking pigs close to every day, 2 sets of valves still had over 50% valve life. I know I put 30,000 hours on those amps easy. My father over 30,000 hours. He had them 20 years. He used the same set of valves all that time. 2 set were never used and I never did wear out the set my father used.

How long do valves last?

I have original Mcintosh valves in a 1968 MC225 less one 12AX7.

BTW you still have to bias most SS ampt too, you know.. :-)

Regards

The i50 is a really nice looking unit, and it's nice to see AR release a product available to those with somewhat more modest means. It is a little strange that they release a product for what could be described as entry-level (per cost...not in the quality sense) with manual bias setting, but requiring it be done by a technician. 

The skeptic in me wonders if this is a way for AR to have consumers build relationships with dealers by design (with the benefit of recurring revenue), a'la John Deere or any modern automobile.