MAC Autoformers?

@bdp24 

I checked out the Roger Modjeski YouTube video.  Awesome.  I've often observed that there are 3 types of smart people in the world, namely:  regular smart; very smart; and scary smart.  Roger is scary smart and down to earth at the same time.  

I think he might have tossed a tongue in cheek zinger at ARC.  Roger snickered that expensive ARC power tubes need to be replaced every 2000 hours.  In contrast, Roger said his early tube amps (RM-9 ??) could run their power tubes for years.  

Not being a EE, I can't say whether tube life in ARC amps is good, bad or neutral.  But I can tell you that ARC charges $200 for a single KT-150 tube.  My ARC Ref 150 SE takes two quads (8 tubes) -- for a total cost of $1,600.  Ouch!  I should mention that while I can buy the KT-150s from well know tube vendors for half that price, I have had problems getting closely matched pairs from one well know vendor.  

If Ralph (Atmasphere) picks this post up, ... what is the tube life is in your amps.  Btw Ralph, …. Roger mentioned a couple of times that negative feedback, *if used properly*, is not all that bad a design feature.  Not taking a position, just passing along what RM said.

BIF

Thanks guys, I appreciate the compliments. Making amplifiers has been my life's work as I find them forever fascintating in all their many forms. Though I have put into producion only three power amps and two preamps, there are lots of designs hanging around waiting for the right person to come along.

@ramtubes. Sorry for not being current with the handles of some of our members, but if you are Roger M, I have a few questions. 

Matching KT-150s in my ARC Ref 150 SE amp is a bitch.  ARC sourced tubes match much better.  When I refer to matching, I am speaking to matched pairs that can be biased within close tolerance, say 3 to 5 mVs.  ARC suggested optimal bias is 65 mVs on the set tube, but the bias on the slave tube in the "matched pair" can be between 57 and 73 mVs (+/- 8 mVs).  

Do you know any 3rd party tube venders who can match KT-150s for ARC amps at ARC standard?  ARC charges $200 per tube; $1600 for a complete retube is outrageous.  Most 3rd party tube venders charge half that amount.  However, a batch of KT-150s that I bought several years ago from a well known tube vender have bias numbers that range from 5 to10 mVs per matched pair, … and in most cases 8 to10mVs.

Btw, do you really think that ARC driven KT-150s have a useful life of only 2000 hours, …. the recommended time for replacement?  How can one tell if a tube has *really* reached the end of its useful life?  If you say, one should replace the tubes when the amp doesn't sound good reminds me of the story of the frog and the pot of boiling water.  

The story goes like this.  If you drop a frog into a pot of boiling water, he will jump out.  However, if you put a frog into a pot of cool water and slowly bring the pot to full boil, the frog will just boil to death. The later variant is like listening to tubes.  The sound degradation occurs so slowly, you don't realize that the tube is spent until the you get the urge to replace.  

Thanks.

BIF 

Amen to this Tube Amp post-we need more engineers on here...

QUOTE  :;"’6-18-2018 2:05amThis is an old topic...literally. In the times when tubes were dominant amplification devises, the output transformer was a necessity (one might say a necessary evil. The tubes are amplifying voltage, so that they work with high impedance loads. The loudspeakers normally have quite low impedance. Thus a transformer is required to match the output of tube amplifier and the loudspeaker. When first transistors have emerged, the schematic design did not evolve immediately. Thus, early transistor amplifiers were very similar to the matured at that time tube amplifiers.

However, over time it was realized that transistors work better and amplifiers of current as opposite for tubes which work better as amplifiers of voltage. Thus, transistor amplifiers can and do work well with low impedance load such as loadspeaker directly.

Now, still transformer (autoformer) can assist with the loudspearkers with various impedance...i.e. 4 Ohm vs. 6 Ohm vs. 8 Ohm providing more stable load to a SS amplifier. However, transformer alone is not a perfect transfer devise and making quality transformer for audio output is difficult and costs a lot of money. The typical issues with output transformers are: reduced damping factor and difficulties with driving complex loads, slew rate reduction, additional distortions, etc.

From my perspective, McIntosh polished out the technology and because of the quality, manages to make very good sounding amps. The majority of other brands simply moved on ( which at this point seem to be the right thing to do) and still makes good amps. At the end of the day it is not the technology but the end result that matters "’