MAC Autoformers?

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 "’

@georgehifi - Yes let's go with the MkIII. I'm sure while ruffling some feathers on the active preamp side, you could ruffle a few more as there have been a few vendors venturing into the passive LDR space at prices quite a bit above what I paid for the Lightspeed. I still consider it one of the best bangs for the buck in this business.

@bifwynne - I work with Roger and handle the online tube store for him (tubeaudiostore.com). I know we have a quad of KT-150s lying around, but I imagine we can get more if necessary. You may want to read some of Roger's articles on tubes and tube matching. You can find them here:

http://tubeaudiostore.com/tubin1.html

Also, Ken Stevens of CAT has trusted Roger to match tubes for his customers CAT amps and preamps who he sends to us on a regular basis. Roger does the matching to Ken's stringent specifications. The cost is not cheap as there is a lot of work involved. As you have experienced, you may be able to find tubes at a lower cost, but you subtract quality from that as well as it relates to proper matching. I have purchased tubes from some very reputable vendors, but IMO no one tests and matches them as well as Roger. In fact, Roger has been working on developing a tube tracer that can be used by the layman to test tubes. It's been a slow process, but it hopefully it will see the light of day soon.