MAC Autoformers?

@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.

HI BIF,

I looked up your amp on the ARC database but no schematic for the 150. Great site for those who are curious about ARC gear. From what you tell me it appears there is only one bias pot per channel. Do you have a schematic for me to look at? I need to know if the driver tube is direct coupled, I do that in the RM-200 and they did in some amps.

I was the first to computer test tube and match them to very tight specs in 1982. I discoverd the Two Point match where once we find the grid volage for a particular current we then find the transconductance. If one matches both of those numbers the tubes will track over a wide range or voltages and currents. Others, as far as I can tell, still set a grid voltage and get a current but they aren't doing what I am doing. 

I also test for grid leakage, which is one aspect of what ARC used to call "Low Gas". What they wanted was tubes that didn't run away when they got hot as ARC tubes do..But tubes usually arent Gassy when new, they get gassy from running hot.

On life all I can say is that I consulted with Sylvania Engineers, spend a full day at the plant in Altoona, PA and learned a lot. I was advised that their power tubes can last 10,000 hours if run at 50% of rated dissipation. Its dissipation that kills tubes, makes them gassy and run away. ARC likes to run tubes at high idle currents such as 60-70 MA and at 500 volts thats can be 35 watts which is the max rating for 6550s. Tube life is not linear with dissipation and max rating can reduce life by a factor of 10, so 1000-2000 hours is typical for many amps, though not for my amps which run them at 15 watts in the RM-9.

So it was good for them to go to the KT120 which is rated higher and the KT 150 higher still. Im not sure I agree with those ratings but they certainly are bigger tubes and have a hight dissipation. I would say the KT150 is a bit of overkill but thats what ARC likes to do. While I keep a large stock of KT-120s on hand I have not yet bought a large number of KT150 as they are much more expensive than the 120s. However I think i can beat their price and know I can do better matching if I can get some demand for those tubes. So lets see if that will occurr..

Yes I am indeed Roger A. Modjeski last time I checked :)

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

HI hemigreg,

Your have brought up some good points here and I hope you take my expansion on them as a compliment. Many of the readers of this thread have been pulled into impedance and power thinking by some new paradigms that I do not particularly agree with. So here goes.

We do need more engineers on here, for one thing to keep the known laws of electricity correct in our discussions. Some manufacturers like to make up stories to defend their equipment's performance, or lack there of. I am always entertained when John Atkinson, a very strait shooter, measures an amplifier and finds horrible results. It is unwise to send JA and amplifier that measures poorly and then have to defend it in the Manufacturers Comments.... which are even more amusing. 

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.

I think its safe to say that when engineers first got transistors they didn't know what to do with them and the evolution of design topologies took awhile to develop into what we have now. I was learning transistors at the same time and had the same early RCA and Motorola books. I was 14 years old and it really wasn't all that difficult to get something that worked. We were all making the circuits out of those manuals which were the most advanced at the time which was 1965. However those schematics did not look like tube schematics at all.

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.

While we consider that transistors are indeed current amplifiers they do not respond linearly to voltage inputs as tubes do. Pentodes act like transdconductance devices which means their current is proportional to their input votage which is a very handy thing to drive a speaker since we start with voltage (from the preamp) and want current for the speaker. Speakers are current driven devices by nature of their physics.

What makes tubes need a transformer and transistors not is the fact that high current transistors come easily but high current tubes do not  The most popular transistor in the world is the 2N3055, which Dynaco, NAD and many others used. For $1.50 it can do 15 amps of current. The best Horizontal Output tubes (mistakenly called Video Tubes because they were in TV sets) can only do one amp of current but they can do this at a very high voltage, thus the need for the transformer. We are really not matching impedances here, we are using the transformer to exchange voltage for current, as all transformers do. Indeed output transformers are difficult to make, more on that if people want to know, and expensive because they have to get the best workers in the factory to make them. I had one guy I trusted and he made all the RM-9 outputs with exceedingly consistant results. 

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.

This is the biggest myth being promoted in this threat. An Autoformer or any transformer cannot fix a difficult load and only affects stability in a poorly designed, on the edge amplifier. A good transformer does not reduce damping factor appreciably, or slew rate or change distortion except at the frequency extremes. M6 iron is extremely linear, low hystersis, and low  eddy current loss. Those fancy amophous irons don't make transformers appreciably better.

What Transformers do is present to the amplifier a particular voltage and current that the output devices like. The taps on tube amps are there for that purpose. You might as well call them A, B and C and choose the one  you like best. At least you have a choice. What an Audoformer does is give you that choice with ampifiers that do not have taps.

The RM-10 produces an unbelievable 40 watts with one pair of EL84/6BQ5 tubes (typically 17 watts) at less than 1% distortion with an 8 ohm load on the 8 ohm tap. Many people do not need 40 watts and they can get 25 by putting their 8 ohm speaker on the 4 ohm tap resulting in twice the damping and 1/10 the distortion. Most people do this, I encourage them in the manual to try it and it works on all well designed tube amps. Try it!

Sorry this is long, but there is a lot of work to be done on this topic.