MAC Autoformers?

I believe Ralph Karsten (Atmasphere) has weighed in on this topic many times in the past.  I recall that Ralph has suggested the use of Zeros, an outboard autoformer, in certain instances, e.g., driving extremely low impedance electrostats.  

As to the points above about tube amps and their use of output trannies, ... the answer there can depend.  For example, my amp is an ARC Ref 150 SE.  It is a tube amp that uses output trannies.  However, ARC also uses some negative feedback to lower the output impedance and extend the bandwidth.  For example, the output impedance off the 8 ohm tap is about 1 ohm or less; about half that off the 4 ohm tap.  

What this means in practical terms is that my amp can perform "somewhat" like a low impedance solid state amp when driving speakers with varying impedance and phase angle curves, …. within reason.

I only use my amp as example.  For those interested in tube amps, check Ralph's white paper that explains the Voltage and Power Paradigms.  For most tubes amps, it is not an all or nothing proposition.  It's a spectrum issue.

BIF   

Ralph wrote:

"Personally I don't think that having an amplifier that behaves as a voltage source is the most neutral way to go because the factor that is left out here is the function of loop negative feedback, which is used in the vast majority of amplifiers. But it is this design aspect that allows amps with output transformers to behave as a voltage source- add enough feedback and almost any amplifier will!"

Based on past posts with Ralph and Al (Almarg), I get Ralph's point.  As I mentioned above, my ARC Ref 150SE uses about 14 db of negative feedback and has "low'ish" output impedances off the 4 ohm taps (about .5 ohms or thereabouts ) and the 8 ohm taps (about 1 ohm or so).  But even still, I can hear a discernable difference in tonality when I play my speakers off each set of tabs because the speakers do not have a flat input impedance function over their frequency ranges.  So much for a flat speaker output frequency response, ... even if that was really ever possible with a pure voltage paradigm amp.  And that doesn't even touch on TIM distortion caused by using negative feedback.  

For pure tube enthusiasts, the only solution is to find speakers that have flat and high'ish impedance functions (say 16 ohms) over their entire frequency ranges.  I do not think there are a lot of beasts like that out there.  Ralph, if you can make some suggestions, please do.   

Btw, another knotty subject that Ralph and Al have posted about some years ago is low damping factor with high output impedance tube amps.  Ralph, I forgot what you posted.  Care to re-educate us?

Thanks.

BIF

I am confused. I get using autoformers on high output impedance tube amps.  The autoformes will make the amp think it is driving a higher impedance speaker load and thereby improve the damping factor, make the load easier for the tube amp to drive and may smooth out the FR of the speaker.

But why would one use an autoformer on a high quality, stable, hog wattage SS amp.  These beasts were (should be designed) to handle demanding speaker loads.  I surmise that most speakers on the market today were voiced to be driven by a high current capable voltage paradigm SS amps.

Maybe autoformers make sense on super low impedance electrostats,but I'd be careful on the front end in matching up any amp to a super low impedance electrostat.  Make it easy in yourself and just short the amp's output terminals and call it a day.   :)

Please explain? 

@Johnto - must have missed an earlier post.  How does your post about Maggies fit in this thread?  I thought this thread was about autoformers.


BIF

@atmasphere 

Ralph, could you clarify a point regarding impedance matching between tube amps and speakers with roller coaster impedance and phase angles numbers.   

I think you mentioned that the function of an output transformer is to step down the high voltage from the output tubes to the amps output taps.  In addition, the output tranny will step up a speaker's impedance presented to the output power tubes at the primary windings of the tranny.

Maybe a simple example will help further clarify my understanding.  My amp is an ARC Ref 150SE which uses KT-150 tubes in push-pull fashion.  I recall that the output impedance of KT-150s is 3000 ohms.  In a theoretically perfect world, if I had a speaker with a perfectly flat 4 ohm impedance function, plugging the speakers into the amps' 4 ohm taps would result in a stepped up impedance of 3000 ohms presented off the primary windings of the output tranny.  That would be a perfect match with the power tube's output impedance.

We all know that in the real world, most speakers have roller coater impedance functions.  As a result, the impedance off the primary windings of the output transformer will vary from the optimal 3000 ohm target.  ARC says, just use the taps that sound the best.  

So here is my question.  What is happening electrically if there is an impedance mismatch?  Is the amp producing distortion if there is an impedance deviation from 3000 ohms?  If so, does having larger wattage and power supply capabilities ameliorate distortion effects?  

How about negative feedback?  In my case, I understand that the Ref 150 uses 14db of negative feedback and as a result, performs like a voltage source amp. Does that feature help to reduce distortion effects?  

I have not touched on phase angle or sonic coloration issues caused by impedance mismatches between the amp's output taps (nominally, the  4, 8 or 16 ohm taps) and the speaker's actual impedance at a particular frequency.  

As to sonic coloration, I recall that the output impedance of the ARC Ref 150 off the 8 ohm taps is around 1 ohm or less.  This results in some sonic coloration, ... but not a lot.  In a Stereophile review of the ARC Ref 150, John Atkinson measured the Ref 150's voltage output variation off the 8 ohm taps to be (+) or (-) .8 db when presented with a synthetic speaker load.  

Btw, are similar issues presented with OTL tube amps?

Thanks for the education.

BIF

@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

@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 

@ramtubes --- Roger, thanks for responding.   Yes, ... there is only one bias pot to control the bias of a pair of KT-150s.  The bias pot is used to adjust the bias in mVs of the set tube.  ARC recommends 65mVs.  The other tube in the pair is a slave.  If the tubes are well matched, the slave will measure 65 mVs, plus or minus 3 or 4 mVs.  

No, … I do not have a schematic and I do not know if the drivers are direct coupled to the output tubes. As you probably gleaned from the ARCDB website, each pair of KT-150s are driven by one 6H30 driver tubes; 4 in total.  

Does the following ARCDB excerpt give you a clue about your direct coupling question?  The ARCDB website states that the " power-supply energy storage has been doubled to some 1040 joules. All-new interstage coupling capacitors using technology and materials first incorporated in our Anniversary Edition preamplifier effortlessly link input stage to output stage, which is powered by two matched quads of Russian KT120 [KT-150s in the SE version] output tubes driven by 6H30 twin triodes. There is more than ample multistage solid-state regulation. Output stage coupling is a combination of “ultralinear” and Audio Research’s patented 'partially cathode-coupled' topology, which is superior to conventional pentode or triode operation."   

The New Sensor website reports that the KT-150 has a maximum plate dissipation of 70 watts.  See https://www.newsensor.com/pdf/tungsol/kt150-tungsol.pdf

I seem to recall reading that ARC runs the KT-150 at half that max amount.  The New Sensor website says that the KT-150 "CARRIES A 70 WATT PLATE DISSIPATION RATING WHICH PROVIDES FOR PUSH-PULL AMPLIFIER DESIGNS UP TO 200 WATTS OUTPUT."  I believe that a quad of KT-150s in the Ref 150SE is configured to produce only 150 watts.  So I surmise that ARC chose the tube because it sounds good and is pretty robust.  And ARC is not pushing the KT-150s to the max Q limit. 

As far as sound is concerned, I ran my amp with KT-120s and KT-150s.  IMO, there is a world of difference in terms of sonic saturation and a sense of bass extension and control.  In plain terms, the KT-150 is a great sounding tube, but expensive.   

Your comment about tap selection is interesting. In a post directed to Ralph Karsten I mentioned that my speaker are nominally rated at 8 ohms, but that is a misnomer. The impedance and phase angle curves are rock and roll.  

See https://hometheaterhifi.com/reviews/speaker/surround-sound-speaker-systems-reviews/a-secrets-speaker...

In particular, impedance drops below 8 ohms between 65 and 700 Hz and below 4 ohms between 70 and 150 Hz.  There is a big impedance peak between 600 and 6000 Hz.  The good news is my speakers are rated at 92 db sensitivity.

Based on a Stereophile article done on the Ref 150 some years back, Atkinson suggested that one try the 4 ohm taps for the reasons you suggested above.  Weird thing is that I have gone back and forth between the 4 and 8 ohm taps any times and I keep settling on the 8 ohm taps.  Distortion and damping factors considerations aside, the 8 ohms taps sound better, even though I sense the amp is adding a little not too unpleasant flavor to the presentation.  

And yes, …. please keep me in mind that if you decide to expand your tube repertoire to include KT-150s. I sense it is the power tube de jure for many manufacturers and as the KT-150s age, the need for replacements will make a market for replacement tubes.

Care to guess what the useful life of the KT-150 is in my amp given what I posted above?  Is there any way to measure tube life and quality with a tester?

Thanks and I love your YouTube presentation.

BIF        

bdp24, … see my post to Ralph and Roger about the same question.   Theoretically, the 4 ohm taps should work better.  In my case, I subjectively think the 8 ohm taps simply sound better and I do not know why.

@ramtubes 


Roger, you wrote that "a typical 100 watt amp puts out 100 watts into any matched tap and somewhere around 75 watts into a tap mismatched by one step. In this case the tube are loafing along, distortion is reduced and damping increased. But this requires that the load does not go significantly below the tap impedance.

However going in the other direction where the load is lower than the tap impedance bad things happen. In that case the amplifier puts out less power, works harder and the tubes get overly hot to the point or radically shortening their life."

Let me understand better what you are saying.  If my speakers have a roller coaster impedance curve (4 ohm to 20+ ohms), is my tube amp "happier" if I use the 4 ohm taps.  I think you are saying that the amp may produce less power, but it will produced better sound with less distortion and less tube wear and tear.

I assume this is so because the back impedance presented to the output tubes off the primary winding of the output tranny will be equal to or be higher than the output tubes' output impedance.  

Is that correct?

BIF

@ramtubes 

Thanks Roger.  Thought you might find the attached websites of interest.  Atkinson tested an earlier version of my amp, the Ref 150.  The test results should be comparable to my amp, the Ref 150SE, because the Ref 150and Ref 150SE share almost identical electrical characteristics.  

Atkinson uses a simulated speaker load, which is actually kind.  See below.  Impedance doesn't drop much below 8 ohms.  Atkinson measured Ref 150 tap impedance as follows:  "The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm."  Running the Ref 150 into the simulated speaker load, he measured FR as follows:  "From the 8 ohm tap (fig.1, gray trace), it was ±0.8dB; the 4 ohm tap offered ±0.4dB, the 16 ohm tap ±1dB." 

https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements

https://www.stereophile.com/reference/60/index.html

So in summary, I gather that even if speaker impedance swings between 3.9 and 20++ ohms at its peak, it is still better to use the 4 ohm taps because it will draw less current off the output tubes and thereby cause less tube wear and less distortion.  Also, the 4 ohm tap is close tothe bass dip impedance, which is good.  Max power may be compromised at higher impedance, but if I am not pushing the amp, that shouldn't be a problem.

Btw, based on Atkinson's measurements, I gather that if my speakers do dip to approximately 4 ohms in the bass, the damping factor should be roughly 7.27 (4/.55), which ain't too bad.

Am I getting it now??

Btw, ARC uses 14db of negative feedback in ultralinear mode to achieve these electrical characteristics.  The amp is also runs in modified AB mode.

Thanks again,

BIF 

@ramtubes 

So Roger, …. bottom line, do you concur that using the 4 ohms taps on my Ref 150SE should yield a flatter FR, less distortion and longer tube life?? I gather you are saying yes. 

P.S. the ARC engineers obviously were trying to balance a lot of design ideas and techniques when putting the Ref 150SE together.  Just enough negative feedback to get the output impedance low enough to handle a host of speakers designed to be driven my low impedance SS amps and a robust power supply (1040 joules) to handle the tough spots that require a lot of current (i.e., low impedance and highly reactive phase angles in the bass region).

P.S.S. -  I have about 1800 hours on my KT-150s.  Can you help me source some matched tubes that will bias well my amp?  I have 8 fresh KT-150s sourced from another vendor sitting in a box.  They don't bias well.  Could you match them for me?  

@almarg - been distracted.  Glad to be back.  Al, if you get a chance, check out Roger's amp design presentation here:

https://www.youtube.com/watch?v=aO1Q281FQxo

The guy knows his stuff.

BIF 

@ramtubes,  I was in contact with Anthony today.  I will ship off 2 quads of KT-150s next week that I bought from a 3rd party tube vendor.  Hopefully, Roger can get some good matched pairs out of these tubes.  Damn expensive!!  Otherwise, it's back to ARC.

Anthony advised that I should "light load" the amp off the 4 ohm tap, which Atkinson measured as having an output impedance of .55 ohms.  Presumably, FR should be flatter and DF higher. The impedance in the bass saddle is a smidge south of 4 ohms. I estimate the DF to be approximately 7 in the saddle.  Hopefully, that will be enough to get the bass kick I like.

Also, Anthony echoed that even if the 4 ohm taps presents an impedance to the output tubes off the primary coil of the output tranny that is higher than the rated 3000 ohms output impedance of the tubes, no harm will occur.  In fact, it will make for less strain on the tubes and lower distortion.  The cost, … about 3db of power.      

I can live with that.  My speakers have a sensitivity rating of 92 db.  They play plenty loud.

Thanks  Roger and Anthony

@ramtubes,

Roger, take a look at the New Sensor specs for the KT-150.  I believe it says the plate resistance is 3000 ohms.

https://www.newsensor.com/pdf/tungsol/kt150-tungsol.pdf

I thought you said before that if the primary impedance presented to the output tubes off the output tranny is less than the plate impedance, it can cause distortion and shorten tube life.  Ergo, use the 4 ohm tap. You called it light loading.  

So if the speaker impedance is higher than the nominal tab impedance, resulting in a primary impedance presented to the output tubes that is higher than the plate impedance, it will result is less distortion and longer tube life. Ergo, you said to use the 4 ohm tap.

Of course, the 4 ohm tap has lower output impedance and therefore a higher DF, which is a good thing. 

What did I not understand??

BIF

Rodger,

Take a looks at the specs for a Tung Sol 6550.  

https://www.newsensor.com/pdf/tungsol/6550-tungsol.pdf

Interestingly, the 6550  "load resistance" is 3000 ohms. But the plate resistance is 15K ohms.  

The KT-150 spec sheet you showed us says that "Tube impedance is    [f]rom 10.0KOhm to 12.5KOhm.

I think the importance point here is that whatever the relevant resistance spec is, I think your point is that if one has a speaker with a flat impedance function of 4 ohms, using the 4 ohm tap will present an optimal load to the output tubes off the primary winding of the output tranny.  

By contrast, if one's speaker has a rock and rock impedance function, ranging from 4 ohms to 20+ ohms, using the 4 ohm tap is still the way to go even if the back impedance presented to the output tubes is higher than the tube's impedance spec, whatever it is. In other word, opt for light loading.  It will result in less distortion, longer tube life, and a higher DF.

BIF