Someone is selling a MAC MA6500 Integrated claiming its superiority over the Ma6600 due to the fact that "it does not have the degrading autoformer design found in the MA6600". That is the first time I've heard a claim that the autoformer was a hindrance to better performance; I thought quite the opposite. What do you MAC Maves think?
I use I am using a Mac C2500 tube preamp mated to a Mac MC 152 amp with the transformers driving GE Triton Reference speakers and this combination has gotten me off the merry go round as it is the best sound I have ever had in my listening room.
My other system uses. Mac MA6600 Integrated with transformers. I was considering a MA6500 but Audio Classics who know more about Mac than anyone else with the exception of McIntosh told me I would lose the lush romantic Mac house sound without the transformers. Needless to say I am thrilled with all my Mac purchases and am done.
@bifwynne , Well, since you asked, the answer is ’distortion vs actual frequency response’.
All amps make distortion. Some distortions (IMD and higher ordered harmonic distortion) are more audible than others. Of the two mentioned, the latter is what makes solid state amps bright and harsh, and does that to tube amps as well, although they are not as harsh as solid state because there is simply less of this form of distortion. In a zero feedback amp, there is respectively (solid state or tube) even less. That’s why they sound smoother. The problem is, solid state amps have non-linear capacitances built into the junctions of the output devices. This capacitance is responsible for higher ordered distortions (brightness and harshness) that still affects a zero feedback solid state amp.
Since the ear uses the higher ordered harmonics to sense sound pressure, it is more sensitive to these harmonics than most test equipment, since it has to cover a 120-140db range! This is an insidious, inconvenient truth that the audio industry does not like to face. This is further complicated by the Fletcher-Munson loudness curves, which place many of these upper harmonics in the most sensitive region of the ear’s response.
That makes things particularly tricky.
The reason zero feedback tube amplifiers exist is that a good number of designers have come to recognize the problem of higher ordered harmonic distortion. One fairly easy way to sort that out is to avoid using feedback, since it is known to add such distortions of its own in the process of otherwise suppressing distortion. So this means that the designer has to use other means to suppress distortion.
This results in an amplifier with a fairly high output impedance.
The thing is, if you chose the speaker carefully, the result is **far** more neutral than is possible with the conventional approach (its not subtle- you can hear it right away, whether an audiophile or not). The reason this is possible has to do with two salient facts: first, no speaker made anywhere at any price is flat. Second, the ear converts distortion into tonality, and has tipping points wherein that tonality *is favored* over actual frequency response.
The latter fact is why two amps on the bench can have identical frequency response but one will sound bright and the other will not!
I’ve found over the years and decades that if the speaker requires the amplifier to employ feedback to get flat frequency response out of the speaker, that the result has no chance whatsoever of sounding like real music! At best it will sound like a very good stereo (this bit is tricky; many people are happy with the status quo of a good stereo, because they think getting it to sound like real music isn’t possible!). Its this difference where I have drawn the line, and why over 42 years we’ve avoided feedback.
**That** is why bother with a zero feedback tube amp. Sorry for the derailment.
excellent explination on the higher order harmonics.... Musicians know this stuff in and out and can appreciate harmonics big time since you tune string instruments for intonations,harmonics etc.... the music we hear or think about is usually more the many order harmonics instead of just a plain note. think Foriear transformans math and wave theory. same for light the eye likes vs the actual Frequency, back to the amps, they should not enhance and particular range or require feedback to quell a too bright range-Just increase current/voltage flatly from 0-30K. design your differential signal work in the pre and equilizer if you want. this can be seen on a RTA live. think when you tune your guitar on how super high the lead sounds when its only a few 1000hz,
A friend brought over a circulotron amplifier and we connected it to my Quad 63s I was immediately overhelmed with boomy one note bass. Knowing the Quads and most ESLs have high impedance in the bass I hooked up a generator to the amp and voltmeter to the speaker and found a very sharp 8dB peak at 50 Hz, the cause of the boomy one note bass. The treble was down several dB for the same reason; the non flat impedance curve.
Cone speakers have similar impedance peaks at resonance in addition to peaks and dips in the entire curve due to multiple drivers and their crossovers. This cannot be ignored unless you really like the modification to the frequency response. Very few speakers have flat impedance curves as this is hard to achieve and still get good frequency response. I know of no speaker maker who designs for high output impedance amplifiers.
One other point. The proper marriage of the ESL and OTL was the KLH 9 speaker and the Futterman amplifier. It was a good marriage for two reasons. The KLH 9 was a 16 ohm speaker that did not go below 12 ohms and the Futterman was a high damping amplifier. The Futterman, like all OTLs love a high impedance load and the KLH was. Now we have ESLs that dip to an ohm which no OTL likes.
I know of no other combination of ESL and high output impedance amplfier that is good one.
@ramtubes Roger, you might want to read this: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php If the Quads were close to the wall behind them, they will often have one-note bass. This often happens when a solid state amp is employed; the customer will note that there is no bass on account of the higher impedance of the speaker into which the amp can't make power, and so will move the speaker around, eventually finding out that if they move it closer to the wall behind it, they can finally get some bass. But this is not how *any* planar is supposed to be set up.
ESLs don't respond so well to voltage rules, depending on the speaker- the Quad ESL57 and 63 being pretty good examples- also the Sound Lab ESLs, AudioStatic and even Martin Logan (if you use a set of ZEROs to allow for their low impedance). The reason is, unlike box speakers, the impedance curve is not also a map of its efficiency, which is pretty much the same across its bandwidth (on account of its impedance being based on a capacitor rather than a driver in a box with resonance). This is typically a 9 or 10:1 change in impedance! If the amplifier output power varies with this curve, the result will be too much highs and not enough bass.
The solution we've found with our customers using Quads is to have them pull the speakers further out into the room, so there is at least 5 feet behind the speaker to the wall. In this way the bass normalizes and customers report that the Quad is playing bass quite well.
I know of no speaker maker who designs for high output impedance amplifiers.
I can name a few- Coincident, Classic Audio Loudspeakers, DeVore Fidelity, Audiokinesis, Lowther, PHY, PureToneAudio and many more. Essentially, any speaker that works with an SET is working with an amplifier of high output impedance.
We are quite a ways off-topic; if you would like to discuss this further it would be a good topic for another thread.
The Eminent Technology LFT-8b, while employing magnetic-planar drivers as do Magneplanars, is rated as 8 ohms by it’s designer, Bruce Thigpen. While Maggies’ impedances are solidly in the 3-4 ohm range, the ET LFT driver is an almost purely resistive 11 ohm load (the speaker’s 8 ohm rating a consequence of it’s dynamic woofer, for frequencies 180Hz down).
Maggies require a LOT of power, very expensive in a tube amp. The matter is exasperated by their 3-4 ohm load, almost all tube amps putting out half as much power there as at 8 ohms (the notable exception being Music Reference/Ram Tube’s Roger Modeski’s RM-200---100w/ch @ both 8 and 4 ohms!). If you bi-amp the ET LFT-8b, you can use a modestly-powered tube amp (the RM-200 works splendidly, as I have heard does the Atma-Sphere M60) on the m-p drivers, and a ss amp on the woofer. The panels and the woofers each have their own binding posts.
Ralph, I did read your white paper before entering this discussion. Lets just agree to disagree. Peter Walker designed the 57 to be used with an amplifier with a damping of 20 and even specified the series inductance. To say that these speakers will play with a damping factor of 1 is not fair to the speaker, no matter that some people like a widely altered frequency response. How can we discuss little differences in distortion when the frequency response has been so altered to make the speaker unreconizable?
Thats a nice little list of expensive speakers that represent a vanishingly small part of the market. I agree a single driver speaker like the Lowther will work just fine with your amplifier.
As an antique radio collector I have given a lot of thought to how early SE amps (the 45 in particular) got relatively flat response without feedback. In that case the driver was a single cone in an open baffle cabinet so the impedance rise was not so severe and relatively flat. These early radios sound pretty good. However that does not represent modern popular speakers.
ramtubes Ralph, I did read your white paper before entering this discussion. Lets just agree to disagree. Peter Walker designed the 57 to be used with an amplifier with a damping of 20 and even specified the series inductance. To say that these speakers will play with a damping factor of 1 is not fair to the speaker, no matter that some people like a widely altered frequency response. How can we discuss little differences in distortion when the frequency response has been so altered to make the speaker unreconizable?
Hear hear, good to see someone else thinks the same, about amps becoming tone controls with wild speaker impedance curves. Low output impedance (damping factor) and current are the only fix to make the amp stay reasonably flat. Or I suppose you could have an "inverse tone control" to counter it, but the amp in question will start to gag itself very early in volume level.
I did read your white paper before entering this discussion. Lets just
agree to disagree. Peter Walker designed the 57 to be used with an
amplifier with a damping of 20 and even specified the series inductance.
To say that these speakers will play with a damping factor of 1 is not
fair to the speaker, no matter that some people like a widely altered
frequency response. How can we discuss little differences in distortion
when the frequency response has been so altered to make the speaker
unreconizable?
You may not remember one of your customers, Bill Toberman (RIP, lived a few miles from me), but he had a set of RM-9s which beat out any ARC introduced to his system. He had Quad ESL63s. At the time I didn't think our amps would work with ESLs, but when Bill convinced me to bring them to his house, I was proven wrong- the MA-1s and Quads proved to be a match made in heaven. No loss of highs either, and this in direct comparison to the RM-9. The ESL57 proved even easier to drive and more accessible, since one of my employees had a set. In a nutshell, the FR is not as unrecognizable as you suggest!
We've also gotten Best Sound at Show using our amps with ESLs... all I'm saying here is there is more to this than meets the eye at first blush.
As we all know, there is theory, but there is also practice and we have many happy Quad customers. The thing is, OTLs can provide far more current than most people realize; I've seen one of our 12-tube output sections take out a 15 amp fuse...
As an antique radio collector I have given a lot of thought to how early
SE amps (the 45 in particular) got relatively flat response without
feedback. In that case the driver was a single cone in an open baffle
cabinet so the impedance rise was not so severe and relatively flat.
These early radios sound pretty good. However that does not represent
modern popular speakers.
If you recall the old days, speakers like Altec, Klipsch, JBL and others had level controls for the midrange and tweeter. These controls were not for room adjustment, they were there because the amplifier to be used had an unknown voltage response. **That** was how frequency response was handled in the old days prior to the introduction of the Voltage Paradigm. This practice persisted into the early 1970s as the industry switched over; KLH had level controls for their tweeters as did the AR-1, the world's first acoustic suspension loudspeaker. Of course, now open baffle speakers are back- one of them is in that list I posted above and its not the only one by any means. Google: https://www.google.com/search?q=open+baffle+speakers&client=ubuntu&hs=UEV&channel=fs&...
Nowadays there are entire generations of audiophiles that grew up thinking that the voltage rules are the only game in town. In mid fi, that's certainly true but in high end the power rules have been in play for decades. That is why SETs are around. We've been making zero feedback OTLs now for over 42 years. The reason all this is going on is due to objectionable distortions- and what kinds the ear doesn't care about. That latter statement is why the tubes/transistor thing has been raging on and on, and is why tubes are still in production (horns too), 60 years on after being declared 'obsolete'...
Just to be clear, we've been working on our own take on a class D amp for the last 2 years; not using anyone's modules, since we realized we had something to bring to the table. That amplifier acts as a voltage source; please don't think that I don't understand what's going on here. The paper was written because many people ask why we don't use feedback, and an explanation was needed that went beyond 'we don't like it'.
The reason 'paradigm' is used is because someone inside the paradigm (which is a platform of thought) will regard that which is outside the paradigm as blasphemy or heresy- it can't possibly be right. But this is history: the Power Paradigm used to be how everyone did it. These days our understanding of physiology has greatly expanded so we can better see why there can be a benefit to this approach.
ramtubes I also would like to note that the OP of this thread, Paul R owned a RM-9 Special Edition. We miss Paul, a really good guy.
Totally, "top bloke" had many late night discussions with him. He really loved his RM-9 Special Edition which he said was a perfect match with my "Lightspeed Attenuator" which he also started thread on, which exploded to over 18 million hits, made my life hell and interesting at the same time, rip Paul miss you heaps.
I posted this on the thread Roger (ramtubes) started to discuss transformers, but thought I would do it here as well.
Roger gave a 90 minute talk on tube amp design at the 2015 Burning Amp Festival. If you own a tube amp, or are thinking of getting one, you will be glad you watched the video. The video is on You Tube, but also on the Berkeley Hi Fi School website.
Go to "berkeleyhifischool.com" and put your browser over "RESOURCES". A drop-down screen will appear---click on "VIDEOS". On the right hand side of the page is a row of "ARCHIVES". Click on October 2015, and the video will appear. Prepare to learn a lot!
Paul was awesome. I had the pleasure at hanging out with him at RMAF several years ago. We met in Bobby P.’s room as Paul was a Merlin owner. It was there that I mentioned to him the Lightspeed and he bought it soon thereafter. In addition to the 9SE, Paul also loved his RM-10 and used the Lightspeed with that amp as well. I can't be certain but I have owned my Lightspeed for around 9 or 10 years.
I can't be certain but I have owned my Lightspeed for around 9 or 10 years.
Wow I think your right, the MkII, was released around 2006 when all MkI's were recalled and converted. I think it's time for a MkIII, what do you think Anthony? Upset the active preamp brigade some more?
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.
I know BIF, Roger doesn’t look any different than a regular guy, but damn! It’s humbling to listen to him talk about amplifier design; I wonder how anyone can have so much knowledge in a normal-sized head. I talked to Bill Johnson a few times, and to Tim de Paravicini (EAR-Yoshino), David Manley, Keith Herron, Ralph Karsten, Frank Van Alstine, Max Townshend, and numerous other really good Hi-Fi engineers. They are all very interesting of course, but I have the distinct impression that Roger Modjeski understands amplifier design as well as any human in history. That he and his products aren’t more well known and widely owned actually doesn’t surprise me any more than the fact that most of my favorite music is made by artists known to only the hardest-core music lovers, like we here on AudiogoN.
The late Brooks Berdan was my Hi-Fi dealer for many years. He sold me Hi-Fi, I sold him vintage drums. He was a Music Reference dealer, and loved Roger and his amps. He told me his wealthier clients would pass up MR products because they didn’t cost enough (no bragging rights), or weren’t being reviewed or talked about enough in TAS and Stereophile, didn’t have the High End cashe’ of ARC, VTL, Jadis (Brooks sold a lot of Jadis), etc. Brooks had a LOT of used ARC amps in his pre-owned racks, their owners tired of the breakdowns, repairs, frequent tube replacement, updates, etc. His tech Tom Carione showed me all the burned circuit boards in the ARC amps he had repaired every time a tube blew. He had never seen a Music Reference amp come back for repairs---never. That’s my kinda amp!---Eric.
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.
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:
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.
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.
@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."
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.
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?
Roger, I'll ask the following for myself, but your answer may well be of interest to others. You rate your RM-200 Mk.2 amp at about 100w/ch at 8 ohms, and, unlike a "normal" tube amp, about the same at 4 ohms. The advantage of using the 4 ohm tap is less distortion, lower output impedance (higher damping factor), and longer tube life. With a tube amp which produces less power at 4 ohms than at 8 (typically half as much), the user forfeits power to gain the advantages afforded by using the 4 ohm tap ("light loading"). Since the RM-200 produces about the same power on both taps, what possible reason is there for NOT hooking up a nominal 8 ohm speaker to the 4 ohm tap? Thanks---Eric.
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.
i am rebuilding a set of ESL63.... to learn but also listen to.... will start out with a rebuilt MC240 ( which probably has the best transformers available in 1961 ! anyway, my mentor Richard Vandersteen always says run it on the best sounding tap” but i can see both a Ralph and a Roger amp in my future....
Speaking of Richard and Roger, Roger says the idea to install the Capacitor "Forming" function into the MK.2 version of his Music Reference RM-200 amp was a suggestion from Richard. Brooks Berdan sold a lot of RM-9's and RM-200's to owners of Vandersteen 2's and 3's, a great matchup.
For the price of retubing an ARC Reference 150, you can just about buy a used RM-200! The four output tubes run in the RM-200 (producing 100w/ch) last about five times longer than those in the REF 150. How many 150 owners have compared their amp to the RM-200? I’m guessing none. You can buy a used PAIR of RM-200’s for less than a single used Ref 75! There’s one on AudiogoN right now for less than $3k, a ridiculously good deal. I have all the power I need from the one I already own or I’d buy it.
In addition to Brooks, John Ruttan at Audio Connection used to sell
Vandersteen and MR combos and IIRC correctly it was the RM-10 and 2s. Given the new cost of the RM-200 a used one for under $3k is quite a bargain. It should also be noted that the RM-200, unlike most vacuum tube amplifiers which lose power while the speaker load impedance drops, increases output power as the speaker load becomes more challenging.
It is stable down to 1 ohm and includes a 1 ohm tap.
While light loading sounds better to me, Roger would also be the first to say use the tap that sounds best to you. There is no reason to not experiment. It's a cheap and easy tweak.
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.
10,000 hours is typical- so we warrant the power tubes for a year on this basis, and always have.
I agree about the feedback 'used properly' (which many designers do not) comment. Proper application of feedback is tricky to say the least, and may not have been possible until the age of personal computing, due to the number of variables involved. Here is a nice primer on the topic:http://www.normankoren.com/Audio/FeedbackFidelity.html
one thing I greatly value is a collegial, challenging, supportive and sharing attitude in the user AND supplier community....I did not know Roger did the bias forming....also..so I will add to my list of collaborative yet competitive firms RM, Vandersteen, Audio Quest, Aesthetix, Brinkmann, ......I am sure the list goes on....
10,000 hours is typical- so we warrant the power tubes for a year on this basis, and always have.
Ralph, What is your dissipation figure as a percentage of rated? How do you figure in high current peaks when the amp is used at full power?
I agree about the feedback 'used properly' (which many designers do not) comment. Proper application of feedback is tricky to say the least, and may not have been possible until the age of personal computing, due to the number of variables involved. Here is a nice primer on the topic:http://www.normankoren.com/Audio/FeedbackFidelity.html
I have spoken with DImitry and a group of people trying to improve the tube models for SPICE. He put the parameters of the RM-10 into their SPICE program and got results that did not even come close to what an RM-10 actually does, and this is only for the midrange where the output tranformer is considered perfect. He did not attempt any feedback analysis as the output transformer is almost impossible to model where it matters.
I know of no tube amp designer who uses SPICE. can you name a few?
Since the RM-200 produces about the same power on both taps, what possible reason is there for NOT hooking up a nominal 8 ohm speaker to the 4 ohm tap? Thanks---Eric.
This is something that needs clarification. The reason we have taps on tube amplifiers is to get the full rated power into different loads. That means when we test an amplifier it produces full power into a load of the same resistance as the tap. Indeed hooking a 8 ohm load to the 4 ohm tap results in reduced power. Usually though only a 30% reduction, not 50% due to other factors (reduced loss in the output transformer, power supply and tube saturation voltage). So 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.
What an RM-200 does is to go into AB2 mode in the above case The tubes stay happy and the reduced load gets extra power in the same way a transistor amp gives more power into a lower load.
I know of no tube amp designer who uses SPICE. can you name a few?
Nope. Maybe one? I know I don't! Victor Khomenko was trying to model our MA-1 in Spice many years ago before he became part of BAT. He called me up saying that the amp didn't run in Spice, but I had to point out to him that the amp actually worked despite what Spice came up with :)
As you can imagine, the ratings of the tubes tend to vary with load- and the amp will draw less power and run less heat if a higher impedance load is used. Also, as you've pointed out, the class of operation is affected. In our case the amp is biased to be class A2 on the proper load, but will be class AB2 if the load impedance gets low enough. We don't get crossover artifacts on that account though.
As far as high current spikes, at full power the tubes still have some dissipation left over, so they can handle spikes, however as the load impedance is reduced, eventually this will cause the cathode coating to fail.
Ralph, Thanks for your answer. I think most of us develop amplifiers in real space verses cyber space. SPICE, by its name, was indeed designed to help people design ICs not tube power amps.
The thing that concerns me about 6AS7s, it that they emit cathode sparks in my Tektronix 530 curver tracer at zero grid.That is consistant with your statement of cathode coating failure as it appears the sparks are bits of cathode coating. They actually look like sparks off of a 4th of July sparkler.
The thing that concerns me about 6AS7s, it that they emit cathode sparks
in my Tektronix 530 curver tracer at zero grid.That is consistant with
your statement of cathode coating failure as it appears the sparks are
bits of cathode coating. They actually look like sparks off of a 4th of
July sparkler.
The 6AS7G begins to develop grid current at about -15V or so; 0 volts on the grid is pushing them pretty hard (a bank of 12 power tubes at 0v on the grid can pop a 15 amp fuse rather quickly BTW). Despite that our driver circuit can push them to +15 volts on the grid while maintaining linearity. Here are some tips for reducing arcing: Precondition the tubes by putting filament current on them for at least 72 hours if you have the Russian variant (6H13C). If you have the Chinese (6N13) or American (6AS7G) tubes you will want to precondition for 96 hours. If B+ is applied before preconditioning or at any time during the process, you may consider the process ended. We built a jig for just this purpose. Preconditioning reduces premature arcing and can double the life of any power tube. The Russian and Chinese tubes hold up much better in this regard. The 6AS7GA and its 6080 variant to the GA should be avoided for use in our amps- the grid heatsink is much smaller and tends to warp with grid current.
@atmasphere - Hi Ralph, I know we have had conversations about the 6AS7GA tubes and your comment above echoes the advice you gave me. However, Roger has a nice stock of those tubes and he asked me if I wanted to try them in the M-60s. So I took it over to the shop and he put in matched quads (2 matched per monoblock). Now I have no idea if the grid heat sink is warping and I wouldn't even begin to know where to look to check, but I have to say these have been the best tubes that I have had in the amp. The Chinese were not very reliable, even after preconditioning, the Russian were much better, and the RCA on par with the Russian. So far I think it has been a good 6 months since those tubes have been in the amp and no issues with arcing or lint shorts (Roger's term) or other type of failure. In addition, Roger didn't precondition the tubes. Now as to lasting 10,000 hours I guess we will see. Obviously still too early to tell.
I neglected to mention that the 6AS7GA tubes I am using are General Electric military spec. In addition, as Roger had performed measurements and had data on the tubes he was able to match them so they were drawing an equal amount of current. Matching the tubes also allowed the position of the DC offset adjustment screw to be set at the middle (near 12 o'clock) of its range to achieve the appropriate reading on the meter. My previous experience with other tubes was such that while you could still set the offset, you may not have had as much flexibility for future adjustments. In some cases I also had to swap tubes around a number of times to achieve the proper DC offset. Matching the tubes made the process a lot easier. So regardless of the type of tubes used, and even though Ralph doesn't require the tubes to be matched, based on my experience I would suggest matching them if you had a means to do so.
If you have the tubes tested in a rigorous manner as Roger does, it makes sense that they would hold up better :) It *also* makes a difference that they are mil spec- that might even be the bigger difference!
That does not seem like the sort of tube of which ample supplies exist. We stay away from the collector market as a general rule but if you mess with NOS tubes of course you can get better results.
We've had such poor luck with the garden variety of GA-style tubes that we don't mess with them. Usually what happens is someone buys a mess of them and installs them in the amps without any sort of testing. Then we get a call about a malfunctioning amplifier, but then it turns out to just be tubes. With the Russian tubes (which is what we've mostly been shipping recently) we have few enough problems that doing a warranty for a year is easy.
After the MkIIIs went into production about 14 years ago, we found we were shipping about 10% of replacement tubes that we were doing before that. Its good that you have a set of American tubes working for you- IMO and from all the feedback we have, they sound the best (and the amp makes more power too, an extra 20 watts).
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.
The tubes will be happiest if you use the tap that is equal to or lower than the lowest impedance of the speaker. So if your lowest is 4 ohms use the 4 ohm tap.
Although David Manley (rest his soul) believed that we match the primary of the tranformer to the tube’s impedance this is not true. Once again its all about the voltage and current that the tube is comfortable with. With that in mind, a reduced impedance will require more current from the tube. If the tube is already at max current minimum voltage (the correct place to be at full power) the reduced load impedance will demand too much current and cause a great increase in voltage drop across the tube and overheat it.
A 10% overcurrent may double or triple the voltage drop aross the tube increasing the dissipation by many times. Its a horrible situation. In fact the RM-9 Special addressed this problem by allowing the output tubes to go into AB2 mode thus saving the tubes. The speaker that prompted this was a Theil whose impedance dropped below 4 ohms in the treble region where loud trumpet music would just bake the tubes in a regular RM-9. The AB2 mode is also used in the RM-200 as i liked what it did.
For normal tube amplifiers it would be best to connect the amplifier to the tap that matches the lowest impedance of the speaker in the region where there is a lot of music.
Speaker manufacturers are not so honest about their impedance range and a curve is the only way to know. If they didn’t do so many tricks in the crossover we would not have this problem. The drivers are not the problem, the crossover is.
In my experience most speaker designers do not know much about electronics or care about what the amplifier may have to do.
These are examples of speakers that will not play well on low damping amplifiers due to wide impedance swings or very low impedance demanding excessive current. The density of music where dips occurr is important to how much the speaker will overheat the tubes. These are just in the order that I found them in Stereophile under floor standing speakers. I didnt have to look for long. these are the first 4 measured in order.
Connected to an amplifier with a damping factor of one at eight ohms the following will happen. Where the impedance approaches 20 ohms there will be a 4-5 dB peak at those frequencies. Where the impedance dips below 3 ohms the respoinse will dip 4-5 dB making a total error in frequency response of 8-10 dB.
In addition if the low impedance occurrs where there is a lot of music the amplifier will current clip and the tubes will get hot.
Click on the link and open in a new tab for a good look.
The Wilson Alexia 2 is still a current-hungry design. Its impedance drops to 2.6 ohms at 84Hz (fig.1), and there is a demanding combination of 5.1 ohms and –44° electrical phase angle at 57Hz, both frequencies in regions where music can have high energy levels.
Eggleston Viginti June 2018 needs to be connected to the 4 ohm tap and will work it hard in the bass where there is lots of energy. Also note how high the impedance is from 1Khz to 8 Khz where a low damping amplifier will cause a 4-6 dB rise in level making the speaker very bright on trumpet music and anything in that region.
Monitor Audio Silver 300, JA notes "Although the minimum magnitude is 3.6 ohms between 150 and 170Hz and there is a combination of 5.4 ohms and –39° at 97Hz, this speaker won't tax the amplifiers with which it is used".
While this speaker won't tax most SS amps, it will bother a tube amp unless connected to the 4 ohm tap. (RAM)
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."
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.
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