MAC Autoformers?

@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.

ramtubes
An Autoformer or any transformer cannot fix a difficult load and only affects stability in a poorly designed, on the edge amplifier.

What I’ve been saying all along, they are a band-aid fix for amplifiers that are "not capable" of doing the job properly without them.

They are also "not able" to make an amp that "is very capable" without them sound better. If anything they make them sound worse.

Cheers George

@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