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MAC Autoformers?

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?
pubul57
ramtubes's avatar
ramtubes
460 posts
 
BIF,

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.


What I have been saying is that the primary impedance chosen has nothing to do with the plate impedance of the Pentode. Ideal plate resistance of a pentode is very high as demonstrated by the flatness of the plate curves above the knee. If the plate impedance of a KT150 were 3000 ohms it would produce something close to a 45 degree line on the curves. I just think the 3000 ohms is a mistake thought is it also shown on the KT-120 data sheet, which is curious it would be the same for both tubes.. Modern spec sheets are not done with the care of the good old days and there are few out there to check them.

RCA, GE, Tungsol and Sylvania all had identical plate curves or at least very close and such a mistake would have been found right away. The printed specs for most tubes are IDENTICAL from one maker to the other. There is no other maker for the KT 120 or 150 to monitor the situation.

Thus is the current state of tubes.

Here is a much better data sheet, made not by the manufacturer but someone else..

http://www.tubeampdoctor.com/images/File/data%20sheet%20KT150%20Tung-Sol.pdf

On the first page are the pentode curves and the plate resistance is the slope of the line above the knee. The top curve has a slope of about 20 ma/200 volts = 10K ohms, the lower curves, where the amplifier typically operates is flatter making the impedance even higher.

Also note on the last page the the the load impedance is given as 3,000 ohms. So that is likely the source of the mistake in the Tungsol data sheet, which was made by the Russians I would think. Perhaps something got lost in the translation.

The simplest way to explain light loading is to look at what heavy loading does. If the load is two heavy (4 ohms on the 8 ohm tap) the tube will have excessive current and voltage across it (ie it wont get to the knee but be to the right of it). That product of voltage and current does two things. It reduces the output of the amplifier and it heats the tube excessively.

bifwynne
2,206 posts
 
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  
ramtubes's avatar
ramtubes
460 posts
 
The KT-150 spec sheet you showed us says that "Tube impedance is   [f]rom 10.0KOhm to 12.5KOhm.


And is even higher at low currents where we are often listening. It really helps for someone get a scope or some kind of monitoring device to see the voltage the speaker is using. Some listeners are at very low levels and some much higher. Anyone who is going through tubes is up at the high endor has a terrible mismatch.This may not be obvious, but once one goes over the knee at 0 grid volts the extra current comes at the expense of large voltage drops across the tube and the amplifier is surely clipping at this point. . 

There is no optimal load for a power tube. The load belongs to the circuit in which it is used and the load that is expected. That is why an RM-9 can do equally well with the KT 88 family of tubes (beam tubes) or EL34 (true pentodes). Since these tubes have very different curves they will certainly sound different. One must realize that installing different tubes, either the same type with slightly different specs, or another type entirely will mostly change the feedback of the amp, thus the damping and distortion. 

In most cases the largest difference will be in the bass and overall frequency response due to the higher damping (more feedback) with the beam tubes.
atmasphere's avatar
atmasphere
11,834 posts
 
What is bifurcation in feedback?
Put very simply, bifurcation is the process where feedback can cause a 2nd to become a 4th and so on. Norman Crowhurst wrote about this and I think later also Baxandall. If you look into Chaos Theory you will see quite a lot of papers on the topic. 
toshko
3 posts
 

Hello, I found the thread and read it very carefully. Presently I am reverse engineering this type of block scheme of power amp.
Here is what I have found:
Damping factor for MC1200 and 2000 in 8 Ohms is about 40 which translates to 0.2 Ohm output resistance. In my experience with transformers, this translates to about 100 windings of 1.2 mm wire over a large cross section of E+I laminated steel. A little more for double C-core.
2 kW into 8 ohms is 126.5V and 15.8A
MAC output devices are actually step up transformers. Why are they called autoformers is not clear to me.
In the power supply 100V filtering capacitors are used, thus the rail voltage must be less than that voltage, lets say 90V or even less than +/- 80V for longevity. (typical commercial capacitors begin at 1200 hours at rated voltage and temperature and there are some rated at 13000 hours, but some people don’t turn off their gear which makes for a mere 77 weeks in the latter case... lowering the voltage and temperature multiplies the life multiple times)
126.5/80=1.575 is the transmission ratio of the transformer.
100/1.575
2000W and 80V translate to 3.2 Ohms load and 25 Amps current.


Note: output transformers are designed based on allot more complicated calculations.

This is very simple and vague calculation.

It is not unthinkable for the primary winding to be sub 2 ohms and the operating voltage of the supply rails to be under 50V with large currents.

The complication in the reverse engineering comes from the separate winding of the NFB loop, which loads both primary and secondary windings additionally and thus reducing their impedances.

From the photo of the autoformer in the McIntosh manual, the wires seem to be in the 0.8-1.2mm range. 1 mm wire can withstand 30 Amperes, which leads to primary resistance of about 2.2 Ohms.

In addition, aluminum or nickel wires are not unthinkable in regards of 22-30 meters of wire having 2-3 Ohm DCR.

Why?
Everyone who have tried to lay down the design requirements of a powerful amplifier have come across the design limitations of high voltages.
Reducing the voltage allows for high-end audio grade components such as capacitors and transistors. At 140V there are no capacitors and a limited choice of transistors.
So the autoformer enables McIntosh to design a no compromise SS amp capable of large current due to the high transistor count and powerful power supply at the cost of some manageable complication.


P.S. Regarding the distortion, it is in octaves and in multiples of the base frequency, thus the 7th harmonic of 100 Hz is 14,800 Hz and the 7th harmonic of 1000 Hz is 148,000 Hz... That is why the H-Fi standard measures only to 3rd harmonic only up to 7 kHz.

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