MAC Autoformers?

BIF,

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.

https://www.stereophile.com/images/718WAlex2fig1.jpg

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.

 https://www.stereophile.com/images/618EggAndrafig1.jpg

B&W 702  The 702 S2's nominal impedance is specified as 8 ohms, with a minimum value of 3.1 ohms

DO NOT connect this speaker to the 8 ohm tap. 

https://www.stereophile.com/images/518BW702fig1.jpg

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)

https://www.stereophile.com/images/318MS300fig1.jpg

Very cool thread ;-)

@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