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Old 8th August 2002, 10:51 PM   #1
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Default Speaker impedances and output transformers

Please help educate a non-techie.

What happens if I connect a four ohm load to the eight or sixteen ohm taps?

I realize output power and frequency range are affected.

Does this do any harm?

I have a speaker switching box which adds eight ohms of resistance to paralleled pairs (1-2, or 3-4) and all the pairs are at least eight ohms.

I've been running the switcher box off of the four ohm tap, with the assumption that this is the safest way to run it.

Does it make a difference? What happens if the output trannies are shown too low a resistance? Will they heat up?

This is something I've always wondered about and I'm hoping one of you might be able to explain.


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Old 8th August 2002, 11:07 PM   #2
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the OPT matches the speaker load to the load (Z) required for the tube.

so if we put a 4 ohm on the 8 ohm tap, it brings the Z much lower than it is supposed to be, which affects frequency response and power (dont know how it doesnt though )

I'll let someone with more knowledge finish it off..
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Old 10th August 2002, 02:21 PM   #3
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Gee, thought I'd have an answer by now....

What happens if you connect a lower impedance speaker to a higher impedance output tap?

For that matter, what happens if you short out the output taps on tube amps?

I've done this inadvertently with no apparent negative results. Do the output windings heat up and will they eventually give out??

Thanks in advance!!

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Old 10th August 2002, 04:11 PM   #4
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Hi Lee,

The short answer is that the load that the tubes actually see is proportional to the turns ratio in the transformer. The output taps are designed so that when a speaker of the nominal impedance of the same value as the tap is connected, the output tubes will run in their optimum (as specified by the designer) position.

The equation is Zp = Zs x (turns ratio)^2

So, if you connect an 8R speaker to the 8R tap, everything is (theoretically) optimum. Same with connecting a 4R speaker to the 4R tap etc.

If you connect a 4R speaker to the 8R tap, the tubes will see half the impedance they would if an 8R speaker were connected. Conversely if a 16R speaker were connected to the 8R tap the tubes would see twice the impedance. In very loose terms, the lower the impedance the tubes are driving into, the power output will increase, as will the distortion and most likely the dissipation in the output tubes. With the output tubes driving into a higher Z, the power output decreases, along with distortion and tube dissipation, and bandwidth increases.

However there are a number of caveats that apply to what I've just said. The chosen operating point, quality of transformer design and construction, primary inductance, frequency, power level and the speaker's actual impedance curve will affect all these things too in a dramatic way.

<b> have a speaker switching box which adds eight ohms of resistance to paralleled pairs (1-2, or 3-4) and all the pairs are at least eight ohms.
I've been running the switcher box off of the four ohm tap, with the assumption that this is the safest way to run it.</b>

If I have read this correctly, when you select, say, pairs 1 <i>and</i> 2, an 8R resistor is added in series with the paralleled pair. In this case spkr1 // spkr2 is 4R, plus 8R in series gives a nominal load of 12R. The OP tubes will be driving into a loaded 3 times easier than they were designed to, so they should be coasting. Connecting the switchbox to the 8R tap would then be fine, and give you some more power. Your amp will be fine running this load on any of the taps, but the 8R would be best. I think the switchbox probably adds a 4R resistor in series with the paralleled speakers to give a total load of 8R

I have a problem with the switchbox. If it's addig 8R (or 4R) as a series resistance, it will throw off all the bass tuning and crossover points on the speakers and the resistor will be absorbing the majority of the amps power. Almost all speakers are designed to see as low a source resistance as possible and I can only imagine they sound like crap in this arrangement. A better idea would be to wire up a big multipole swich, or better a set of relays, so that when only one pair of speakers are in use, the 8R tap is utlised, and when 2 pairs are used, the 4R tap is connected.

I hope that's clear, as it's very late and I'm tired and not feeling very coherent. If you want something clarified, or I misunderstood the detail of the switchbox, ask further questions and I'll elaborate.

In the meantime, if you want some more detailed reading on loadlins try <a href="http://www.siteswithstyle.com/VoltSecond/General_Load_Lines/General_LOAD_LINE.html">Loadlines</a> and
<a href="http://www.siteswithstyle.com/VoltSecond/211_Load_lines/211_LOAD_LINE.html">Effects of Inductance on Loadlines</a> , both from VoltSecond's page.

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Old 10th August 2002, 07:25 PM   #5
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Hello Brett!

Excellent info and links. Thanks!

I now understand that running a load on a lower rated tap will cause the tubes to run out of voltage before their optimum output.

The link you recommended was right on topic, says it's better to go that route than to run out of current.

My box adds 8 ohms between #1&2 and between 3&4. If I run 1 and 3, they are paralleled. Same with 2&4.

I don't notice any serious degradation of sound, other than lower volume when the extra resistance is added. I can't say I don't lose some bass, though.

Maybe the vintage drivers and their relatively simple Xovers are not quite as sensitive to impedance fluctuations.

The one question I still don't have firmed up in my mind is what happens to the output trannies if they are shorted or accidentally fed say a 2 ohm load?

Thanks again for tackling this query for me!!

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Old 10th August 2002, 10:13 PM   #6
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Another way to look at it is if you don't need the volume you can "sometimes" achieve better performance by putting a 16 load on the 4 tap. Using very efficient speakers in a small space you tend to not need much power to get things as loud as you want them.
So to improve performance pentodes(most of the later power tubes) are run in triode. A simple thing to do.
Then if you still have more than enough power you can put a heavier load on than what the tube thinks it is getting. For reasons unknown to me tubes tend to like high impedence drivers best.
Not being able to design my way out of a wet paper sack I just follow directions on things to try, given to me by someone I trust. Do not trust me. Though I can hear results I usually don't have a clue as to the physical reason for them.
I know what I hear and accept it, not being too curious as to why it happens as long as it does. Of coure I am not stupid or I would be dead. 400 volts with lots of capasitance can ruin your whole day.
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Old 10th August 2002, 11:45 PM   #7
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Originally posted by Thatch_Ear
For reasons unknown to me tubes tend to like high impedence drivers best.
With the high impedance taps on the OPT there is "less" transformer between the driver and the output tube.

community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
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