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How critical is an exact match of impedance from the output tap to speaker? My binding posts are wired to the 8 ohm taps on my OPT's, but I would like try a set of speakers that list impedance as 4 ohms minimal, 6 ohms nominal. I am not sure if I want to switch to the 4 ohm tap permanently, I just want to try the speakers out. Can I connect to the 8 ohm tap without risk? Or is it time to take the bottom cover off?

Using the 6 ohm speakers on the 8 ohm taps should not hurt anything.

mesherm wrote:Using the 6 ohm speakers on the 8 ohm taps should not hurt anything.

The above is correct. But if you are listening carefully the sound may be different on the 8 vs. the 4 ohm tap. The tubes will see a higher impedance on the primary if you use the 8 ohm tap on the secondary. With the 8 ohm tap I'd expect lower overall distortion (maybe) but less overall power available from the amp.

If the goal is an "apples to apples" A/ B listening test you have to factor this in. That said, at low volume level I doubt this makes much difference

The tubes will see a higher impedance on the primary if you use the 8 ohm tap on the secondary. With the 8 ohm tap I'd expect lower overall distortion (maybe) but less overall power available from the amp.

If I am not mistaken, you have it backwards.
Whatever load is connected to the secondary is "reflected" onto the primary side by the impedance ratio between the primary and the secondary.

Suppose the transformer has an 8000 Ohms: 8 Ohms impedance ratio.
If the a 6 Ohm load is connected on the 8-Ohm tap, the "reflected" impedance on the primary side is:
6 Ohms * (8000 Ohms)/(8 Ohms) = 6000 Ohms.

If, on the other hand, a 6 Ohm load is connected on the 4-Ohm tap, the "reflected" impedance on the primary side is:
6 Ohms * (8000 Ohms)/(4 Ohms) = 12000 Ohms.

Matt

Keep in mind the speaker impedance as listed on the speaker is a nominal one and that the actual speaker impedance is going to be lower on the low frequency end and higher with high frequency. Sweep a speaker with an impedance meter and you will see what I am talking about.

"Nominal" impedance is a bit of a misnomer. According to the IEC standard, I forget the document number, RATED IMPEDANCE is calculated as not being 20% higher than the minimum which includes DRC of the coil. 4*1.2 = 4.8 Your speakers are really 5 Ohm. This also doesn't take into account the complexity of the load for reasons of power factor which is completely lost in current specs. http://www.stereophile.com/reference/707heavy/index.html

Ah Oh.. I'll get on the soapbox.. I'd better stop now

Safest tap is the 4 Ohm one.

There is a convention to the use of the term "nominal impedance", and if the impedance over the majority of the bandwidth, specifically covering the range in spectrum where majority of the musical spectral power occurs, it's 8 ohms. A single number cannot tell all there is to tell about an impedance that varies with frequency.

You must keep in mind that 'nominal impedacnce' is not defined in IEC. Indeed, the electronics industry was advised when the Trade Descriptions Act was introduced, that the word 'nominal' should no longer be used in specifications. That is why the IEC concept of 'rated value' is so useful. There is a very detailed definition and explanation of this term in IEC60268-2.

The IEC standard (IEC60268-3) allows any "increase" above the rated value, but limits the "decrease". The standard does not allow the impedance to fall below the 80 % of the nominal value at any frequency, including DC.

Taken directly from the IEC page.