I misspoke - I hooked the return run up to the 8 ohm tap. Sorry about that. :-)
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I am no expert but I do not see how you could have hurt the speakers or the Amp.
The polarity would have been reversed to the speakers and the output impedance may not have been 4 ohms. (4 ohm load tap being more negative with respect to the 8 ohm tap.)
As for the Amp, it still seen a connected load....
I don't think you would have done any damage during the brief time you had it connected that way, especially if the volume wasn't particularly loud. But it's not a good idea, and could conceivably result in damage to the amplifier in the long term, as I see it.
Basically what you are doing is connecting the speaker across fewer turns of the transformer secondary than it should be connected across. That amounts to "light loading" the amplifier, which if carried to an extreme would approach the kind of effect that running a tube amp with no load can have, namely damage to the transformer and/or output tubes resulting from inductive kickback.
According to my calculations what you probably did was to light load the amplifier by a factor of about 5.8, corresponding to what would happen if you were to connect a hypothetical 23 ohm speaker between the 4 ohm tap and ground. As I say, it's very doubtful that would hurt anything in the short term, but it is not a good idea.
Al, Thanks for your comments. This amp has an adjustable feedback loop and its effect on the sound when I had it mis-connected was huge. That's how I knew something was wrong, apart from a tonal balance shift towards more of everything except highs. It was interesting though (after I had correctly terminated the speakers) to hear what my electronics from the late 80's and early 90's sounded like with my Bolero's. Not bad! Grannyring is right. :-)
This amp has an adjustable feedback loop and its effect on the sound when I had it mis-connected was huge. That's how I knew something was wrong, apart from a tonal balance shift towards more of everything except highs.Hi Newbee,
That all seems understandable.
Probably the mismatched loading was resulting in abnormal amounts of distortion, which would have been partially corrected by feedback, to an increasing degree as the amount of feedback was increased.
Also, the change in the transformer turns ratio that was being used would have reduced the output impedance of the amplifier, causing a more solid state-like tonal balance. Given the impedance curve of your speakers, shown at the bottom of this page, that would mean a de-emphasis of everything above around 2 kHz.