dummy speaker load.

[Alan0354]

I went ahead and just use a 7.5 ohm resistor and a SPDT to choose between one of the speaker and the resistor. I am surprised how good the result is. I had the amp up to 5.5 with resistor and about 3.5 with the speaker. It has more base with two speaker, I verify by using a 4 ohm resistor to even take more load off the speaker. I got even less bass with 4 ohm. So the observation of using resistor reduce the bottom a little. With a 10 ohm, it's just very little difference.

Two speaker do sound fuller than a single. But I think that is normal as I was in a small room with the amp less than 3' from me. So having two speaker do widen the wall of sound quite a bit. It's like you crank the balance of the stereo to one side. Even though you know the stereo sounds exactly the same, just because you lose one side, it's just sound not as full. I think in order to have a more representative test, I have to have it in my living room where I can stand at least 10' away.

As for the sound, I actually think it sound slightly better than using my THD Hotplate attenuator at -4dB. The THD seemed to lose a little punch even at -4dB.

[cbarrow7625]

This topic made me ask myself a question to which I do not know the complete answer, I'm hoping one of you smarter guys will be able to fill in the blanks.

Assuming the two speakers in the amp are in parallel, for a (modern) solid state amp we could just disconnect one speaker and get a theoretical 6dB reduction in level (assuming the voltage output stays constant from the amp); 3dB decrease in voltage sensitivty changing from 4 to 8 ohms and 3 dB decrease from mutual coupling of the speakers. If we did the same in a tube amp we wouldn't get the same results because the load reflected back to the power tubes would double as a result of the speaker impedance doubling. Will that cause the output tubes to generate more voltage or less voltage? I assume it will be more voltage? How much more? I can't imagine it would double. How much would you anticipate the real-world reduction in output would be? For a solid-state amp, real world is usually somewhere between 4-5 dB.

Also, since most guitar amps seem to tolerate running with the load doubled (8 ohms on a 4 ohm tap, for example), why not just unplug one of the speakers instead of putting any kind of dummy load in place? I know that at very high levels that can be a problem for the OT, but it seems like it is something done successfully quite often.

Please educate me!

[eschertron]

This topic made me ask myself a question to which I do not know the complete answer, I'm hoping one of you smarter guys will be able to fill in the blanks.

I do not the complete answer either, so we will both see what that is...

What I can say is that changing the load impedance, by removing one speaker (or swapping cabs, or whatever), will cause the load line on the output tubes to rotate. The primary effect this has is to change the *character* of the sound, as the operation of the tube(s) is now different than it was. For a doubling of the load impedance, the primary Z will also double, causing the load line to flatten. What that will do to the voltage depends on the actual load line and operating points, but it will likely go up a little bit, nowhere near double though.

[Alan0354]

I can tell you the amp does not go down much in loudness when I disconnect one speaker. The main thing that the sound change is more like it is not as "wide" for obvious reason..........you cut out half the area that produce sound!!! I think sound change from increasing impedance is very minor, but you don't lower the loudness much by cutting out one speaker.

On the other hand, replacing one speaker with a resistor do lower the loudness more obvious. Tube amp is not like SS that you truly half the power by doubling the load impedance.