MattHooper
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So you'd need an idea of the "average' output impedance of the tube amp. I've no idea what magnitude this typically has - this thread suggests in the region 1 to 4 ohms.
Typical output impedance for tube amps
I would like to take the tupe amp output impedance into acount when I design my loudspeakers. can anyone tell me in what range, the typical output impedance of a push-pull and a SET amp is?www.diyaudio.com
So let's take 2ohm.
So you are going to need a 2 ohm resistor. Lets say you are dumping up to 50W into an 8 ohm speaker. That will be about (I = sqrt( P/R)) - about 2.5A
That 2.5A through 2 ohms will be 12.5W (unsurprisingly in the ratiio of the two impedances) . This is a minimum - I'd go for 4 x that to stay well on the safe side, so 50W
So you are going to need a 50W 2 ohm resistor in series with the speaker. All you need to do is solder one of the speaker wires to one end of the resistor, and connect the other end of the resistor to the speaker terminal using a short length of wire. At the amp end or at the speaker doesn't matter. Suitable attention to prevent shorting of bare resistor terminals to nearby kit should be taken.
With the (potential) power you are talking about, you'll probably need a metal clad resistor (example), bolted to a heatsink. Though you could try it without a heatsink first assuming you are listening in the "first watt" range.
Bear in mind these types of resistor are wire wound, so will have some inductance of their own. Ive no idea if this is enough to have an audible impact.
Also bear in mind. The output impedance of an amp will not be a fixed resistance. It will vary with frequency like the impdeance charactersitic of a speaker. Again I have no idea how much - probably much less than a speaker does. But it means a resistor won't provide the exact same effect.
EDIT - looking at your amp measurements above, I think around a 3db variation was seen in the frequency response curve.
If we assume the 8 ohm speaker here varies from say 5 ohm, up to 20ohm, then with a 2 ohm series resistor, you would get voltage variations from 70% (at 5ohms) to 90% at 20 ohms. This would be a 2.2dB variation. If the speaker drops as low as 4 ohms, then the variation would be 2.7dB. Alternatively changing to a 3ohm resistor would result (in the 5 to 20 ohm case) in a variation of about 3dB
Thank you for taking the time to write a clear and detailed description.
Given what is involved, I can see that’s not something I would endeavour to do in my system.
But I’ll be passing this information on to my friend.
Thanks again!!!