Speaker attenuation question

[Silvertone Jockey]

I have on hand a bunch of high wattage wire wound resistors .85 ohms each, and 175 watt dissapation. They are wound with relatively few turns compared to other wire wound resistors and the wire is zig zaged (see picture)

So can I make a load box out of these or will the inductive reactance be a problem for the amp? My idea is to series enough of them to achieve a series parallel arrangement where the speaker receives only 1/4 to 1/3 of the total output wattage, so I can practice with the amp cranked into the sweet spot. That is if I won't kill my amp. I know how to calculate the resistance, it's the unknown (to me) effects of the inductive reactance on my tubes that I need to learn about. Thanks in advance for the help

[R.G.]

So can I make a load box out of these or will the inductive reactance be a problem for the amp?... That is if I won't kill my amp. I know how to calculate the resistance, it's the unknown (to me) effects of the inductive reactance on my tubes that I need to learn about.

The inductane of these will likely not be a problem. After all, *speaker coils* are coils of wire, too. Don't sweat it.

However, whether this will kill your amp depends on other things, like how long your amp would live if it was run full tilt for long periods of time into even real speakers. But that's another story.

[olddawg]

I have on hand a bunch of high wattage wire wound resistors .85 ohms each, and 175 watt dissapation. They are wound with relatively few turns compared to other wire wound resistors and the wire is zig zaged (see picture)

So can I make a load box out of these or will the inductive reactance be a problem for the amp? My idea is to series enough of them to achieve a series parallel arrangement where the speaker receives only 1/4 to 1/3 of the total output wattage, so I can practice with the amp cranked into the sweet spot. That is if I won't kill my amp. I know how to calculate the resistance, it's the unknown (to me) effects of the inductive reactance on my tubes that I need to learn about. Thanks in advance for the help



[ATTACH=CONFIG]30185[/ATTACH]

I doubt it will make a good attenuator, but it seems like 10 of them in series like would make a decent bench load bank. You could even tap it for a for ohm load.

[Alan0354]

For use as a load, I think it's ok. But if you try to wire series/parallel for attenuation, it is not going to sound that good. I did experiments with this. I use an adjustable resistor as a rheostat to attenuate the signal into the speaker, it did not sound good. So far, I found if I use a 10 ohm resistor and substitute one of the 8 ohm speaker in my Pro Reverb, I attenuate the output by about half. The sound was very true, just a little less bass. That can be compensated by turning the bass knob up a little. I even try different value, if I use 4 to 6 ohm, the bass was cut too much.

So far, this is the only way I attenuate about 3dB and the sound is very true to the amp, even better than my THD Hotplate at -4dB.

[Silvertone Jockey]

Thanks for the replies and info

[loudthud]

In a guitar amplifier (especially a tube amp) there is a complex interaction between the power amplifier and the speaker. When a simple voltage divider is used as an attenuator between amp and speaker, this interaction is nullified to a large degree. The amplifier sees a near constant impedance because of the resistive load and the speaker sees a near constant low impedance driving it. The result is a change in tone.

Many things have been tried to restore the interaction. None are perfect. The hardest thing emulate is the non-linear output impedance of the power amp at clipping. Before the output clips the source impedance seen by the speaker is (usually) fairly low, a function of the output tubes and the feedback circuit. When the amp clips the feedback becomes ineffective and the output looks more like a current source or at the extremes an open circuit except for the parasitic elements of the output transformer.

One thing that helps bring back the tone is simply a resistor in series with the speaker so that the driving impedance looks more like a current source. Something on the order of 10 to 50 ohms between the resistive voltage divider and the speaker helps bring back the tone. It's far from perfect, but a step in the right direction.

[Alan0354]

That's a very good point that when the power amp clips, the NFB loop open, the output impedance of the power amp goes up and become kind of a non ideal current source.

Even if you put a resistor in series, it's still questionable whether it will sound the same. The impedance of the speaker is not constant, so the voltage across the speaker is frequency dependent. That alone will distort the sound. Not to mention to high impedance a load might damage the tube power amp.