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I have a Peavey LTD 400 steel guitar amp. I want to overdrive the tubes in it, but it is a very loud amp. To practice I would like an attenuator between the amp and the speaker. I've seen a lot of DIY attenuators on the net for 50 and 100 W amps, but not for anything like this amp. If there is a cheap solution to my problem, I would love to hear about it. I can solder, and have a degree in math, so I'm not useless, but I'm not up on electronics. Can you help?

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Two power resistors can do this. For example, 8 Ω across the speaker and another 4 Ω in series with both would put only 1/4 of the output power into the speaker while still presenting a 8 Ω load to the amp.

Consider the power requirements of the resistors. In the example above, only 1/4 of the output power will go into the speaker, so 3/4 will heat up the resistors. Half the total power will be dissipated by the 4 Ω resistor, 1/4 in the 8 Ω resistor, and 1/4 in the speaker. If the amp dumps 40 watts, for example, then you need 20 W and 10 W resistors minimum, respsectively. In practise you want to get resistors rated for somewhat more than the maximum power you plan to have them dissipate. Those big ceramic wire wound types are probably what you will end up with.

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  • \$\begingroup\$ I assume you picked your resistors for the load they would present, but impedance matching does not matter at those frequencies, just spreading out the load. \$\endgroup\$ – Kortuk Nov 26 '12 at 3:56
  • \$\begingroup\$ Impedance matching is important for vacuum tube guitar amplifiers. The output transformer usually has several impedance taps so that the output tubes see their desired impedance. Changing impedance will also alter the sound, which is undesirable for a poor man's Power Brake. \$\endgroup\$ – markrages Nov 26 '12 at 7:01
  • \$\begingroup\$ Ah, I see this amp is solid state with presumably near-zero output impedance. Now, consider the impedance looking the other way: Loudspeaker damping is dependent on the driving impedance, and frequency response. See a impedance-vs-frequency plot, such as the one at celestion.com/product/5/cf18vjd. Clearly, changing the driving impedance will change the audio frequency response. \$\endgroup\$ – markrages Nov 26 '12 at 7:07
  • \$\begingroup\$ Olin's L-pad maintains the proper impedance to the amplifier, but a T- or pi-pad is necessary to present the proper impedance to the loudspeaker as well. \$\endgroup\$ – markrages Nov 26 '12 at 7:10
  • \$\begingroup\$ @Kortuk: I tried to keep the load on the amp at 8 Ohms not so much for impedance matching as to keep the amp operating as close as possible to what is was with just the speaker. Too low impedance, and it might not be able to deliver the current and clip. Too high, and it can't dump enough power onto the load. \$\endgroup\$ – Olin Lathrop Nov 26 '12 at 14:15
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I downloaded the PDF. Sorry, I do not see any tubes in the schematic.

Your amplifier is based on bipolar junction transistors.

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    \$\begingroup\$ yup, no tubes there. Overdriving a solid-state amp is generally considered to sound unpleasant, but there's no accounting for taste. \$\endgroup\$ – markrages Nov 26 '12 at 7:11

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