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Hey guys, I was just wondering what the preferred method of loading an amp when you're only testing the preamp, that is you're trouble shooting Amp A so you have the pre-amp out going to the input of Amp B. How do you load Amp A to ensure that the sound you hear is only coming from Amp B?
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If it is a solid state output amp it doesn't matter. An open load will work fine. If it is a tube amp you better have a load on it, usually a big resistor or bank of resistors. If you have the master turned down or the input to the power amp disconnected you can get away with a lower wattage resistive load, but be careful. -
It depends on the amp. Many preamp outs cut the signal to the power amp automatically, so no need to "load" the amp. On others, you can just turn down the master volume.
Usually I dont just test the preamp. I put the amp on a resistive load with one scope trace on the output. If the amp has "preamp out" and "power amp in" jacks, I put a signal into the "pwr amp in" jack and check the power amp to make sure it is up to spec, and set the bias. Then I put a signal in the regular input jack. If I can't get a good clean channel out of the power amp then I know it is not the pwr amp and start checking for signal at each gain stage til I find the culprit. All this happens after the amp has passed a basic inspection for correct tubes, burnt parts or pcbs, previous botched repair or mod work (see that a lot - eBay amps are a specialty of mine, I offer repair work and counseling..)Comment
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Pull the power tubes. Variac it down if the higher voltages are a concern.
Or use a dummy load in the form of a large resistor.Comment
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