2. amps-preamps
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Loading Tube Equipment for Audition/Comparison

Hello. I'm at one of those "accumulation points" where I have a bunch of equipment that I need to sort through and make some decisions on which to keep. I'd like to listen to amps and preamps back-to-back for the purpose of comparison, and I'd like to keep the equipment warm so I am swapping in a warmed up piece of gear during the comparison. I know some pieces of equipment can be unstable if they are not attached to some sort of load. Here are my general questions:

1) Is this true at all with preamps? I heard that tube preamps are typically stable whether loaded or not. In that case, I can keep both preamps on during the comparison. Just turning them off to swap out cabling.

2) What "side" do amps need to be loaded on - inputs or outputs? If a tube amp is connected to speakers (or some other load), does it alo need loading on its input - or is loading generally just needed for the output.

3) Is there a way to load the output of an amplifier with just a simple resistor? Can one get an 8 ohm resistor that handles high watts to provide the load. If so, what are the approprirate specs and where would one find such a resistor.

Thanks very much for good input. Peter
peter_s

Showing 2 responses by atmasphere

atmasphere's avatar
atmasphere
12,120 posts
 
Your understanding of preamps is correct.

If you have no input on an amplifier, generally you won't need a load on it either. The best way to accomplish that is with a shorting plug on the input so there is no possibility of a signal.

Some amps are unstable with no load so this is only a generalization. If there is doubt, check with the manufacturer or test the amplifier with a DVM set to the AC scale, across the amplifier speaker terminals. If you only see a few milliVolts, you will be just fine.
atmasphere's avatar
atmasphere
12,120 posts
 
Hi Peter, I have met a few amps that were unstable with no input. That's why I recommend a shorting plug, at least until you sort that out.

#2 is correct.

#3, with a shorting plug in place and no speaker load, measure the AC voltage coming out of the amp with a DVM. If only a few millivolts (less than 5), then no worries. If you see more than that, it might be a good idea to load the output of the amp with either a speaker or a resistance.
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