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
3 responses Add your response
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.
peter_s OP
1,348 posts
 
Thanks Ralph! A few specific followups:

1) Is it correct to say that if you DO have an amplifier loaded on the outputs, you don't have to worry about the inputs (you can have nothing on the jacks)? I'm also asking because I want to test for the source of hum on a tube amplifier.

2) One can easily make a shorting plug with a cheap RCA patch cord by cutting off the plugs and soldering the two wires together just above each plug, correct?

3) When checking the amplifier with a DVM set on AC voltage, do you do this with the speakers hooked up, so you don't endanger the amp if it is unstable without a load?

Thanks again. Peter
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.
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