Driving a load resistor

[rf7]

I've noticed that some tube amps don't like to drive a load resistor with a 1K sine wave, they go into some crazy distortion spikes on my scope and the transformers make noise. Is that normal, or am I doing something wrong? Or is the amp messed up? It works fine with a guitar and a speaker.

[Enzo]

It might help top know what amp you are talking about. Normally a resistive load offers no problems for an amp. When you say it works fine with a guitar and speaker are you scoping it? What does your sine wave do into a speaker? Might look the same. And since guitar signals aer not sine waves, we won't necessarily expect to see the same things anyway.

Guitar amps are anything but flat, and the result of overdrive and distortion is indeed spikes and peaks and stuff. What level signal is going into the amp input? I would limit my test signal to 100mv. That is 1/10 volt.

[drewl]

Running into a load with the amp cranked you can hear the 1k being amplified....1k is an annoyting frequency to me, so I usually dial down to like 600-700 hz

[drewl]

Running into a load with the amp cranked you can hear the 1k being amplified....1k is an annoyting frequency to me, so I usually dial down to like 600-700 hz

[Enzo]

I usually use 100Hz for that same reason. Lot easier to listen to.

[rf7]

It might help top know what amp you are talking about. Normally a resistive load offers no problems for an amp. When you say it works fine with a guitar and speaker are you scoping it? What does your sine wave do into a speaker? Might look the same. And since guitar signals aer not sine waves, we won't necessarily expect to see the same things anyway.

Guitar amps are anything but flat, and the result of overdrive and distortion is indeed spikes and peaks and stuff. What level signal is going into the amp input? I would limit my test signal to 100mv. That is 1/10 volt.

It was an Ampeg V4 head. I was not scoping the guitar, just listening to it, and I did not listen to the sine wave. Maybe my test signal was too hot, as you suggested. I wanted to see how many watts it would put out before clipping occurred. I've got to try it with a smaller test signal.

[bob p]

if you're getting unexpected distortion, one useful thing to do would be to trace the signal path through the amp to see where the distortion is occurring. if you've got a clean sine wave going into the grid on the first preamp tube, and a distorted signal coming out at the plate, that tells you that the distortion is occurring in that stage. as Enzo mentioned, check the voltage amplitude on the input to make sure that you're not overdriving the input.