Weird loopback response from a tube amp

[atharva]

Hi,

I've been using a solid state amp with UR22C for loudspeaker measurements. I decided to try to see how a tube amp would work. While doing the loopback calibration I'm seeing a very weird response. I've tried both channels. It appears that there is mains (50Hz) distortion and harmonics. I had noticed this before with the solid state amp but as you can see the solid state amp is clean now but the tube amp looks terrible.

Is this expected from a tube amp or is this an issue with my amp?

Thanks in advance. I'm using the latest REW version 5.20.13

 

[John Mulcahy]

The scale for the loopback result is very unforgiving, but uneven frequency responses, hum and noise are not particularly unusual in tube amps.

 

[atharva]

Thanks John.. If the scale is unforgiving does it mean one can use such an amp for measurement if one had to? I mean if one used a tube amp for listening then I suppose I should use the same amp for measurement.

 

[John Mulcahy]

Can't say I follow the logic of that. The measurement system should be as clean as you can make it, it sets the limits on what you can measure.

 

[atharva]

When you said the scale was unforgiving did you mean that the response wasn't all that bad?

 

[John Mulcahy]

Difficult to tell, but it looks like the low frequency extension is poor for something that might be used for measurement, particularly the right channel. Tube amplifiers also exhibit significant frequency response variation with load due to their high and frequency-dependent output impedance. In general not a good option for measurement.