A little over a week ago, Fuzzy Beard brought his Bassman 20 over for the amateur RTA treatment and here's what we found. As I stated the last time I did this with my 6G6-B Bassman, please feel free to comment, critique, offer interpretations etc. These are my amateur untrained observations. (Also, please ignore the 60 Hz spike of my mains) The Bassman 20 has just four controls - Treble, Mid, Bass and Volume. We started out by profiling the individual controls to get an idea of the frequency range that each one affected. The following graphs were captured with a speaker-level direct box sitting between the amp and cabinet and feeding into TrueRTA. We discovered in this process that setting any of the tone controls to 1 effectively disables that control with the response so far down in the noise as to be unnoticeable. This first graph combines the Treble and Mid control measurements. Settings are as follows: Orange line - Treble 1, Mid 5, Bass 1 Blue line - Treble 1, Mid 10, Bass 1 Purple line - Treble 5, Mid 1, Bass 1 Yellow line - Treble 10, Mid 1, Bass 1 As you can see, there is considerable overlap of the Mid and Treble controls. The Mid set to 10 brings the high end up almost to the same level as when the Treble is set to 5. I was also struck by how identical the curves were as the controls were increased. With the exception of the Treble curve flattening out above 1.5kHz when the dial was set to 10, the curves remain fairly uniform and seem to raise all frequencies in the spectrum about the same amount. I also noticed the roughness of the bass frequencies when Treble was set to 10. Having looked at Mid and Treble, I was surprised at what we found when profiling the Bass control. Settings are as follows: Green line - Treble 1, Mid 1, Bass 5 Purple line - Treble 1, Mid 1, Bass 10 The Bass control only affects frequencies up to around 200 Hz. For our last control profile, we measured the Volume control. This was especially puzzling to me since, above a setting of 2, the Volume only seemed to affect the bass frequencies. In this graph, the controls are set as follows: Treble 1, Mid 6, Bass 4 for all measurements. Purple line - Volume 2 Each line above Purple is the Volume raised in increments of 1. I'm not sure how to explain this result. For our next measurement, we raised all the tone controls together in increments of 1 starting with all tone controls set to 2. The Purple line is Treble 2, Mid 2, Bass 2. The yellow line is Treble 3, Mid 3, Bass 3 etc. You can see a fairly consistent curve with the high end flattening out above 1.5kHz at settings of 9 and 10. Finally we measured the speaker response against the amp output curve. In this graph, the Green line is Fuzzy's preferred tone setting of Treble 1, Mid 6, Bass 4 and a volume setting of 4. The Purple line is the speaker response, smoothed by 1/6th octave, measured with a calibrated measurement mic at 36", with the amp on a stand in the middle of the room. The room is untreated and there are artifacts galore but you get the general idea. I was struck by how evenly the speaker response rose with the amp. As you can see, the speaker was done above 3kHz. This is a very cool little amp. We noticed the same bump around 100 Hz or so that I measured in my 6G6-B Bassman but it was more pronounced in the Bassman 20. After that, the similarities end. The amp sounds fantastic but I'm not sure I fully grasped the interaction of the tone controls. Even after all this measurement, I don't think I could dial in a specific curve using the numbers without a lot of trial and error. Also, the amp is working against a couple of disadvantages. First, at 18 watts, it's not very loud. Second, it runs a 15' speaker in a VERY small enclosure. A better speaker in an enclosure of suitable dimensions would make this an entirely different amp. I neglected to do an SPL measurement and I'm still trying to figure out if the Volume control measurement had a mistake in it somewhere but I had a blast working with this amp.