Back with some more measurements.
I now have a proper microphone stand, so I can position the microphone consistently.
I've taken a handful of measurements since my initial posting, and found that the process is very sensitive. I was initially doing these measurements in my office, which is
not acoustically treated and fairly small (about 10 x 11 feet). The only "treatment" is an area rug, but all other surfaces are hard (wood or glass). Most would call this an acoustically poor room.
So in the measurements that follow, I moved the speaker cabinet out into the foyer of my house. This is one of the larger, more open areas of my house, so I'm hoping it reduces the room effects to some degree.
I also realized this REW software has a button to average multiple response measurements. So, I took all measurements three times, and averaged them. The graph that follows has three measurement curves. Each curve represents a different speaker paired with the Triwatt at the same settings. The microphone is about 1 inch from the grille cloth. I tried to center the microphone at the speaker's center (though I eye-balled this, can't claim any precision). Triwatt configuration is as follows:
- JJ 6V6 power tubes
- New production Tung-Sol 12AX7 in v1
- Ruby 12AX7AC7 HG in v2, v3 (supposedly the same as TAD 7025-S, Preferred Series 7025 and Gold Lion Standard pin 12AX7)
- Sovtek 12AX7LPS in v4 (phase inverter)
- All components in the circuit are per the build documentation (recall, I had a cap in the tone stack swapped out in my previous measurements, that cap has been reverted back to stock value)
- Overdrive off (and turned down to 0)
- Master volume at 9:00
- All other controls at 12:00
- Using Link input
The three speakers tested are as follows:
- Reeves Vintage Purple (green line) - this is in the 1x12 Triwatt combo cab that Trinity built for me
- 12" Eminence (blue line) - not sure which model, this is a Blackheart 1x12 cab that I got many years ago to mate with a Blackheart BH5H cab
- WGS Retro 30 (orange line) - this is in a 1x12 combo cab built by Sour Mash; it currently houses that speaker plus a Ceri@tone Dizzy30 amp
Attachment:
File comment: Triwatt: Reeves Vintage Purple (green) versus Eminence (blue) versus WGS Retro30 (orange)
triwatt_purple_eminence_retro30_2048x781_20200708.jpg [ 363.83 KiB | Viewed 671 times ]
I'm curious how big of a difference can be seen in the frequency response of different amps with the same speaker. To my ears, the Dizzy30 and Triwatt sound
very different. Spoiler: their frequency response graphs are very different. This is all for the ef86 channel of the Dizzy30. I used the half-power setting, "high" input, preamp gain at noon, cut at noon, and master around 8:00. The only other control is the tone knob, which is a 6-way switch that changes the coupling capacitor between amp stages (between ef86 and phase inverter IIRC). I took two measurements, one at each extreme of that switch:
Attachment:
File comment: Dizzy30: green: tone knob at left-most position, about 10:00; blue: tone knob at right-most position, about 3:00
dizzy30_retro30_tone_minmax_2048x781_20200708.jpg [ 325.29 KiB | Viewed 671 times ]
What follows is the same graph, but add the response of the Triwatt with the WGS Retro30 speaker (orange line). In other words, the cabinets and speaker are the same, but the amps are different.
Attachment:
File comment: Same as previous, but with Triwatt using Retro30 speaker (orange line)
triwatt_retro30_vs_dizzy30_retro30_tone_minmax_2048x781_20200708.jpg [ 361.37 KiB | Viewed 671 times ]
Last is just a repeat of the Triwatt with the Reeves Vintage Purple speaker in isolation (for better visibility):
Attachment:
File comment: Triwatt with Reeves Vintage Purple
triwatt_purple_2048x781_20200708.jpg [ 266.56 KiB | Viewed 671 times ]
I'm curious what others think of this. My biggest surprise is how ragged it is from 1 kHz and up. My expectation, based purely on Internet hearsay of people calling the Hiwatt "hi fi" and sometimes even "sterile", I would expect a much flatter and even response in the range where the human ear is most sensitive (say 1kHz to 3 or 4 kHz). But this is definitely not a graph of hi fi equipment! Not saying that's a bad thing, not by any means - just saying it's different than what I would have predicted. I suspect it's the ragged frequency response that gives any guitar amp its signature tone.
Eager to hear others weigh in on this!