Nice analysis! Yeah switching tubes makes direct A-B comparisons more difficult. You’ve got a good set-up there.More info for people that are on the fence regarding the 6C8G "mod"...
First, I would like to preface by saying I do NOT have golden ears, and never been musically trained, and so I may not always hear very small nuances or differences.
Since I got a CTH loaner from @CEE TEE (thanks again buddy), I had a unique opportunity to A/B without going through tube rolling cycle (turn amp off... wait a couple minutes... remove tube, and place new tube, turn on amp... wait 1 minute for it to start outputting sound). Even better: I have a high quality A/B/C switch and high quality set of cables for 1/4" <--> RCA. That basically allows me to run both amps in parallel, connected to same A/B/C switch, and single headphone. So, I can switch mid-song between amps while the headphones never leave my ears!
So... I have done some A/B-ing over the weekend, comparing stock 6922 to several 6C8G tubes (mostly RCA and Ken-Rad). To make sure the amp itself not playing any role, I also swapped the 6922 and 6C8G between the 2 amps and repeated the experiments with similar conclusions.
What were my conclusions then? (had to create some cliff hanger for you guys)
For MY (non-golden) ears - in most tracks I could not hear any difference at all. Tried dozens of tracks (various artists, genres, etc), and VERY few had any difference I could notice! That difference was very subtle, in favor of the 6C8G. I think the subtle difference I hear is "richer" or "fuller" sound on 6C8G. Maybe a little bit wider sound stage. Mind you it was not a real blind A/B, so placebo effect could be blamed here (when I favor 6C8G in the few tracks I heard something a little different).
Let me repeat that TO MY EARS, and I forgot to mention I have been using Sennheiser HD600, most of the time I could not tell any difference, and when I could - it was subtle.
Used my Metrum Amethyst DAC during those A/B tests, fed from Coax. Mostly FLAC 44/16 tracks, with a few albums in FLAC 96/24.
Next, as a "control" experiment, I have asked my GF (who unlike me WAS musically trained in her teens and young adult past) to compare the 2 amps. For her, it was a blind test as she knows nothing about amps and tubes...After 10-15 minutes switching back and forth - she announced they sound the same to her 95% of the time. When pressed to choose a "better" one - she chose the stock 6922 tube noting some instruments sometimes sound more natural on it.
The 2 amps were volume matched, I measured 88dB for 1KHz tone (on HD600), and there is zero lag when switching between amps.
Some last notes and comments:
- I have only used 1 headphone (HD600). Anyone here thinks those are not good enough to tell difference between the 2 amps?
- My A/B method may be flawed. I was switching between amps mid-song, on average every 1-2 minutes. Does it make more "scientific sense" to listen to entire song, then switch and listen to same song again?! My assumption was if I do that - my brain would require a much bigger difference to be able to favor an amp or be able to tell me there is a difference at all...
- My lossless audio files and (~$1100) DAC cannot be a "quality bottleneck" IMHO.
My final conclusions (as of today):
- If you're considering this mod, and have golden ears, or can easily hear differences between tubes no matter how subtle they are -> go for it!
- If you enjoy modding and tinkering -> go for it!
- If your headphones are sensitive / very easy to drive -> you may have hum, so think twice...
- If you've compared 2 amps / DAPs / tubes in the past that people said are very different, but you could not hear a difference to save your life -> skip this one
I will do more A/B-ing, use different headphones, and report back with an update.
I am open to suggestions and comments - what should I do differently? which tracks should I try that might have more pronounced difference? Any other suggestions?
I’ve tried two tubes other than the stock Electro Harmonix. One is a Reflektor 6922; I felt like it had a touch more bass, but the presentation seemed “pointy” and didn’t have the same smooth feel of the stick tube. But this is after maybe 30 hours burn-in on the Reflektor so any sound differences may just be due to different usage of the tubes. But overall the extent of change was minimal and they seemed 95% the same.
With the 6C8G though I thought there was a bigger change, in dynamics mainly and the sound feels a bit more live. The tube I have is 2nd hand and looks like it’s had a long life, so I’m presuming it’s well burned-in.
In all, I think Massdrop have done well with the stock tube and any differences I’ve heard have been adjustments rather than full alterations of the sound. I did play around the other night with switching between the 3 tubes, but with the few minutes delay in between, so granted I could have some expectation/placebo effect going on.
I’m going to try and experiment and see if I can keep the 6C8G as the main tube I use. I’ve got a cable and plug on the way I can use to ground the chassis.
