[runeight]

Quick update on the 6C8G "mod" and hum:
I am very grateful to @CEE TEE who has sent me a loaner CTH, so I can test to see whether the hum was "by design" (as @runeight suggested it could be) or just my unit...

FWIW, it seems that there may be other factor(s) at work than just the headphone impedance and efficiency...I tried this mod with a pair of LCD-X (impedance around 20 ohms, 103dB/1mw), and I cannot hear any hum at any volume level. YMMV of course.

Good way to put it FLguy.

Just to be clear, Zachik, the CTH is NOT designed to hum and so far as I know it does not hum when used in its factory condition and properly attached to a good source.

I have been (gently) trying to tell you that you are using the amp outside of its designed space and you are sticking in many different kinds of adapters to try to make this mod work.

The issue is not the CTH, it is the mods and the variations among the mods among different people and the surrounding sources and equipment. Some mods work and some introduce conditions that pick up hum.

A better way to go about this would be for those whose CTHs do not hum, if they don't mind, to post the exact configuration of their systems and the exact mods and mod components, and the environment in which the CTH is living. This way you all can at least know what seems to work and what doesn't seem to work.

MHO

 

[Zachik]

FWIW, it seems that there may be other factor(s) at work than just the headphone impedance and efficiency...I tried this mod with a pair of LCD-X (impedance around 20 ohms, 103dB/1mw), and I cannot hear any hum at any volume level. YMMV of course.

Interesting. Forgot to mention I tried 3 different tubes as well. Same behavior.
BTW, as for your "any volume level" comment - the hum is constant and same volume level regardless of amp volume level. It is very clearly there even when no music is playing, and volume is set to zero!

 

[Phantaminum]

Quick update on the 6C8G "mod" and hum:
I am very grateful to @CEE TEE who has sent me a loaner CTH, so I can test to see whether the hum was "by design" (as @runeight suggested it could be) or just my unit...

After testing every permutation between 2 CTH, 2 power adapters, and 2 tube (6C8G -> 6922) adapters - the conclusion is clear (IMHO):
The "6C8G mod" is only meant for high impedance headphones (as noted a few posts ago by @Phantaminum).

So far, I tested with 3 headphones:
1. Campfile Cascade - sensitive, easy to drive and efficient. Un-usable with 6C8G as hum is way too loud.
2. Mr. Speakers AEON Flow Closed (AFC) - less sensitive / less efficient. Hum still there, but substantially lower. Not recommended with 6C8G but could work for some.
3. Sennheiser HD600 - much higher impedance and lower efficiency. This one pairs very nicely with 6C8G.

I have several more cans in my stable - will try some of them with the 6C8G in the next couple days and report back...

Great job putting in some elbow grease and finding out what works and what can be affected. Thanks @Zachik

 

[Zachik]

Just to be clear, Zachik, the CTH is NOT designed to hum and so far as I know it does not hum when used in its factory condition and properly attached to a good source.

Alex - I know it is not designed to hum. In my "by design" comment, I was trying to convey that the amp is working as designed, but with this mod - it could be expected to hum. English is not my native language - might have chosen the wrong words or just step over my own tongue...

I have been (gently) trying to tell you that you are using the amp outside of its designed space and you are sticking in many different kinds of adapters to try to make this mod work.

Tried 2 different sources:
- Massdrop RDAC pre-production unit
- Metrum Amethyst
Connected by very good quality RCA cables (built by Triton).

A better way to go about this would be for those whose CTHs do not hum, if they don't mind, to post the exact configuration of their systems and the exact mods and mod components, and the environment in which the CTH is living. This way you all can at least know what seems to work and what doesn't seem to work.

Good point! @Guidostrunk @Phantaminum @FLguy and anyone else who has tried the 6C8G tubes on CTH - can you PLEASE post exact sounds chain leading to the CTH (plus which headphones you've tried it with)?

 

[FLguy]

... A better way to go about this would be for those whose CTHs do not hum, if they don't mind, to post the exact configuration of their systems and the exact mods and mod components, and the environment in which the CTH is living. This way you all can at least know what seems to work and what doesn't seem to work.

As far as I can tell, I'm using the same components referenced by Phantaminum, with the addition of a second set of tubes (RCA), although I'm now back to Tung Sols. The end result is that I've tried 2 different sets of tubes, and heard no hum with either.

A couple of variables which I will experiment with when time permits:
1) My CTH is plugged into a cheap UPS
2) Of course, my house power (and EMI environment) is likely also not identical to others (not much I can do to test this one)
3) I'm running the LCD-Xs out of the XLR output of the CTH

I'll experiment with items #1 & 3 when time permits, just to see whether those make any difference, and will post back.

 

[Phantaminum]

1. Furman power conditioner
2. PC
3. Schiit USB
4. Schiit Eitr
5. Blue Jeans Coax cable
6. Schiit Gumby
7. Schiit PYST RCA cables
8. MTCH + Tube Monger Socket Saver
9. Chinese 6c8gs adapter
10. 6c8g tubes
11. Senn HD650/Auteur/AFO/E-Mu Teaks

 

[FLguy]

...@Guidostrunk[/USER] @Phantaminum @FLguy and anyone else who has tried the 6C8G tubes on CTH - can you PLEASE post exact sounds chain leading to the CTH (plus which headphones you've tried it with)?

I've mostly used the MCTH in the following system...
1. APC UPS
2. Dell laptop PC (USB output)
3. Matrix S-SPDIF
4. Balanced XLR (Cardas Clear Light)
5. Brooklyn DAC (powered by HDPLEX LPS)
6. RCA cables (Cardas Clear Light)
7. MCTH + Tube Monger Socket Saver
9. Chinese 6c8gs adapter
10. 6c8g tubes (Tung Sol, currently)
8. XLR out to Audeze LCD-X (and a variety of other, higher impedance headphones: HiFiman HE560, Senn HD58X/6XX, Beyer DT880 250 ohm)

I hope this helps...cheers

 

[Guidostrunk]

Android phone(otg)> Spotify (premium), some downloaded flac files, mimby,MCTH, Sennheiser 58x jubilee (150ohm). Zero hum!

Good way to put it FLguy.

Just to be clear, Zachik, the CTH is NOT designed to hum and so far as I know it does not hum when used in its factory condition and properly attached to a good source.

I have been (gently) trying to tell you that you are using the amp outside of its designed space and you are sticking in many different kinds of adapters to try to make this mod work.

The issue is not the CTH, it is the mods and the variations among the mods among different people and the surrounding sources and equipment. Some mods work and some introduce conditions that pick up hum.

A better way to go about this would be for those whose CTHs do not hum, if they don't mind, to post the exact configuration of their systems and the exact mods and mod components, and the environment in which the CTH is living. This way you all can at least know what seems to work and what doesn't seem to work.

MHO

 

[Zachik]

1. Furman power conditioner
2. PC
3. Schiit USB
4. Schiit Eitr
5. Blue Jeans Coax cable
6. Schiit Gumby
7. Schiit PYST RCA cables
8. MTCH + Tube Monger Socket Saver
9. Chinese 6c8gs adapter
10. 6c8g tubes
11. Senn HD650/Auteur/AFO/E-Mu Teaks

Thanks @Guidostrunk and @FLguy for the quick responses!
@Phantaminum - no hum with AFO? It is pretty close to the AFC that I tried with, and it does hum (granted much much less noticeable compared to the Cascade)...

 

[someyoungguy]

Thanks for the updates everyone, looks like this mod prefers a harder to drive headphone. Previously I was using some HD6XXs with the stock electro-harmonixs tube, which were a nice combo, but pity I sold them before getting the 6C8G combo.

I’m more into closed backs, so will proceed with the plan of earthing the chassis etc to see how it goes, but also look around for some harder to drive closed backs that won’t break the bank. Maybe Beyerdynamic DT770s or ZMF classics.

 

[Marlowe]

1. Dell PC USB (Tidal HiFI)
2. AudioQuest Cinnamon USB cable
3. Schiit Eitr
4. Blue Jeans coaxial cable
5. Schiit Modi Multibit
7. Blue Jeans RCA cables
8. CTH (plugged into a power strip connected directly to household outlet)
9. Chinese 6922 to 5670/2C51 adapter
10. Tung-Sol 2C51 tube (NOS)
11. Sennheiser HD 700/Sennheiser HD 6XX

I've used the above for around three weeks with no hum or other issues.

 

[Zachik]

1. Dell PC USB (Tidal HiFI)
2. AudioQuest Cinnamon USB cable
3. Schiit Eitr
4. Blue Jeans coaxial cable
5. Schiit Modi Multibit
7. Blue Jeans RCA cables
8. CTH (plugged into a power strip connected directly to household outlet)
9. Chinese 6922 to 5670/2C51 adapter
10. Tung-Sol 2C51 tube (NOS)
11. Sennheiser HD 700/Sennheiser HD 6XX

I've used the above for around three weeks with no hum or other issues.

Did you compare the 2C51 to stock 6922 ? How do you think they compare?
Just curious - probably won't be investing in yet another adapter and more tubes.....

 

[Guidostrunk]

2c51 is better than the best CCa tube. There's no comparison. My subjective experience. 2c51 led me to 6c8g.

Did you compare the 2C51 to stock 6922 ? How do you think they compare?
Just curious - probably won't be investing in yet another adapter and more tubes.....

 

[Zachik]

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?

 

[Marlowe]

Did you compare the 2C51 to stock 6922 ? How do you think they compare?
Just curious - probably won't be investing in yet another adapter and more tubes.....

2c51 is better than the best CCa tube. There's no comparison. My subjective experience. 2c51 led me to 6c8g.

Well, to begin with, I have only seriously got into this hobby a couple of years ago and don't pretend to be expert in discerning differences or in describing them in standard audio terms. (Also, my ears will be 65 years old in three days.) TBH, I could not say--for me--that there was "no comparison" between the stock 6922 tube and the T-S 2C51. It's not night and day. Depending on the track, I did feel that there was a bit more impact (I know that's a fairly imprecise word here) and slightly more separation of the instruments with the 2C51. As I mentioned in an earlier post, my best example of the tracks I compared is Out on the Weekend from Neil Young's Harvest.The track begins sparely with the drums and bass predominant. It sounds great with the 6922, but with the 2C51 I felt both the bass and drums hit just a little bit harder. Similarly, on Echo & the Bunnymen's Heaven Up Here, a terrific album with great bass playing and drumming, I felt the bass and drums were somewhat more visceral. YMMV of course.

And I already had the adapter and 2C51 tube on hand in my Schiit Valhalla 2, so it it cost me nothing in money or effort. It's not an expensive mod though. (FWIW, a socket saver is not required for this mod as it is for the 6C8G, since there is no cable to connect from the adapter to the tube. FWIW, I'm not using one, though some recommend one even with this mod.) The adapters cost me about $18 for two; you can buy just one for roughly half. The tubes cost me $35 for a pair, though I'm not sure if such a good deal on NOS 2C51s is available ATM. However a bought a few NOS GE 5670s (5670, 2C51, 396A are all the same tube and use the same adapter) for $4 each and they are still available on eBay. I haven't tried them yet in the CTH, but I thought they sounded pretty good in the Valhalla 2, though not quite as good as the Tung-Sol 2C51.