[Zachik]

I do like tube roll on my DAC though.

Your DAC was designed for tube-rolling. The PCM56 DAC by @L0rdGwyn is NOT designed to be a tube roller's DAC...
Replacing the tubes in 5-10 years - that annoyance is negligible

 

[Monsterzero]

Your DAC was designed for tube-rolling. The PCM56 DAC by @L0rdGwyn is NOT designed to be a tube roller's DAC...
Replacing the tubes in 5-10 years - that annoyance is negligible

Yup I read that tube rolling isnt intended. FWIW Ive gone thru 5 rectifiers in 2 years as up until last month my DAC was on 14-16 hours daily.

My new house has built in speakers throughout as well as on the front and back porches so my Lampi only gets used a few hours daily now. Using a Modi MB for the whole house system.

 

[L0rdGwyn]

Right, this isn't really a DAC for tube rolling! Pretty much designed for a specific tube. However, they won't last 5-10 years! Unless you are using the DAC very little, but they should last a long time, long enough that it shouldn't be too much of a nuisance.

I've decided I'm going to try one more thing on the output stage before I finalize the chassis - rather than using a low gain, low resistance output tube, can instead use a high-gm pentode strapped as a triode. Gain of the tube will be higher, so must lower the I/V resistor on the transformer secondary to still hit 2Vrms. Linearity of the tube will be higher, so H2 output distortion will go down. The tradeoff here is a higher output impedance, likely on the order of 1-1.2K rather than ~750ohm of the 12B4A. Most importantly will need to see which setup sounds better. Probably won't finish this today, but going to go get the batteries I need for biasing.

 

[Zachik]

Right, this isn't really a DAC for tube rolling! Pretty much designed for a specific tube. However, they won't last 5-10 years! Unless you are using the DAC very little, but they should last a long time, long enough that it shouldn't be too much of a nuisance.

I am curious: how long would you estimate the tubes last?
Also, how do you know when it is time to replace them? Would the DAC stop working (i.e. no sound coming from the RCAs)?

 

[Monsterzero]

Also, how do you know when it is time to replace them?

Not sure if this applies to all DACs and amps for that matter, but I lose dynamics and staging depth on my TRP when a tube needs to be replaced.

 

[Zachik]

Not sure if this applies to all DACs and amps for that matter, but I lose dynamics and staging depth on my TRP when a tube needs to be replaced.

I guess the challenge is: if that deterioration is slow and gradual - there is no "right moment" to replace then...

 

[triod750]

Is the deterioration of tubes frequency linear? Or do you lose more gain in the treble end?

 

[Monsterzero]

Is the deterioration of tubes frequency linear? Or do you lose more gain in the treble end?

again, speaking only about my experiences with my DAC, the gain hasnt really suffered but the bass response def suffers as does the holographic presentation. YMMV depending upon your gear.

 

[bcowen]

When there is no reason to tube-roll...

I've heard of this phenomenon, but thought it was just another urban legend.

 

[Zachik]

I've heard of this phenomenon, but thought it was just another urban legend.

Some tubes, like the 4W one in my freezer, only need to be rolled when they burn out

 

[L0rdGwyn]

HMMMMMM. Hmm.

Well I had to try the triode-strapped *tetrodes in the DAC tonight.

The big reveal! It's the 6E6P-DR.



Very linear when triode-strapped, mu of 33, plate resistance 1.2K.

Wired up another pair of tag boards, ferrite beads on all electrodes to discourage oscillation. Thought I'd do a whole new pair of tag boards in case the change isn't to my liking.



There it is. I/V resistor reduced for the same output voltage, batteries changed from 12V to 1.2V, CCS loads reduced from 30mA to 25mA for my chosen bias point.



So what does this do for us - well the THD came down significantly - with a -6dB undithered 1kHz sine (again), THD is 0.018% compared to 0.07% with the 12B4A.

I haven't measured it yet, but the output impedance will increase from ~750ohm to ~1.2K, but really not an issue.

The high frequency response, however, is more rolled off in this setup. This is likely due to the increased input capacitance of the 6E6P-DR relative to the 12B4A.

20kHz is down -2.7dB relative to midband compared to -0.9dB using the 12B4A.



Subjectively though, I do think the clarity and detail retrieval increased quite nicely. I really don't get the impression the top end is rolled off. Tonally I would say the change shifts the sound of the DAC more toward neutral if I were to call the 12B4A setup warm. In addition, the noise floor is lower and the 6E6P-DR is less microphonic than the 12B4A.

Despite the measurements, I would call it an improvement. I think I will leave it as is for now without making a final decision and listen for a few days. I still have the 12B4A tag boards, so could switch back for another listen, which I will likely do!

 

[Zachik]

Subjectively though, I do think the clarity and detail retrieval increased quite nicely. I really don't get the impression the top end is rolled off. Tonally I would say the change shifts the sound of the DAC more toward neutral if I were to call the 12B4A setup warm. In addition, the noise floor is lower and the 6E6P-DR is less microphonic than the 12B4A.

Despite the measurements, I would call it an improvement. I think I will leave it as is for now without making a final decision and listen for a few days. I still have the 12B4A tag boards, so could switch back for another listen, which I will likely do!

From my experience, often I preferred a worse-measuring piece of gear compared to a better measuring alternative.
I mean isn't that what tubes is all about?? Solid state measures often better, but is not as much fun or as musical.

 

[L0rdGwyn]

From my experience, often I preferred a worse-measuring piece of gear compared to a better measuring alternative.
I mean isn't that what tubes is all about?? Solid state measures often better, but is not as much fun or as musical.

Agreed, and the rolloff isn't so egregious that it is a true issue. I can't hear beyond 16-17kHz anyway the distortion is lower, which is likely contributing to the increased clarity.

I won't make any final call right now, will give it some time, then switch back to 12B4A maybe next weekend and see how I feel about going back.

 

[Zachik]

Agreed, and the rolloff isn't so egregious that it is a true issue.

I think it is usually the distortion, not roll-off, that measures worse but sounds subjectively better.

I can't hear beyond 16-17kHz anyway

If I had to take a guess, I would say my hearing probably rolls-off at 13-14kHz
In 15 years - you'll likely experience the same...

 

[L0rdGwyn]

I think it is usually the distortion, not roll-off, that measures worse but sounds subjectively better.


If I had to take a guess, I would say my hearing probably rolls-off at 13-14kHz
In 15 years - you'll likely experience the same...

Yeah I'm sure it's coming! NOS DACs often have a rolled off top end, but that doesn't stop people from feeling they sound more musical