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Has anyone measured the effect (if any) of tube rolling?

Dogen

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#1
I’m very much enjoying my new DarkVoice 336se tube headphone amp, and part of the fun is rolling in different 6SN7s I have on hand with my Sennheiser 6xx cans. Perhaps it’s folly, but I do think I detect differences in sound depending on the tube. Ken-Rads tend to have deeper bass, RCAs are darker, Sylvanias as brighter. I’m talking different 6SN7s, not substitutes. So, in Objective Land, has anyone measured the effect of tube rolling? Do measurable difference show up when it’s looked at?
 

SIY

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#2
The effect of different tubes is HIGHLY circuit dependent. For example, in a circuit with lots of feedback, the differences will be minimal, but for a circuit run open loop, the differences can be quite substantial.

There's a lot of data in Morgan Jones's excellent book "Valve Amplifiers" on brand variation in several different tube types, using a standard test circuit. Likewise, Merlin Blencowe has quite a bit of data in his preamp book. I've done some further work on a few other types which I've posted in various places over the years.
 

amirm

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#3
As it happens, I have a Darkvoice in for review and have another set of tubes to test with it.

It is a challenge to test tube differences due to need for warm up and circuit drift.
 

Dogen

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#4
As it happens, I have a Darkvoice in for review and have another set of tubes to test with it.

It is a challenge to test tube differences due to need for warm up and circuit drift.
Looking forward to that! Not expecting great measured performance but I do like the sound of this amp. It’ll be interesting to discover what tubes you have.
 

SIY

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#5
It is a challenge to test tube differences due to need for warm up and circuit drift.
In a decent circuit, that's not as hard as you'd think. New tubes stabilize with 30 minutes or so after their first use, and it takes a while before the deterioration with age is apparent. I had no problem getting comparable results to Morgan's 6SN7 measurements with a similar test setup.
 
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#6
As it happens, I have a Darkvoice in for review and have another set of tubes to test with it.

It is a challenge to test tube differences due to need for warm up and circuit drift.
Looking forward to that review as well :D. There aren't a whole lot of good reviews or measurements of tube amplifiers and I think the Darkvoice is a fairly popular recommendation. Would also be interesting to see how an entry level tube amp compares to higher end options like those from Woo Audio.
 

gvl

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#8
I had one but sold it recently. It is a fun amp until you get tired of tube rolling, iirc there is no negative feedback so with this amp every tube sounds different. As for measurements it would be interesting to see but subjectively comparing to the JDS Labs O2/Atom it will only be of academic interest. It is an OTL circuit, it only plays well with high-impedance headphones.
 
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#9
I am packing up a Dynaco ST-70 with two different rectifier tubes to send in for testing .... I am a tube rolling skeptic for any circuit with NFB but am interested to see if a rectifier tubes makes any difference as claimed by so many. Looks like we are actually going to get some measurements on tube rolling rather than the endless subjective and sighted tests that are all over the internet.
 

amirm

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#10
I have a Cayin HA-300 with a number of different tubes I would drop off for measurements.
That would be great. That is one beast of a headphone amplifier! Let's schedule to get it done in February.
 

SIY

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#11
I am packing up a Dynaco ST-70 with two different rectifier tubes to send in for testing .... I am a tube rolling skeptic for any circuit with NFB but am interested to see if a rectifier tubes makes any difference as claimed by so many. Looks like we are actually going to get some measurements on tube rolling rather than the endless subjective and sighted tests that are all over the internet.
I've built and modified perhaps a hundred of those amps over the years. The major difference in rectifiers for that amp is voltage drop, especially under load. So driven hard, there can indeed be small differences in power output and power supply modulation. The real issue is the electrolytic power supply caps and the bias rectifiers, which inevitably need replacement.

7199s are all over the place in performance, even within the same brand, but with feedback applied, the differences reduce markedly. The idea that "RCA sound smoother" or whatever is nonsense- there's more difference among a lot of (say) RCA 7199s than the average difference between (say) a lot of RCAs and a lot of GEs.

Ditto output tubes- matching is far more important than whether the EL34s are Mullard or RCA or whatever.
 

rebbiputzmaker

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#13
I've built and modified perhaps a hundred of those amps over the years. The major difference in rectifiers for that amp is voltage drop, especially under load. So driven hard, there can indeed be small differences in power output and power supply modulation. The real issue is the electrolytic power supply caps and the bias rectifiers, which inevitably need replacement.

7199s are all over the place in performance, even within the same brand, but with feedback applied, the differences reduce markedly. The idea that "RCA sound smoother" or whatever is nonsense- there's more difference among a lot of (say) RCA 7199s than the average difference between (say) a lot of RCAs and a lot of GEs.

Ditto output tubes- matching is far more important than whether the EL34s are Mullard or RCA or whatever.
ST70 transformers can also vary somewhat depending on vintage and who wound them.
 

GGroch

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#16
I very much enjoy tube rolling on my Little Dot Mk III. It seems to be a cheap way to tweak the sound to my tastes...but I strongly suspect that the differences I hear are due at least as much to expectation bias as to objective differences. It is not that measurable differences do not exist (as in cables), its just that the tube amp forums I visit always have many posts on subjective differences, and no measurements.

As a result there are strongly held beliefs on these boards that may or may not be built in fact.

These include:

1. That stock Chinese tubes invariably perform poorly compared to new or vintage Russian, European, and American tubes. This one is interesting because I have purchased highly rated vintage Soviet era tubes that appear identical to poorly rated modern Chinese tubes, my guess is the tooling was sold. Why should modern Chinese tubes be invariably worse? There seems to be almost universal belief that they are.

2. That tube brands have a house sound. Or (as mentioned earlier) are unit to unit variations greater than differences between brands. If so, then the results of tube rolling is a crap shoot.

3. In general, do vintage tubes outperform modern tubes...and if so, what vintage represents the golden age?

4. Do military grade tubes have any real advantage or disadvantage in audio applications.

5. What is the audible impact of tubes that test higher on a tube tester (as long as both are within spec).

I am sure any testing done here will not answer most or perhaps any of these questions...but if objective measurement was more widely available then a lot of myths could be dispelled.
 
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SIY

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#17
Well, there's lots of objective measurements out there, but it's not like that's made any difference. This is religion, not science.

I particularly like the idea of "brand sound," considering that "brand" correlates to the shape of the printed ink on the box and tube envelope- and nothing else. I posted pix a few years back of some ECC83/12AX7 which all clearly came from the same factory. They were branded Amperex, Radio Shack, Mullard, RSI, and one or two others I don't recall at the moment. Country of origin prints read West Germany, England, and Holland, depending on the brand. And from what I understand, they were actually made somewhere in Eastern Europe.

In any case, differences among tubes can indeed, depending on the circuit, verge into the audible. And clearly, noise variation from tube to tube will be audible in low level circuit positions (e.g., phono or mic preamp input).
 

Dogen

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#18
I very much enjoy tube rolling on my Little Dot Mk III. It seems to be a cheap way to tweak the sound to my tastes...but I strongly suspect that the differences I hear are due at least as much to expectation bias as to objective differences. It is not that measurable differences do not exist (as in cables), its just that the tube amp forums I visit always have many posts on subjective differences, and no measurements.

As a result there are strongly held beliefs on these boards that may or may not be built in fact.

These include:

1. That stock Chinese tubes invariably perform poorly compared to new or vintage Russian, European, and American tubes. This one is interesting because I have purchased highly rated vintage Soviet era tubes that appear identical to poorly rated modern Chinese tubes, my guess is the tooling was sold. Why should modern Chinese tubes be invariably worse? There seems to be almost universal belief that they are.

2. That tube brands have a house sound. Or (as mentioned earlier) are unit to unit variations greater than differences between brands. If so, then the results of tube rolling is a crap shoot.

3. In general, do vintage tubes outperform modern tubes...and if so, what vintage represents the golden age?

4. Due military grade tubes have any real advantage or disadvantage in audio applications.

5. What is the audible impact of tubes that test higher on a tube tester (as long as both are within spec).

I am sure any testing done here will not answer most or perhaps any of these questions...but if objective measurement was more widely available then a lot of myths could be dispelled.
These are excellent observations and questions:

1) the anti-China bias gets all caught up in political and jingoistic attitudes. Some Chinese tubes are excellent, others are not.

2) I don’t see how a brand can consistently have a sound. But these biases operate within me; RCA tubes are going to be warm and fat. It really makes no sense! Nobody was voicing tubes back then...

3) I do believe tubes were manufactured with more quality in the past, when avionics and wartime communication depended on them. But whether that translates to better audio performance is an open question. In general, I think they can be expected to last longer.

4) in my experience, military grade has no consistent impact on sound quality. Perhaps they are more “rugged” but may not mean much more.

5) this is a good question and I’d like to know more. It may indicate likely lifespan...

If one doesn’t lose his head, tube rolling can be a cheap and harmless way to have fun in this hobby. But the BS surrounding the issue is strong.
 
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