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Author Topic: Underheating tubes in microphones  (Read 15613 times)

AusTex64

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Underheating tubes in microphones
« on: May 07, 2015, 09:53:19 PM »

I would like to hear other's experiences with underheating tubes in mics. I understand there are only benefits - reduced self noise, longer heater/tube life, and higher impedance presented to the capsule in most cases. I've underheated the Telefunken PCC85 in my CS-1 build from 9V to 7V and it sounds great with very low self noise. I run the EF802 in my U47 build at 5.5V instead of 6V, it sounds great and is the most quiet tube mic I've listened to so far. Quieter in fact than my mid '70's U87. Learned about underheating from the late, great Oliver Archut.

Does anyone know how much more life one can get from the tube by underheating? I know with tungsten light bulbs, underheating the filament by 5% will increase life by 40% (saw this in a GE paper 20 years ago, I'm in the lighting business). Since tubes have tungsten heaters, I don't see why the same metric would not apply.

Do any of the manufacturers underheat tubes in mics like C12, M49, M147, Manley, etc?

Robert Mokry
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soapfoot

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Re: Underheating tubes in microphones
« Reply #1 on: May 08, 2015, 08:53:06 AM »

Underheating the tube will change its transfer characteristics somewhat. Klaus or someone else will correct me if I'm wrong, but I'm not sure it's advisable to underheat a tube unless the mic was designed around the tube in its underheated configuration.

Also, there's a point beyond which underheating is detrimental to tube life, not beneficial. Up to 10% is usually safe for the tube.

Also worth noting-- preserving the life of the filament itself is not the primary concern. The filament is rarely the part to fail when a tube fails.
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #2 on: May 08, 2015, 11:29:26 AM »

There are a series of articles in the old Audio Express magazine on this subject. Most of the subjects involved 6.3 volt heaters. After several tests the authors determined that a slightly reduced 6.1 volts did add significant life to the tube without any audio problems.
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klaus

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Re: Underheating tubes in microphones
« Reply #3 on: May 14, 2015, 12:24:04 PM »

(...)After several tests the authors determined that a slightly reduced 6.1 volts did add significant life to the tube without any audio problems.
Confirmed, from 30+ years of observation.
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Klaus Heyne
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Re: Underheating tubes in microphones
« Reply #4 on: May 15, 2015, 05:03:54 PM »

Is that to say that 6.1 would be optimal but any less is not advisable? As noted earlier, I would imagine this to be circuit dependent beyond a certain point, yes?
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klaus

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Re: Underheating tubes in microphones
« Reply #5 on: May 15, 2015, 07:28:47 PM »

I find a heater voltage measured at the tube of slightly above 6 VDC, but below 6.3VDC ideal in the use as an impedance converter (not amplifier).

But all of this stuff is "seat of the pants": who has the time and smarts to figure out an objective, absolutely identical test setup where you heat not the same, but a similar tube, at many different levels and, enough of them to have a statistically relevant sample size? Not me. So I just go by empirical data.

With that in mind, I have found through the years a lot more busted tubes where the heating voltage was on the high side, and a few busted/noisy ones on the extreme low side, and the least amount of tube failures or noisy ones in that 6.1VDC range.
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Klaus Heyne
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AusTex64

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Re: Underheating tubes in microphones
« Reply #6 on: May 17, 2015, 10:24:15 PM »

Thanks to all for the thoughtful comments. I Googled "vacuum tube under heating" and found this: http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

I thought this part especially relevant to this thread:

Tube life is sharply dependent on temperature, which means that it is dependent on filament or heater operating voltage. Operate the heater/filament too hot, and the tube will give a shortened life. Operate it too cool and life may be shortened (especially in thoriated filaments, which depend on replenishment of thorium by diffusion from within the filament wire).

A few researchers have observed that the lifetime of an oxide-cathode tube can be greatly increased by operating its heater at 20% below the rated voltage. This USUALLY has very little effect on the cathode's electron emission, and might be worth experimenting with if the user wishes to increase the lifetime of a small-signal tube. (Low heater voltage is NOT recommended for power tubes, as the tube may not give the rated power output.)

Operating the heater at a very low voltage has been observed to linearize some tube types-- we have not been able to verify this, so it may be another worthy experiment for an OEM or sophisticated experimenter.
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klaus

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Re: Underheating tubes in microphones
« Reply #7 on: May 17, 2015, 11:16:33 PM »

That advice contradicts my experience and measurements for all microphone tubes but the Telefunken VF14 and UF14 steel tubes.
Underheating the filament of a 6.3VDC EF86 by 20% would mean a heater voltage of barely over 5VDC, and for the AC701 3.2VDC.

There's no chance of any acceptable noise floor for tube mics with these tubes powered that way.

But microphone tubes, like the VF14, whose filament is underheated by 40%, work superbly.
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Klaus Heyne
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AusTex64

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Re: Underheating tubes in microphones
« Reply #8 on: May 18, 2015, 12:58:15 AM »

Interesting, especially the comments about increased noise with under heating. I run the EF802 in my AMI U47 build at 5.05V per Oilver's instructions, and it has lower self noise that my '70's U87. See this schematic, where Oilver suggests this voltage: http://www.tab-funkenwerk.com/id68.html
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #9 on: May 18, 2015, 11:11:47 AM »

Noise has a stew of contributors, low frequency hum and its harmonics, midrange grunge, shot/spot noise plus white noise from resistors, tubes, transistors and random electron movements.

Any comprehensive noise tests/comparisons are best done with a graphical analyzer like Audio Precision that can show a 20 hz~20k hz spectral view of the noise. Wideband S/N measurements will sum all those noise sources together but will not tell which is the main contributor.

If you can see the dominent noise source then you can formulate a plan to reduce it. White noise/hiss is attacked with lower source noise tubes or semiconductors plus resistors. Low frequency hum and harmonics are usually from power supply and or grounding causes.

A few hours with that test gear and a few test subjects would remove all doubts about heater voltages, noises and their relationship. So far I've not seen that done so guessin' is the next option.
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klaus

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Re: Underheating tubes in microphones
« Reply #10 on: May 18, 2015, 01:16:29 PM »

I have not found it much easier using test gear in determining microphone-generated noise than using ears. And I use relative dB comparisons mostly when communicating with clients (they seem to feel better when disaster is clothed in decibels.

For one thing, extracting external noise from the equation, i.e. separating unwanted sound that enters through the capsule, from internal, microphone contributed, noise is harder than just reading numbers off a display. It takes extreme isolation (professionally designed anechoic chambers) to eliminate low-level environmental noise bleed. Disconnecting the capsule, and substituting with a fixed capacitor, as is often recommended to those who don't own a six-figure anechoic chamber, also has its drawbacks: the mechanical-to-electrical gain structure of the two sections is not accurately reproduced with a fixed capacitor: different capsule types and brands have different gain before the impedance converter.

Between the two, the anechoic chamber test is preferable, but also has other flaws: comparing noise between mics and weighing its relative annoyance is hard. A 3dB rise in one frequency range might not be as noticeable or as unpleasant to the ear compared to a 1.5 dB rise in another.

I plead again, as I have done so often, for acoustic, subjective testing and the confidence for the listener that derives from that.

An example: I test a lot of clients' VF14, to determine whether they will be usable "in the field". There are specimens of this tube which have a relatively minor (in dB) rise in tube hiss, but the spectrum of that noise is centered in an annoying range, which would be disturbing to the ear in quiet recording passages. Other VF14 whose noise, again measured in dB, is centered in an octave below the previous example pass the subjective test.

The true test for microphone tube noise is whether the noise interferes with quiet music passages, i.e. low spls. The same subjective approach can be used with determining tube microphonics. I have not found an objective test one could refer to at what level and/or frequency microphonics are unacceptable.
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Klaus Heyne
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #11 on: May 19, 2015, 11:20:53 AM »

Ears are a rather non-linear way to determine small noise differences. Plus they sum all the noises together so seperating low and high frequency noise and their relationship is difficult. If ears were completely reliable, test equipment wouldn't be necessary. That is akin to global signal to noise measurements in the test gear where all the noises are summed to give a broad band reading. Break that down into a spectral image and then you see everything.

When measuring the tube and system noise, bypassing the capsule is needed to remove the acoustic contributions to the measurements. Then the readings are illuminating as nothing gets by an analyzer with a 140 db measurment range. The human ear is limited to about 110 db of differentiation, far less than that test equipment. Anyone that has spent some time with that level of test gear will realize it offers a microscopic view of non-linearites. The human ear will have a very difficult time matching and repeating of the same data without errors or human subjective input.
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klaus

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Re: Underheating tubes in microphones
« Reply #12 on: May 19, 2015, 02:06:21 PM »

If ears are so unreliable as bullshit detectors of noise, offensive frequencies, etc. as you claim, why use them at all? Why not just sit in front of your TEF analyzer and watch Beethoven's Ninth that way?

The world of sound and other realms of sensory perception does not respond so well to its analysis by means of test equipment. It is therefore not analogous to the world of the hard sciences- physics, chemistry, etc.  Take medicine, where the detection and elimination of pathogens, measurable only with sophisticated equipment, can make the difference between life and death. The ultimate destination of all things audio is our hearing, in all its seeming limitation and, from a measuring standpoint, "unlinear" beauty. It is not compliance with an objective standard.

And, if the human ear is limited to a dynamic range of ca. 110dB, as you say, what's the point of measuring another 30dB beyond that? If you can show that that extra lowered noise floor which your machine indicated will serve the our hearing of music, I'd like to understand how.

Yes, ears are indeed very non-linear in sound perception. And, thankfully so. If linearity of audio transmission were the goal, B&K mics would rule every recording venue. But their (truly helpful) role is pretty much limited to the precise detection of sound levels (noise measurements of aircraft, to define local noise ordinances, for example).

Bypassing the capsule's contribution to noise is not logical for the reasons I had stated earlier: capsule gain is a significant part of the overall s/n of a mic, and its contribution to overall mic noise, then, is neglected or distorted by substituting it with a fixed capacitor bypass.


In sum, if the human ear is such a poor judge correctly identifying what sounds 'right' from what sounds 'wrong', or what is an objectionable noise level from an acceptable one, what then is the point of "objectifying" sounds via measuring equipment, if the bottleneck (hearing) cannot be eliminated?

I continue to insist in my work and my approach to microphone repair and refinement that the ultimate end control a microphone must pass must remain the human ear.

But I also agree that if, for example, a certain type of distortion is measured by using analyzing equipment it sometimes can speed up troubleshooting. However, if the tester is not capable, or does not trust his/her hearing, as final quality control, all is lost in the end.
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Klaus Heyne
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #13 on: May 20, 2015, 11:27:00 AM »

Test equipment is not intended to replace hearing for subjective analysis. It is intended to find faults and errors, some below the human hearing abilities. In that realm, it is not only useful, but required for manufacturing a consistant product.

If test equipment like TEF, Audio Precision, Rhodes and Schwartz and Prism were not useful and required, companies like Sennheiser wouldn't use them.

Anyone attempting to refine a design or minimize noise contributions without that level of test equipment will be missing a lot of data. I'll leave that to the readers to determine whether that is the wisest decision. I realize these test rigs are not cheap, many I know would love to have them but the price of admission is very high.
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radardoug

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Re: Underheating tubes in microphones
« Reply #14 on: May 20, 2015, 04:52:42 PM »



When measuring the tube and system noise, bypassing the capsule is needed to remove the acoustic contributions to the measurements. Then the readings are illuminating as nothing gets by an analyzer with a 140 db measurment range. The human ear is limited to about 110 db of differentiation, far less than that test equipment. Anyone that has spent some time with that level of test gear will realize it offers a microscopic view of non-linearites. The human ear will have a very difficult time matching and repeating of the same data without errors or human subjective input.
Where do you get that 110 dB from?
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #15 on: May 20, 2015, 10:20:49 PM »

I did a lot of testing with playback systems and noise/THD measurements. I found most listeners could detect errors/differences down to around -110 db or so. Below that only a few could, the golden ear types. Great listeners can detect subtle differences that do not show up on the analyzer at normal listening levels but way down below the resolution of most digital converters.

Audio analyzers can measure THD via FFT to around -140 db or so. Using a network analyzer, some math and a careful set up THD has been measured at National Semiconductor by the late Bob Pease down to -154 dbu.
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radardoug

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Re: Underheating tubes in microphones
« Reply #16 on: May 21, 2015, 04:56:23 PM »

So what was your acoustic reference zero level?
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boz6906

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Re: Underheating tubes in microphones
« Reply #17 on: May 21, 2015, 06:52:09 PM »

Really good discussion of human hearing from MIT, describes dynamic range ca. 120 db:

"The dynamic range of the auditory system, which is the interval between the softest and loudest sounds that the ear can hear, is more than 120 decibels."

The description of the basil membrane is relevant to our topic, noting that: " ...the cochlea acts as a spectrum analyzer, separating different frequencies of sound from each other.":

"An important property of the basilar membrane is that each region is tuned to a particular frequency. The basal end is tuned to higher frequencies. That means that the point of maximum vibration for higher frequency sounds is at the base. As a high frequency pressure wave enters, it vibrates with a maximum amplitude at a point near the base, and quickly dies out as the wave continues inward. Lower frequency sounds continue on inward until their maximum point of vibration, and quickly die out after that point. Still lower frequency sounds produce maximum vibration at points close to the apex. Therefore, the nerve fibers which are located near the base contain the higher frequency components of the sound, while the fibers located near the apex contain the lower frequencies components of the sound. In this manner, the cochlea acts as a spectrum analyzer, separating different frequencies of sound from each other. It is thought that the outer hair cells contribute to the sharpness of this tuning, with each cell selectively elongating and contracting only in response to its favorite frequency, and not others."

http://web.mit.edu/2.972/www/reports/ear/ear.html

Perhaps a trained ear can differentiate between low freq AC power noise and high freq capsule noise, etc.
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Uwe

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Re: Underheating tubes in microphones
« Reply #18 on: May 21, 2015, 08:46:30 PM »

Let me repeat what I previously stated in another thread:

One accurate measurement is worth a thousand expert opinions!

If I have learned anything during my 50+ year career in electro-acoustics, relying on your listening alone will inevitably lead to inconsistent and highly subjective results. There is very good reason for many of the quality manufacturers having developed early on their own measuring equipment and procedures. Among others in the field of audio devices, these come to my mind, but surely are not the only ones: B&K, Neumann, Sennheiser, Telefunken, Grundig, Panasonic, Goldstar (LG). All made major contributions and advances in equipment, standards and procedures for conclusive measurement of various electro-acoustic devices. Add to this roster specialty test equipment providers like Rode & Schwarz, Audio Precision, Tektronix, H&P, Agilent, Stanford Research, Hameg and others.
Meaningful measurements always correlate to objective listening experiences. Toward this goal most notable manufacturers collaborate in professional trade associations to develop, establish and continuously improve such measurement methods and standards. Why, if listening alone by one or some self proclaimed experts would be sufficient?
Aware of the capabilities of contemporary test equipment, I'll go as far as turning around the opinion expressed on this forum  and postulate that we can measure and quantify everything we hear, but we can not hear everything we are able to measure. However, nobody can measure what someone thinks to be audible. And here we quickly get into the quagmire between objective VS subjective characterization of audio performance and the unwillingness or inability to properly understand test results and correlate data to actual listening experience. The goal of measurements in most cases is to support and improve the ultimate listening experience. The technical data derived from measurements do quantify specified properties of the equipment under investigation and allow objective comparisons. In this capacity they will always complement and support the listening expectation and ultimately the listening experience.
To overcome reliance on listening alone, any advance development and performance improvements in electro-acoustic devices will necessitate parallel improvements in test equipment and methods.

Getting back to the original subject of this thread after my introductory rant, the contribution of different tube filament-, or more correctly, cathode-temperature to self noise can be measured precisely and correlated to audibility with appropriate noise weighting filters as defined in the applicable industry standards.
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klaus

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Re: Underheating tubes in microphones
« Reply #19 on: May 22, 2015, 02:28:27 AM »

[...] the contribution of different tube filament-, or more correctly, cathode-temperature to self noise can be measured precisely and correlated to audibility with appropriate noise weighting filters as defined in the applicable industry standards.
So let's hear from you what your test equipment found in regards to under-heating of filaments?

(Not that I agree with you about the conclusions of your self-described "rant". But it seems pointless- we are both very much convinced of our positions. Still, I have to ask you: what do we make of your former employer's position vis a vis "objective" testing and resulting improvements of microphone models through the years when they reissue a past model that measures, shall we put it politely, "not all that well" but nevertheless is highly sought-after for its sound?)
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Klaus Heyne
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soapfoot

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Re: Underheating tubes in microphones
« Reply #20 on: May 22, 2015, 10:03:19 AM »

I value objective measurements if (and only if) placed in the proper context, and kept in perspective.

I like what H.H. Scott said best--

"If it measures good and sounds bad, -- it is bad. If it sounds good and measures bad, -- you've measured the wrong thing."
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #21 on: May 22, 2015, 11:07:52 AM »

In my experience, those that profess that subjective hearing is superior to objective hardware analysis are usually those that have little or no experience in the use of these advanced tools. Ears come for free but these audio tools are very expensive and out of the reach of most audio workers.

Perhaps the readers would like to know how the moderator has determined that the subjective analysis technique alone is superior than the use of objective analysis tools to suppliment and verify the subjective analysis?
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #22 on: May 22, 2015, 11:19:05 AM »

Yes, ears are indeed very non-linear in sound perception. And, thankfully so. If linearity of audio transmission were the goal, B&K mics would rule every recording venue. But their (truly helpful) role is pretty much limited to the precise detection of sound levels (noise measurements of aircraft, to define local noise ordinances, for example).

There are excellent recordings using B+K mics done over the years. For pop work, George Massenberg's catalog is full of those mics.

For jazz work, Todd Garfinkle's excellent catalog must be heard to be appreciated, a pair of matched B+K mics, my mic preamps and the performer is all you hear.
www.marecordings.com
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boz6906

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Re: Underheating tubes in microphones
« Reply #23 on: May 22, 2015, 12:02:25 PM »

Gentlebeings, I think we are discussing two different aspects of sound recording.

1. The accurate and repeatable pick up and translation to electrical signal of a sonic event (music, etc).

2. The creation of an estheticly pleasing tone or timbre through the combination of a sonic event with a particular transducer whose transfer characteristic imparts a subjectively pleasing 'vibe' to the combined sound.

The first can be measured with an RTA, etc; for the second one needs something more...
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Piedpiper

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Re: Underheating tubes in microphones
« Reply #24 on: May 22, 2015, 12:39:39 PM »

Nicely put! I would add that Todd's work is stellar, in the camp of #1, where emphasis is on getting the instruments in the space to sound right and then capturing it. #2 includes the transducers and electronics in the artistic process as musical instruments themselves and the recordist becomes much more of an artistic collaborator than is afforded by #1, though #1 certainly does not completely preclude them from this role.
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soapfoot

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Re: Underheating tubes in microphones
« Reply #25 on: May 22, 2015, 03:39:14 PM »

Gentlebeings, I think we are discussing two different aspects of sound recording.

1. The accurate and repeatable pick up and translation to electrical signal of a sonic event (music, etc).

2. The creation of an estheticly pleasing tone or timbre through the combination of a sonic event with a particular transducer whose transfer characteristic imparts a subjectively pleasing 'vibe' to the combined sound.

The first can be measured with an RTA, etc; for the second one needs something more...

It could be argued that there are two different types of "fidelity" as well. Fidelity to the physics of the original event, and fidelity to the emotional content of the event. The first can be empirically proved. The second is more mutable, and related to the first, but doesn't always track with it 100%.

For instance-- I've definitely felt, subjectively, that on many vocalists a good U47 allows me to feel more "in the room with the performer" energy even when compared to other mics that can be empirically proved to be more linear, quieter, etc.

I can't explain that. But it's not mere euphony or "coloration" that appeals to me in that case. At least it sure doesn't feel like that. It feels as though there is less of a barrier between myself and performer. Not in a "technicolor" sense, but in a "I can feel emotions transmitted from the original performance and it makes my hair stand up" sense. I feel like I've observed that some mics that measure objectively better can still fall short in this "emotional fidelity."
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AusTex64

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Re: Underheating tubes in microphones
« Reply #26 on: May 23, 2015, 12:37:06 AM »

So let's hear from you what your test equipment found in regards to under-heating of filaments?

I would also like to know what Uwe found when measuring under heating of tube filaments.

As regards measurement vs. listening, obviously both are useful tools in the development and quality control of microphone construction. Seems the issue is "do better freq response, noise, linearity, sensitivity etc. specs always sound better?". Better being very subjective. For me, sometimes yes and sometimes no. Depends on what I'm after when trying to capture the emotion of the song, timbre of the instrument and room, etc. Sometimes a tube mic with a worn out tube and crusty capsule might serve that purpose better (to my ears) than a super flat, low distortion measurement mic. Horses for courses as the Brits say....

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Jim Williams

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Re: Underheating tubes in microphones
« Reply #27 on: May 23, 2015, 10:59:58 AM »

The beauty of recording is that you do have those choices these days, a fully euphonic interpretaion of the event or a "you are there" experience. Both are equally valid.
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Piedpiper

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Re: Underheating tubes in microphones
« Reply #28 on: May 23, 2015, 01:39:49 PM »

It is worth noting that IME, even creating an optimal illusion of a "you are there" experience requires manipulative technique that diverges from what one might assume from an overly simplistic viewpoint, and that is where it heads in the direction of the other approach, deciding what is optimal and how to get there. There is, of course, a continuum between the two approaches.
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gtoledo3

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Re: Underheating tubes in microphones
« Reply #29 on: May 23, 2015, 05:15:13 PM »

At what point of under heating can one typically expect the cathode to cake up?
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #30 on: May 24, 2015, 12:55:35 PM »

It is worth noting that IME, even creating an optimal illusion of a "you are there" experience requires manipulative technique that diverges from what one might assume from an overly simplistic viewpoint, and that is where it heads in the direction of the other approach, deciding what is optimal and how to get there. There is, of course, a continuum between the two approaches.

The MA catalog defies that concept. There are true 3d effects going on without any electronic assistance, it's all timing cues from 2 quality microphones and a preamp. On the "Further Attempts" CD the bass can be heard moving forwards and backwards as the player rocks the instrument back and forth while he plays.

Phase manipulation with electronics is not the same as the complex timing cues provided by a quality stereo mic pair and a high end preamp/converter following it.
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Piedpiper

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Re: Underheating tubes in microphones
« Reply #31 on: May 24, 2015, 09:33:37 PM »

That's not what I meant. I am very familiar with Todd's work, as well as Chesky and Waterlilly and others, and I have done quite a bit of similar purist audiophile live to stereo mic recordings myself. My point was that one must work quite hard at the placement in a way that is not necessarily what you'd think on first blush, as in just putting the mic in what seems like the ideal audience seat in a concert situation.
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klaus

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Re: Underheating tubes in microphones
« Reply #32 on: May 24, 2015, 10:58:00 PM »

The MA catalog defies that concept.
Please explain what "MA" stands for.
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Re: Underheating tubes in microphones
« Reply #33 on: May 26, 2015, 08:02:59 PM »

MA Recordings is the name of Todd Garfinkel's record label that Jim referred to earlier, specializing in audiophile recordings, live to a stereo pair of B&K microphones.
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Jim Williams

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Re: Underheating tubes in microphones
« Reply #34 on: May 27, 2015, 10:33:39 AM »

Please explain what "MA" stands for.

This is from the MA CD "Rediscovered Memories":

MA derives from the Chinese character ma, aida, gen or kan (depending on its reading), and means "space", "between", "interval" and "relationship".
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