From the National Valve Museum: EF86 @ The National Valve Museum
"The EF86 is the standard early stage audio pre-amplifier valve. This exhibit was made by Mullard who designed the EF86. It was first used in their 5-10 amplifier from 1954.
Its design is low noise and anti-microphonic. In use it is recommended that it is fitted into a vibration resistant base. This has a synthetic rubber mount.
The landmark amplifier designs from Mullard's application laboratories all specified the EF86, and today it is still the valve of choice for this work. Before the advent of the EF86 the Mullard valve of choice for pre-amplifier stages was the EF37A
Mullard describe the EF86 as a low noise pentode intended for use as a RC coupled AF voltage amplifier, particularly in the early stages of high gain audio amplifiers, microphone pre-amplifiers and magnetic tape recorders.
Hum. When used as a normal voltage amplifier with a line voltage of 250 V and an anode load of 100 kΩ and a grid resistor of 470 kΩ the maximum hum level of the valve alone is 5 μV, the average value being about 3 μV when operated with one side of the heater earthed. This can be further reduced by centre tapping the heater to earth. Under these conditions the maximum hum level is 1.5 μV. The low level of hum attained with this valve can be completely masked by that due to an unsuitable valve-holder, in which excessive leakage and capacitive coupling between pins will introduce considerable hum.
Noise. The low-frequency noise generated by a valve is most conveniently specified as an equivalent voltage on the control grid for a specific bandwidth. For the EF86 under normal conditions, I.e. line voltage of 250 V and an anode load of 100 kΩ, the equivalent noise voltage is approximately 2 μV for the frequency range 25-10,000 Hz.
Microphony. Care in the design of the valve to ensure that the electrode structure and its mounting are as rigid as possible has reduced the microphony of the EF86 to a very low level. There are no appreciable internal resonances at frequencies below 1 kHz. At higher frequencies the effect of vibration is usually negligible on account of the damping provided by the chassis and the valve holder. In high gain applications such as tape recording care should be taken in siting the valve, particularly when a loudspeaker is present in the same cabinet or where a motor is mounted on the same chassis. In such cases a flexible mounting for the valve-holder or a separated weighted sub-chassis is advisable."
I would suggest that while care was made in constructing this high-gain valve a little more robustly given the above..that they didn't quite get it right somehow, and needed to rely on a number of other external remedies...location of tube, sub-chassis placement, isolation socket dampers, covers etc. etc. to minimize the microphonic tendencies of the EF86...yet the New issues of this tube haven't really improved upon this IMO...were there more robust versions of this tube ie: built to JAN specs or something along those lines?
I can understand why guitar amps might want to utilize and insanely high gain tube..but why audio designs of the day? Quads, Leaks etc...what is is about the EF86 design that made it special apart from its very high voltage gain potential?
I was hoping to get this in my own dull-witted manner, as this seems to be at the heart of this tube's Achilles heel...maxxing out the tube's gain potential--instead of holding it back a little to make it less susceptible to these problems.
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Some people prefer the 6BR7 - also available with a grid topcap as 6BS7. I think it was a Brimar valve, so may be relatively unknown outside UK? Similar to EF86 but reputed to be quieter. I have no personal experience of either.devilsindetails said:
Interesting...spent a few hrs this afternoon with a friend using the EF86 in a reverb spring return similar to yours, but in a Baldwin organ...he complains bitterly about the EF86's microphonics (his are Hammond marked 60s vintage Blackburn plant Mullards) We substituted several of mine, new and NOS, and the problem either got worse or remained the same.
As per Eli and J Fahey's earlier heads-up I looked at the socket mounting used in this organ chassis and did notice that the socket seems to be sitting on some rubber grommets, but which have become rock hard with heat age...we replaced these with what was on hand as an experiment...added some silicone O rings over the tube envelope as improvised tube dampers and then added a tube shield (the old one had gone missing) and that seemed to tame the microphonics by a substantial proportion.
Hi-Q and many many others of my own personal acquaintance seem to agree, this tube was somewhat problematic in this regard from the get-go particularly when used in confined high-vibration locations such as tape recorders and guitar amps.
But as Eli and those enthusiasts enamoured of this tube are wont to point out, under the right conditions this tube is just fine...
Now...please remember I am just "speculatin' on a hypothesis" here: does the ambient temperature of the glass envelope contribute to the EF86s propensity for microphony? Or for that matter all tubes?
I have no way to prove this with thermal imaging or elaborate heat detecting sensors applied to the envelope of the tube but, in the case of my friend's Baldwin reverb return: as the hours of operation of the tube increased so did its apparent sensitivity to microphony. ie: after two hours of running at full load the microphony problem seemed to hit its apex So if we were to keep the tube envelope cooler or allow it to dissipate its heat a little more quickly and more uniformly would it's microphony also too not level out to a less acute level?
The reason I ask is the owner of the Doctor Z guitar amp I first mentioned also mentioned this to me when first complaining about the microphonics of the EF 86. As a few have pointed out the problem doesn't seem to be as acute in audio applications where the tube has a lot of room to breathe as opposed to the constricted confines of a guitar amp or old tape recorder.
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Wow..DF96..I would have thought u had tripped over the EF86 on your side of the pond a lot more than we colonials😀
Anyone else have any experience with the 6BR7 as an alternative?
Pick a horse for the course. Squeezing 40 dB. of gain out of a resistively loaded bottle is far from "chump change". I fault the designer of the "Doctor Z". That person was too cheap, too poorly trained, or both of the above, to do things "right".
To my mind, microphonics needs to be designed out using as much physics as possible - which can be just as enjoyable a design process as the circuit design for that part, and should not be left as a poor 'design' cousin when the application needs attention.
Bandying about with one physical 'solution' or another, without appreciated how the vibration is getting to the tube, and how to dampen the tube's vibration itself, is somewhat hit-and-miss for a noticeable improvement, just like choosing one valve over another. 50 years ago they were observing 20dB variation between EF86 tubes from the same batch.
Valve Microphonics
I agree with the observation on glass temperature - any change in physical tolerances (eg. between the two mica plates and glass) will change the rigidity of that coupling.
Bandying about with one physical 'solution' or another, without appreciated how the vibration is getting to the tube, and how to dampen the tube's vibration itself, is somewhat hit-and-miss for a noticeable improvement, just like choosing one valve over another. 50 years ago they were observing 20dB variation between EF86 tubes from the same batch.
Valve Microphonics
I agree with the observation on glass temperature - any change in physical tolerances (eg. between the two mica plates and glass) will change the rigidity of that coupling.
That was part of my question about the EF86...the variation observed in the 60s should have promoted tube manufacturers like Mullard, Philips, TFK et al. to improve the construction and the uniformity of performance of this once ubiquitous AF voltage-gain tube. But it seems they didn't and nor have the current manufacturers based on my comparative observations.
trobbins;3454637 [url=http://www.thevalvepage.com/valvetek/microph/microph.htm said:
Thks for that link...a very interesting read and more proof, IMO, that Mullard like Philips and probably others realized the engineering of this tube was not optimal.
Well I'm just a tube tinkerer at best, but it would seems to me that a lot of obvious design considerations simply got ignored (adequate ventilation and auxiliary cooling, isolation from external vibration) in guitar, organ and other enclosed tube amplifiers) or known design problems were masked through some questionable and only partially effective band-aid remedies. (Tube rings, rubber mounting grommets for sockets etc.)
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Devilsindetail, I suggest you don't appreciate the effort put in to making tubes such as the EF86 low in microphony levels, and that comments about 'improving construction' and 'not optimal' are essentially naïve.
Certainly point a finger at manufacturers of end-user equipment if the result was only ever going to work with a '-20dB' end of the spectrum tube. But remember that we are all happy to tweak circuitry and grounding and powering based on new parts and new design/performance information - and microphonics seems to fit in to that profile.
Certainly point a finger at manufacturers of end-user equipment if the result was only ever going to work with a '-20dB' end of the spectrum tube. But remember that we are all happy to tweak circuitry and grounding and powering based on new parts and new design/performance information - and microphonics seems to fit in to that profile.
Points well taken trobbins...but the deviation in microphony of this particular tube even among those made in the same production run as observed in the Mullard analysis previously noted, would seem to indicate some kind of quality control issue to me. But u r correct..time for me to deal with those things I have some control over.😉
Yes, I view the issue as how best to get what we can out of what we've got, and what we now know, and that's where the interest and intrigue lies.
Unfortunately its a difficult subject to throw quantitative forum discussions at, as most of the decision making or views are based on a simple "I can hear it" or "I can't hear it" result - no meters, or spectrum analysers, or simulations or waveforms to bandy about.
Unfortunately its a difficult subject to throw quantitative forum discussions at, as most of the decision making or views are based on a simple "I can hear it" or "I can't hear it" result - no meters, or spectrum analysers, or simulations or waveforms to bandy about.
The mechanical construktion of the "modern" high gain tubes includes thinner and tighter as concept.
With that comes both the challenge to get tubes within specs and keep all the fragile parts steady.
Starving the tube is a way to calm things down if you don´t need to squeze the last drop out of it.
Suspended sockets is a quest......if i only had the formula for it 😀.
With that comes both the challenge to get tubes within specs and keep all the fragile parts steady.
Starving the tube is a way to calm things down if you don´t need to squeze the last drop out of it.
Suspended sockets is a quest......if i only had the formula for it 😀.
Link is to one person's attempt at quietening an EF86.
https://www.youtube.com/watch?v=z7yHtoxkdb8
I found it interesting in that it shows the breadth of fixes that are out there. But to me of more importance was that he indicated it was glass vibration that was the dominant cause of microphony, and that would be the take home message by many.
What he missed elaborating on was that his special valve base 'spring' has a certain stiffness, and that the mass of the valve probably needed to be increased in order to lower the natural resonant frequency of the 'spring and mass' to below where the EF86 microphonic frequencies start to make an impact.
Which of the 'solutions' by themselves (spring/mass or glass flexing) would be more significant is not easy to quantify - but certainly the combination is a technique that has been commonly applied by many.
https://www.youtube.com/watch?v=z7yHtoxkdb8
I found it interesting in that it shows the breadth of fixes that are out there. But to me of more importance was that he indicated it was glass vibration that was the dominant cause of microphony, and that would be the take home message by many.
What he missed elaborating on was that his special valve base 'spring' has a certain stiffness, and that the mass of the valve probably needed to be increased in order to lower the natural resonant frequency of the 'spring and mass' to below where the EF86 microphonic frequencies start to make an impact.
Which of the 'solutions' by themselves (spring/mass or glass flexing) would be more significant is not easy to quantify - but certainly the combination is a technique that has been commonly applied by many.
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Well, this comes "straight from the mouth of the horse".
Who can comment better on EF86 than VOX themselves, who made it famous in the Guitar Amplifier world?
This comes from a Vox History page:
The full page can be read at:
All That Gear That Made (and still makes) It All Happen
So in a nutshell: yes, they are and they were microphonic, so much so that they had to be dropped by their most famous user.
Who can comment better on EF86 than VOX themselves, who made it famous in the Guitar Amplifier world?
This comes from a Vox History page:
The full page can be read at:
All That Gear That Made (and still makes) It All Happen
So in a nutshell: yes, they are and they were microphonic, so much so that they had to be dropped by their most famous user.
I haven't read the thread but most small signal pentodes are microphonic when you set them up for high gain.
Doesn't a slight amount of microphony in a guitar amplifier color the sound in a desirable way? Just like that infamous "tube distortion" that makes its special sound.
Well as a simple "tinkerer" I am limited for the most part to observation and experimentation and a tenuous reliance on the objective scientific measurements that exist (although my ability to comprehend and apply that understanding is more than somewhat lacking😱)
The EF86 was a tube I hadn't had too much experience with, but it immediately seemed problematic in the context of re-occurring complaints from others and in my observations in particular applications.
JFahey mentions the Vox AC30 and that's a particular application where I have some experience...which gets back to my point that either as Eli and others have mentioned...the tube's gain was being wrung-out too far or based on Mullard's own analysis, the variation in tubes was far too high...indicating to me, at least, some form of quality control problem.
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I note that we're all happy to throw extra electronic components at a particular valve in order to improve its performance - such as grid-stoppers, or ccs cathode circuitry, or power supply regulators, or extra filter capacitance. So why not throw some extra mechanical components and design effort at an input tube if it is being pushed to amplify with high gains - it's just another design exercise for that application like choosing a certain type of low noise ww resistor, or ensuring suitable electromagnetic screening or separation, or 0V grounding.
With respect to QC efforts - I often note that many people lament that tubes made nowadays are lacking in many technical areas - so one could comment that manufacturing quality has decreased from its heyday peaks in the 50-60's. It isalso worth appreciating that certain valves were made with almost zero microphonics and huge acceleration withstand specs - but cost and mass manufacture would have ruled the day, especially for a problem that had effectively been designed out for most applications. I suggest VOX could have thrown some mechanical effort at their EF86 incarnations to continue using that valve - but again a cost and manufacture assessment and an almost zero difference to the end user resulted in them moving to 12AX7.
With respect to QC efforts - I often note that many people lament that tubes made nowadays are lacking in many technical areas - so one could comment that manufacturing quality has decreased from its heyday peaks in the 50-60's. It isalso worth appreciating that certain valves were made with almost zero microphonics and huge acceleration withstand specs - but cost and mass manufacture would have ruled the day, especially for a problem that had effectively been designed out for most applications. I suggest VOX could have thrown some mechanical effort at their EF86 incarnations to continue using that valve - but again a cost and manufacture assessment and an almost zero difference to the end user resulted in them moving to 12AX7.
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Agree.
Although let's remember that EF86 were'nt manufactured by VOX, they were a standard and respected Audio tube, well established, made by the millions, of which VOX may have bought, ..... how many? ... a Thousand a year? .
They used just one per amplifier, which were expensive, top of the line.
Very much doubt Mullard would have modified production or design for a few guitar amp makers who must have bought less than 1% production.
They were also used , besides Hi Fi, in mic preamps, mixers, tape recorders, etc.
The only case I know of a manufacturer bowing to MI makers was the famous Sylvania 6L6 STR (Special Tube Request) made for Fender, with reinforced, thicker mica spacers, to handle vibration, and meant to be used "upside down".
Although let's remember that EF86 were'nt manufactured by VOX, they were a standard and respected Audio tube, well established, made by the millions, of which VOX may have bought, ..... how many? ... a Thousand a year? .
They used just one per amplifier, which were expensive, top of the line.
Very much doubt Mullard would have modified production or design for a few guitar amp makers who must have bought less than 1% production.
They were also used , besides Hi Fi, in mic preamps, mixers, tape recorders, etc.
The only case I know of a manufacturer bowing to MI makers was the famous Sylvania 6L6 STR (Special Tube Request) made for Fender, with reinforced, thicker mica spacers, to handle vibration, and meant to be used "upside down".
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