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Old 14th April 2013, 06:14 AM   #1
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Default Are EF86 Tubes inherently microphonic?

I have been experimenting with some SET Audio Amps and a couple of guitar amps that use EF86s and noticed that these tubes all seem to be inherently microphonic.

I have rolled NOS Phillips, Telefunken, Mullard EF86/6267 as well as new issue JJs, Mesa, Sovtek and others and they all seem to be microphonic to some degree...a problem that doesn't seem too acute using my little audio SET Amps but a real pain in the #** in applications like guitar amps that rely on the EF86 like the Doctor Z... tube dampers help somewhat, but I'm interested to know if there is something about the actual physical construction of this tube that makes this tube more susceptible to microphony?

Thks Leo
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Old 14th April 2013, 07:21 AM   #2
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Aren't all tubes microphonic to some degree?

For what it's worth, the 4 April 1956 Philips EF86 datasheet suggests that Philips did pay attention to microphony when designing the EF86, as you would expect for an audio preamplifier tube:

"A sensitivity of 0,5 mV for an output of 50 mW (resp. 5 mV for 5 W output) 1s permissible in those equipments where an output of 50 mW in the loudspeaker does not produce an average acceleration on the tube greater than 0,015 g at any frequency higher than 500 c/s and greater than 0,06 g at any frequency lower than 500 c/s"
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Old 14th April 2013, 08:05 AM   #3
Merlinb is offline Merlinb  United Kingdom
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All components are inherently microphonic. But pentodes are particularly bad because they have more spindly grids to wobble about. The EF86 has an unfairly bad reputation because people try to wring too much gain out of it in low-level preamps. It's simply not a good choice for a guitar amp front end; much more suited to lower-gain or less 'abusive' conditions!
https://www.youtube.com/watch?v=z7yHtoxkdb8
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Old 14th April 2013, 08:17 AM   #4
M Gregg is offline M Gregg  United Kingdom
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Are EF86 Tubes inherently microphonic?

Only when connected to electricity..
It was one of the design parameters..some are even closer to spec and rattle while in use..

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M. Gregg
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Old 14th April 2013, 09:06 AM   #5
JMFahey is offline JMFahey  Argentina
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Judging by the experience of those who use them, yes, they are microphonic.
Forget them in high gain monsters, as Merlin states, but Service Techs repairing (usually old, British) regular gain amps, have big problems finding usable replacements.
We are talking amps which worked properly with them since the late 50's, and obviously nobody complained way back then.
Unfortunately going NOS is not the solution, because all the "good ones" have already been used, by definition what remains is the rejects.
Damper rings help, but more important is to shock mount the sockets; making them "float" on a couple rubber grommets.
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Old 14th April 2013, 10:10 AM   #6
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As Mr. Fahey said, you adapt to the lie of the land, as you find it.

Don't use a ceramic socket. Mica loaded phenolic conducts fewer vibrations.

Perhaps a damper more capable than silicone rubber "O" rings will help. Herbie's Halos have gotten "good press".

There are 2 good EF86 variants currently being produced by New Sensor: the EH EF86 and the "TungSol" EF806SG. The 50% less costly EH would be my choice, in a guitar amp.
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Old 14th April 2013, 05:48 PM   #7
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Quote:
Originally Posted by MarcelvdG View Post
Aren't all tubes microphonic to some degree?
Ok. Let's say they are to differing degrees. But it seems that the EF86 was particularly prone to this judging by anecdotal evidence of others and my experiments with the New Stock I've used to compare the problem to the NOS and used EF86s I have on hand.

Quote:
Originally Posted by MarcelvdG View Post
For what it's worth, the 4 April 1956 Philips EF86 datasheet suggests that Philips did pay attention to microphony when designing the EF86, as you would expect for an audio preamplifier tube:

"A sensitivity of 0,5 mV for an output of 50 mW (resp. 5 mV for 5 W output) 1s permissible in those equipments where an output of 50 mW in the loudspeaker does not produce an average acceleration on the tube greater than 0,015 g at any frequency higher than 500 c/s and greater than 0,06 g at any frequency lower than 500 c/s"
Perhaps that's where the real problem exists, when the tube is being used to wring too much gain out of a circuit...ie: in guitar amps like the Doctor Z...

But it's interesting that even the New Stock will ring loudly..as loudly as some of my 50 yr old examples...which tells me...at least...that the tube structure seems more susceptible to this symptom than many other tubes...
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Old 14th April 2013, 05:51 PM   #8
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Quote:
Originally Posted by Merlinb View Post
All components are inherently microphonic. But pentodes are particularly bad because they have more spindly grids to wobble about. The EF86 has an unfairly bad reputation because people try to wring too much gain out of it in low-level preamps. It's simply not a good choice for a guitar amp front end; much more suited to lower-gain or less 'abusive' conditions!
https://www.youtube.com/watch?v=z7yHtoxkdb8
Which begs the question: Why do boutique amps like the Doctor Z and others seem to insist on using the EF86? Seems like a guitar amp environment with high SPL would only exacerbate its highly microphonic tendencies.
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Old 14th April 2013, 06:05 PM   #9
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Quote:
Why do boutique amps like the Doctor Z and others seem to insist on using the EF86?
Perhaps it's because you can get a lot of gain out of a single stage, especially if you ignore HF bandwidth considerations.
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Old 14th April 2013, 06:33 PM   #10
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Quote:
Originally Posted by M Gregg View Post
Are EF86 Tubes inherently microphonic?

Only when connected to electricity..
It was one of the design parameters..some are even closer to spec and rattle while in use..

Regards
M. Gregg
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