Author Topic: Valve microphonics  (Read 392 times)

Offline Agaton Sax

Valve microphonics
« on: June 09, 2018, 09:22:47 AM »
For those who doubt the significance of  microphony I thought the article by Keith Howard in this months Hi Fi News rather interesting. He refers to this research by Mullard:

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Re: Valve microphonics
« Reply #1 on: June 09, 2018, 09:54:04 AM »
Very interesting, Sax! I used mainly Philips or Mullard valves during the construction of my 5-20 amps when I just built them for a hobby and although Mullard valves were fine, minimal (if any) microphonics, the Philips EF86 was notorious for producing terrible microphonics. A slight tap on the gl;ass case produced a rather hefty ringing thump from the loudspeaker. Brimar EF86 valves were fine, as good as the Mullards but I avoided Philips-manufactured EF86 valves after that.



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Offline Agaton Sax

Re: Valve microphonics
« Reply #2 on: June 09, 2018, 11:20:18 AM »
Normally I stare cross-eyed at Keith Howard's articles but this one on tube microphony in audio amplifiers in the July 2018 Hi Fi News is fascinating.

Offline Ampdog

Re: Valve microphonics
« Reply #3 on: June 12, 2018, 01:32:36 AM »
The article is indeed fascinating as said by Agaton Sax - thanks for showing this here!

I was hoping others would contribute here, noticing what I did. That is some difficulty in connecting what is portrayed with actual audibility, particularly because in another test that I read a few decades ago, no audible effects were noticed while subjecting various valves to real-time sound waves.

So?  What I found here is that in several instances conclusions were drawn regarding a specific electrode but comparing the results from two different valves (the getter test, anode construction detail etc.) It is not explained how the effects found were uniquely related to the element under dicussion and not the rest of the valve(s).

Members' tests revealing microphonics while tapping a valve can be an indication of trouble. But even a light tap directly to the valve envelope can result in acceleration a hundred times more severe than from a passing sound wave. Comparing several valves like that: Maybe, if it is certain that the 'taps' were similar.

I am not rejecting the possible influence of microphonics (everything that has some mass and compliance will  resonate at some frequency), only warning that, as the saying goes, 'apples must be compared with apples'. A home test can be done with one system generating the sound while the system-under-test is monitored, preferably with an oscilloscope (it would be impossible to detect influence on the tested system by ear with the test signal bellowing away in the same room!).       
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

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Re: Valve microphonics
« Reply #4 on: June 12, 2018, 10:07:50 AM »
Thanks for the article Agaton Sax. I use the EF86 every now and then; as an input valve in push-pull amps, and as a driver of single ended amplifiers. As such I really like it, being representative of a whole family of pentodes. I did not find so much microphonics problems. I did notice in the Mullard 3-3 circuit, that there were quite a few EF86's that did not bias properly in the circuit, and the EL84 ran hot. This circuit relies on the EL84 to bias the EF86, and the EF86 is DC connected to the EL84 input. If the EF86 is off spec, the circuit goes ot of kilter. I was surprised how many of my stock EF86 (russian, phillips, mullard and a few telefunken) did not make the grade.

Nevertheless, I will continue to use them in my amps. It is just a matter of tube rolling to find the right match. Not all circuits are as sensitive to EF86 characteristics as the Mullard 3-3

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