Question About a Microphonic Tube

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Question About a Microphonic Tube

Postby vez on Mon Jul 10, 2017 2:50 am

I have a problem with an amp where every tube I try in the V1 position is microphonic. Has anyone seen this before?
Thanks.
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Re: Question About a Microphonic Tube

Postby Clanger on Mon Jul 10, 2017 8:26 am

Which means they are probably all ok.

Is it by chance an EF86? in pentode mode they have a high gain and will pick up quite easily on vibration, whether from chassis vibration or being poked with a stick.
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Re: Question About a Microphonic Tube

Postby pdf64 on Mon Jul 10, 2017 10:41 am

I think that all tubes will be microphonic to some degree, if there's enough gain after them; so a tap with a fingernail on the envelope of the input stage tube will almost always be heard through the speaker.
So super high gain gains may just not accommodate stable operation with every knob at max, whilst there's a speaker in close proximity.
The issue with microphonics is whether it causes a problem in normal operation, eg there's a objectionable 'ring' carrying on after the output of the guitar has been muted.
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Re: Question About a Microphonic Tube

Postby vez on Mon Jul 10, 2017 10:48 am

Clanger wrote:Which means they are probably all ok.

Is it by chance an EF86? in pentode mode they have a high gain and will pick up quite easily on vibration, whether from chassis vibration or being poked with a stick.

No it's a 6SL7. I think it has a gain of 70 or 80. It's not squeezing but you can hear the chopstick when i tap it. stock it comes without a grid stop resistor. I wonder if I it would help if it had one.
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Re: Question About a Microphonic Tube

Postby Clanger on Mon Jul 10, 2017 5:59 pm

Quite an old design of tube and physically large for a twin triode.

As pdf says most tubes are microphonic to some degree, in terms of make a noise when prodded then so am I. If it's not interfering with the amp in operation I would leave be.

Grid stoppers are for radio intereference.
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Re: Question About a Microphonic Tube

Postby vez on Mon Jul 10, 2017 6:48 pm

Clanger wrote:Quite an old design of tube and physically large for a twin triode.

As pdf says most tubes are microphonic to some degree, in terms of make a noise when prodded then so am I. If it's not interfering with the amp in operation I would leave be.

Grid stoppers are for radio intereference.


pdf64 wrote:I think that all tubes will be microphonic to some degree, if there's enough gain after them; so a tap with a fingernail on the envelope of the input stage tube will almost always be heard through the speaker.
So super high gain gains may just not accommodate stable operation with every knob at max, whilst there's a speaker in close proximity.
The issue with microphonics is whether it causes a problem in normal operation, eg there's a objectionable 'ring' carrying on after the output of the guitar has been muted.

Hey thanks guys for replying!
It was a bad solder joint. At first i thought it was the coupling cap because the plate side joint of the cap was particularly loud when prodded so I replaced the cap and its not nearly as loud. But the original cap was an Astron and I thought about it and reinstalled it thinking maybe it was the solder joint. It was.
I really like every amp I have that uses these old 6S*7 tubes. They overdrive nicely.
Thanx!
Rick
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