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It is currently 28 Mar 2020, 10:31

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 Post subject: KT120 Issue
PostPosted: 28 Jan 2020, 20:52 
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Joined: 04 Jun 2008, 20:59
Posts: 4097
Location: Arizona, USA
Hi Everyone, as most of you know I use lots of tubes. Just this week I ran into an issue with KT120s that was rather surprising. I had moved my quad of amps to a shelf behind the the main speakers and slightly off to the side of the subs. The sound was off. I heard a sort of harmonic distortion in the approximate 2K range that was not there before. I accidentally bumped one amp and a sound came from the speaker on that side. I then tapped the tubes. Sure enough all 8 of the KT120s were microphonic. Some badly so. The sound I heard was them "singing" when the music got loud. All are early production ones and have many hours. I don't know how wide spread the issue is ...but if you are using some and get some strange sonic effects you might check it out. Putting the amps where they could not pick up any vibrations stopped the problem. Newer production ones may not do this.

Good listening
Bruce

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 Post subject: Re: KT120 Issue
PostPosted: 26 Mar 2020, 08:30 
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Joined: 29 Mar 2013, 20:37
Posts: 152
Location: Italy
hi Bruce, i know it has been months but i've had one tube out of 4 (tung-sol kt120) with this issue. luckily the seller was kind enough to replace it, apparently it's not that uncommon by a quick google search


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 Post subject: Re: KT120 Issue
PostPosted: 26 Mar 2020, 09:25 
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Joined: 01 Jun 2013, 09:05
Posts: 901
Hi Bruce,

This has always been an issue to greater or lesser extent in the past.
Hence the reason for some people using different materials on chassis builds. (chassis resonance)

Many of the manufacturers used ideas to try to overcome this. As an example think guitar amp tubes right next to the speaker.

An example of this was Tube technology, they mounted all power tubes and preamp tubes on suspension.
I use wood chassis and mount tube sockets on silicone washers.
It also helps using wood side cheeks between top and bottom chassis with feet isolated.

Years ago sorbothane was the "in thing" and you could even get feet made from the stuff but its a problem being weight dependant and compression.
EF86 from way back used rubber mounted Mcmurdo bases, or circuit boards on suspension in early tube radios.
It was pretty standard for people to put grommets though chassis or tag board and the mounting pin through the grommet.

In this case we are working towards production issues, and so it goes.
But some tubes just get microphonic with age so I try to build and allow for some degree of change.

The only problem with isolation materials is aging so you have to be careful about materials.
So they don't fall apart or fail.
Obviously sound deadening can isolate parts of equipment so you need a braided ground strap to bridge the isolation,


Regards
M. Gregg

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 Post subject: Re: KT120 Issue
PostPosted: 26 Mar 2020, 21:27 
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Joined: 23 Feb 2017, 02:02
Posts: 614
Sometimes if you have 4 tubes and push pull or parallel operated.
Swapping one from left to right channel may solve the noise or microphony issue.


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