Horrible noise coming from amp

[rodrigo_echeverri]

My Mesa 20/20 makes this horrible high-pitch noise when I turn down the volume knob for one of the channels... what can it be?

[AlexL]

Re: Horrible noise coming from amp

Replace your preamp tubes.


- E.

[jgcable]

Re: Horrible noise coming from amp

+1

[rodrigo_echeverri]

Re: Horrible noise coming from amp

What tubes should I use?

[AlexL]

Re: Horrible noise coming from amp

Like, what type? Pull your old ones out and look at them. They are probably 12AX7 tubes. Slight chance they could be something else, but in 99% of all cases, they are 12AX7.

As far as what brand, it all depends on how much you want to spend. Groove Tubes are a popular brand that are reasonably priced.

Note that preamp tubes are easy to replace becaause they do not need to be biased; just pull the old ones out, pop in the new ones, and you are good to go. Power tubes are a different story; after replacing power tubes, the new tubes have to be biased.

But the preamp tubes are the ones that cause the high-pitched squealing you described.


- E.

[Newc]

Re: Horrible noise coming from amp

[ QUOTE ]

Horrible noise coming from amp

[/ QUOTE ]

Learn how to play [img]/images/graemlins/poke.gif[/img] [img]/images/graemlins/laugh.gif[/img]

Just kidding, but I couldn't resist [img]/images/graemlins/laugh.gif[/img]

[DSS3]

Re: Horrible noise coming from amp

Yeah, it sounds like a microphonic preamp tube; replace it with another 12AX7.

[SeventhSon]

Re: Horrible noise coming from amp

[ QUOTE ]
[ QUOTE ]

Horrible noise coming from amp

[/ QUOTE ]

Learn how to play [img]/images/graemlins/poke.gif[/img] [img]/images/graemlins/laugh.gif[/img]

Just kidding, but I couldn't resist [img]/images/graemlins/laugh.gif[/img]

[/ QUOTE ]

Yep, if I didn't see a detailed description in the post, I was going to reply in the same way. [img]/images/graemlins/laugh.gif[/img]

[yabba]

Re: Horrible noise coming from amp

swap out the small tube closest to the output jacks first rodrigo (it's a 12AX7, it's shared as a preamp tube for both channels.) If that doesn't work, then try replacing either the one next to it, if the squealing is on channel B, or the one on the opposite end if the squealing is on Channel A. (those are the phase inverters)



the 20/20 tube layout is:



(starting from the tube farthest from the output jacks)

12AX7 (phase inverter for channel A)
EL84 (power tube for channel A)
EL84 (")
EL84 (power tube for channel B)
EL84 (")
12AX7 (phase inverter for channel B)
12AX7 (shared input stage for both channels) (most likely culprit)