Need help....Microphonic Squealing problem...

[Mailman1971]

Just got in a Modded Marshall JCM SLX 900.
Sounds great!!! .......until I turn it up to gig level with the gain past noon.....
Then it gets this 'weird' sounding squeal. Not really a feedback....but it only happens when I turn up the 2nd gain past noon.
If I keep it lower gain it sounds and plays fine but I really want to get the most out of this amp.
Pre-amp tube??
Any help is greatly appreciated!

 

[roadifier]

Probably a microphonic preamp tube

 

[Mailman1971]

Probably a microphonic preamp tube

How can I check it?
Tap it with a pencil eraser when on?
thought I read that someplace or maybe I am just nuts....

 

[glip22]

Probably a microphonic preamp tube

How can I check it?
Tap it with a pencil eraser when on?
thought I read that someplace or maybe I am just nuts....

Take them out Dan. A pencil never helped me. I just start swapping until I find it. It is usually V1 though.

 

[roadifier]

Probably a microphonic preamp tube

How can I check it?
Tap it with a pencil eraser when on?
thought I read that someplace or maybe I am just nuts....

yep thats how ya do it,
one thing weird thing with my triple, is when it is up louder, you hear a hiss that sounds phased when no phaser is on

 

[Mailman1971]

Probably a microphonic preamp tube

How can I check it?
Tap it with a pencil eraser when on?
thought I read that someplace or maybe I am just nuts....

Take them out Dan. A pencil never helped me. I just start swapping until I find it. It is usually V1 though.

so it will be safe to run the amp with one pre-amp pulled out?
just go down the line till I find the culprit?
I will start it out with the V1 though.....
had the same problem with an older 5150 I had a few years back. But I just bought all new pre-amp tubes and it was fine since.

 

[Motorpud]

Probably a microphonic preamp tube

How can I check it?
Tap it with a pencil eraser when on?
thought I read that someplace or maybe I am just nuts....

Take them out Dan. A pencil never helped me. I just start swapping until I find it. It is usually V1 though.

so it will be safe to run the amp with one pre-amp pulled out?
just go down the line till I find the culprit?
I will start it out with the V1 though.....
had the same problem with an older 5150 I had a few years back. But I just bought all new pre-amp tubes and it was fine since.

Just take one that you know is good and put it in V1 and if it doesn't stop just move on down the line swapping them out one at a time with the good one until it stops.

 

[jlbaxe]

Replace them all.....it will help when you re list the amp for sale "FS: JCM900 all new tubes"

 

[steve_k]

Probably a microphonic preamp tube

How can I check it?
Tap it with a pencil eraser when on?
thought I read that someplace or maybe I am just nuts....

Take them out Dan. A pencil never helped me. I just start swapping until I find it. It is usually V1 though.

so it will be safe to run the amp with one pre-amp pulled out?
just go down the line till I find the culprit?
I will start it out with the V1 though.....
had the same problem with an older 5150 I had a few years back. But I just bought all new pre-amp tubes and it was fine since.

it is safe to turn it on with no tube in it....like gary said, just start until you find the culprit. but, you can easily take a new tube and use it for testing in different spots. It will probably be V1 or V2 in this amp.

steve

 

[Ventura]

Serves ya right for not sticking with yer Diezel...

V.