2:90 issue

[Ibajem]

Hi people on grailtone!

I got a Mesa Boogie Triaxis connected to a G-major and finally into the 2:90. I bought the 2:90 about a month ago, the guy i bought it from told me it had brand new ruby tubes. The thing is that i am experiencing a lot of hum from the 2:90 lately, the weird thing is that when i turn the presence control on channel B the sound of the hum changes, i currently only use the "A" channel mono into my ENGL 2x12 cab. I also plugged my guitar straight into the 2:90 and the hum was still there so im sure that the 2:90 is causing the problem. When i changed to the "B" channel i got a lot of hiss, and it sounded like my cab would explode.

Anyone got any ideas of what the scource to this problem is?

 

[gummx97]

At one time i was getting a hum on channel A on my 2;90, the previous owner replaced all the power tubes with no help. try opening up the 2;90, you'll see 3 preamp tubes in there (12AX7), try replacing one at a time or all of them at once since you already have it opened. when i replaced those preamp tubes, the hum was gone!

previous owner couldn't believe his 2;90 had preamp tubes, neither did i until i looked up the schematics. was in shock seeing preamp tubes in the schematic, so i just had to open the amp up.

Try it, might work for you. If it doesn't, at least you eliminated that possibility.

 

[bjoneill74]

1 of the 3 12AX7 tubes is an input driver and the other two are phase inverters.