Tube Amps / Music Electronics
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|1/13/2004 12:25 PM|
|drewl||Mesa Boogie Fets?|
Anyone know why/what purpose is served by having a J175 fet on the input to one of the gain stages on a Dual Rectifier?
they don't show up in any of the schems....reason being, is on channels 2&3 (orange & red) the gain pots were scratchy (dc on them) and turning up the gain was varying the plate voltage of the tube!
I removed the fet from the input and problem is gone....so I assume it's some sort of amplitude clamping/clipping device.
Just a little info in case any of you come across a similiar problem.
|1/13/2004 12:45 PM|
I'd think it's a shunt switch to mute that channel when it's not in use.
Does your channel switching work right?
|1/13/2004 4:42 PM|
Yeah, channel switching is fine. I thought the LDR's (or relays would mute or open the channels not being used.
Makes sense though...
|1/13/2004 6:23 PM|
I don't have a schem in front of me as I'm at school, but I believe it's probably a mute switch. Boogie has used the j175 for a lot of low signal switching. It is true that they use LDRs like candy. I'll check later today....... I'm actually surprised here because I've reviewed the Dual Rect on a couple of occasions and never recall seeing any FETs.
|1/13/2004 7:23 PM|
I know, I've worked on quite a few of these and they never seem to match any schematics I've found!
Like it's a Dual Rectifier Solo, but the schem doesn't have the clean (green) channle in it!
If you have a print i'd love a copy please...no rush.
|1/13/2004 11:31 PM|
Here's everything I got:
|1/14/2004 1:55 AM|
Drew, it has been a while, but IIRC there is a few fets, one on the grid of a V2 side and one other in the 2000 duel rectifiers, It had taken an obseen amout of time to find it as the trouble source, I called mesa and asked why, he said, oh just take it out it does not even need to be there. It was being used as a gate in the switching scheme, but did little. Yours maybe the same thing. fwiw
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