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Mesa Boogie Fets?


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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
Don Symes

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
drewl
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
jeff

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.  
 
Jeff
 
1/13/2004 7:23 PM
drewl
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.  
thanks  
al.
 
1/13/2004 11:31 PM
jeff

Here's everything I got:  
 
http://s88926151.onlinehome.us/dualrect.rar  
 
Jeff
 
1/14/2004 1:55 AM
Dan McCue

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  
 
DM
 

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