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Marshall 1987X biasing question

4/13/2000 11:23 AM
Ken McGill
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Marshall 1987X biasing question
Greetings all. Having been a lurker I am impressed  
with the knowledge shared here and wanted to get  
some advise.Last evening I took my Marshall reissue  
over to a friend and coworkers house to give my  
recently acquired new amp a bias job since I'd  
heard these come cold when new.I know little about  
doing these things and placed total trust in my  
friends abilities.He seems a pretty sharp guy with  
a good reputaion as an FAE in the electronics industry and he likes to play guitar and fool with  
tube amps. So off I go with the amp ,guitar schematics for only the models 1987 and 1987T  
found in the Histry of Marshall. It doesnt show a  
1987x schem.We open up the amp he looks at the schematics to get familiar with it and we discover  
some inconsistancies with whats in front of us  
compared to the schematics.No big deal.I also brought the specifications for the EL34 that has  
pinouts etc.We end up hooking up the Fluke meter,  
find the trimmer and start adjusting and while  
we get idle readings I play the guitar through it  
listening to the tonal changes.While the amp sounded good as it was it really warmed up when  
turning up the bias. Trying the different ranges we  
settled on a setting that had the Plate voltage at  
480 and the screen voltage at 390.Readings that were  
taken from the 2 500v rated blue caps.This setting  
dimed that trim pot. I ask him what else needs to be measured and he said voltages are with in spec  
of tube and caps so it should be OK at this setting.  
While I love the sound this is putting out I surely  
dont want to fry this thing.Do you think I should  
take it to a professional amp tech or will it be OK  
as is until I get new tubes and rebias. What damage  
or wear and tear is it going through biased this hot.IF recommended I noted the orignal setting of the pot and could reset to original position.Hopefully I can leave it as is.  
 
Thanks for sharing your knowledge  
Ken

 
Replies:
Rebel420 q{Trying the different ranges we -- 4/13/2000 1:58 PM
Bob S II
Hiya Ken,q{So off I... -- 4/14/2000 6:09 AM