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Marshall 1987X biasing question

4/13/2000 11:23 AM
Ken McGill	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

4/13/2000 11:23 AM

Ken McGill
email

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