Make normal channel same as treble 1987x

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artiewylde

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I was wondering if I could or if anyone has ever modded a 1987x to have both channels the same so they could be A/B 'd for volume change ??? If so, besides adding a bright cap to the normal pot, what would I need to do?
 
Well...if you have the 1987x reissue, its actually pretty simple, given that the circuit in the reissue is a bit of a mash up of lead spec values.
Both V1 coupling caps in the reissue are .022uf, and both V1 cathode resistors are 820ohm, so all you would need to do is replace the large electrolytic cathode cap (C2) with a .680uf film cap, and add a 470pf cap over the 470k mix resistor ( R10) in addition to the 4700pf bright cap you already mentioned adding to the volume pot.
If it were a normal lead spec 50 watter, you'd have to do a bit more, but in this case, Marshall's lack of historical accuracy helps you out a bit.
 
Thanks for the info, it is a reissue. I'm going to give it a try.
 
artiewylde said:
I was wondering if I could or if anyone has ever modded a 1987x to have both channels the same so they could be A/B 'd for volume change ??? If so, besides adding a bright cap to the normal pot, what would I need to do?
Click to expand...

It could work if you don't run the individual channels into overdrive. Once you start overdriving either channel you don't hear much, if any, change in perceived "loudness". You just get more OD and compression.
 
It works but like you said
Dave M said:
It could work if you don't run the individual channels into overdrive. Once you start overdriving either channel you don't hear much, if any, change in perceived "loudness". You just get more OD and compression.
Click to expand...
It works but like you said it just adds more gain with a very small volume increase. Still cool to have the 2 channels. I'm going to mess with different bright caps to give more variety. Thanks for the help.
 
fusedbrain said:
Well...if you have the 1987x reissue, its actually pretty simple, given that the circuit in the reissue is a bit of a mash up of lead spec values.
Both V1 coupling caps in the reissue are .022uf, and both V1 cathode resistors are 820ohm, so all you would need to do is replace the large electrolytic cathode cap (C2) with a .680uf film cap, and add a 470pf cap over the 470k mix resistor ( R10) in addition to the 4700pf bright cap you already mentioned adding to the volume pot.
If it were a normal lead spec 50 watter, you'd have to do a bit more, but in this case, Marshall's lack of historical accuracy helps you out a bit.
Click to expand...
You want to REMOVE the 470pf cap not add another one..
 
If I was doing that I'd make the 2nd volume a little different (either brighter or mellower). If one was for cleaner tones give it a low value bright cap and see if it can be tweaked more to Superbass without messing with the other volume too much. On a '69 build I did many moons ago, I found not using the bypass cap on V2A made the EQ a little more useable, gave me a better range of in-between dirt sounds but when cranked up to 6 or higher it was still an absolute fire breather.
 
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If your 470k mixer resistors both have treble peaker caps, you can lose some isolation between channels through the unused gain pot if it's set low
 
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