Adrian R
Well-Known Member
A month or so ago I purchased a used 91' 900 MKIII 2100 on c/list. This amp has been quite a project as it was kind of abused, but I got it for a very good price. With a bit of elbow grease and care I got her now looking and sounding new!
Anyway, the amp was desgined for 6L6s as there was the 5881 only sticker on the amp's chassis, and of course the kid who had it before me put JJ E34Ls in it. The sound, although was okay, didn't have that killer warm midrange roar the 900 Marshalls are known for.
Anyway, I ordered a set of JJ 6l6s put em' in, biased them up..and it acted very strangely. Not only did it sound harsh, but it wouldn't completely go on standby!
I thought one of the tubes were bad so I sent the set back and got a new set of JJs. The new tubes did the same thing againagain!
I then sent THAT set back and replaced them with some new EH 6L6s...The EHs did the trick...nice tone, and no standby issues..
I then began to dial in the correct bias setting. I did the simple math, and based on the amp's PV, shot for around 35mA. With the amp at this setting it had good balls..lots of gain...but lots of noise..not to mention too muddy for my liking.
I then deep sixed the bias probe and began backing off on the bias, testing, adjusting again..like 4 times untill I got it right. I also looked at the tubes and made sure they were not glowing to brightly and giving off excess heat. The amp is now totally clear, tack sharp, warm..and VERY balsy.
So..the enlightenment for me here is that even though it is good to do the math and use it as a guide ..its also good to let your ears and eyes be the final judge.. The JJs were great in my other 900s..but this odd 900 didn't like them..weird...
Its just amazing how different an amp can sound just by adjusting it's bias.
This 2100 MIII with the new tubes is now one bad-ass M/F&^%$#!
Anyway, the amp was desgined for 6L6s as there was the 5881 only sticker on the amp's chassis, and of course the kid who had it before me put JJ E34Ls in it. The sound, although was okay, didn't have that killer warm midrange roar the 900 Marshalls are known for.
Anyway, I ordered a set of JJ 6l6s put em' in, biased them up..and it acted very strangely. Not only did it sound harsh, but it wouldn't completely go on standby!
I thought one of the tubes were bad so I sent the set back and got a new set of JJs. The new tubes did the same thing againagain!I then sent THAT set back and replaced them with some new EH 6L6s...The EHs did the trick...nice tone, and no standby issues..
I then began to dial in the correct bias setting. I did the simple math, and based on the amp's PV, shot for around 35mA. With the amp at this setting it had good balls..lots of gain...but lots of noise..not to mention too muddy for my liking.
I then deep sixed the bias probe and began backing off on the bias, testing, adjusting again..like 4 times untill I got it right. I also looked at the tubes and made sure they were not glowing to brightly and giving off excess heat. The amp is now totally clear, tack sharp, warm..and VERY balsy.
So..the enlightenment for me here is that even though it is good to do the math and use it as a guide ..its also good to let your ears and eyes be the final judge.. The JJs were great in my other 900s..but this odd 900 didn't like them..weird...
Its just amazing how different an amp can sound just by adjusting it's bias.
This 2100 MIII with the new tubes is now one bad-ass M/F&^%$#!
