Yup -they're twistlocks. Another thing to look at is what color are the caps - if blue - they are probably CDE's - these are a known problem with this era ARC - they probably aren't blue simply because the odds are they would have failed if CDE's - grey caps are 400uF @ 400VDC - I think the sleded caps ARC sells are 450uF's - but don't remember the working voltage? Replacing the caps with the ARC supplied caps will be relatively straightforward - toughest part of the job will be de-soldering the old caps - you'll need to have the wife hang onto the chassis and pull the old caps out as you de-solder the old ones and suck the solder out of the plated thru holes they go into.This is one of those tasks that really does require two people to do and it will probably be handy to have her hold the new caps onto the board when you solder 'em in.The problem with trying to do it by yourself is a tendency to overheat things while trying to get every single atom of solder out of all the holes ( four if I remember right per cap). She would also come in handy when you are biasing the amp - she can adjust the bias pots while you hold your probes into the test points ( do get the MCM probes - well worth $40 especially with 6550's which are taller and wider than 6L6 series tubes -gonna be harder to get at the test points.BTW - in a minor whine about this amp -it bugs me that every other set of bias pots adjusts opposite in rotation - clockwise is increasing bias on one pair of tubes - next pair is counterclockwise for the next pair of tubes.
BTW - if doing it again - I'd probably go with the Russian Tung Sol 6L6GC's instead of 5881's in a 90B. Your power tubes do need to be matched pairs - not necessarily matched quads. My 90B came with semiworn out 6550's in it when i bought it . Lesson here is that with used tube products - never assume what's there is what should be there - a lot of old tube gear has glowing glass plugged into 'em that passed only two tests -1.) No fire 2.) makes sound .
House -shoot me your name and address via e-mail and I'll mail the 7 old caps off to you for the cost of the postage- while the price is right - very questionable given the age and inherent quality of these caps if they are worth bothering with.
Would strongly suggest waiting until you have the manual in front of you before resuming work on this amp.Really - talk to Leonard at ARC.
BTW - if doing it again - I'd probably go with the Russian Tung Sol 6L6GC's instead of 5881's in a 90B. Your power tubes do need to be matched pairs - not necessarily matched quads. My 90B came with semiworn out 6550's in it when i bought it . Lesson here is that with used tube products - never assume what's there is what should be there - a lot of old tube gear has glowing glass plugged into 'em that passed only two tests -1.) No fire 2.) makes sound .
House -shoot me your name and address via e-mail and I'll mail the 7 old caps off to you for the cost of the postage- while the price is right - very questionable given the age and inherent quality of these caps if they are worth bothering with.
Would strongly suggest waiting until you have the manual in front of you before resuming work on this amp.Really - talk to Leonard at ARC.