HAMPAMP TUBE AMP SERVICES

No, don't change all of the electrolytics, just give them a check if it still hums after trying new tubes. I was just giving you some areas to check if your hum problem still remains. As far as the bias circuit, changing the the elecrtolytics would not change your sound. The motor-boating problem is another story. If one of your by-pass cap have drifted off, it could possibly be increasing the gain enough in the bass frequencies to cause motor-boating. If you have a meter that will measure capacitance, just check and see if one has drifted way up in value. Parts do drift and that is what makes old marshalls sound sweet so I'm not saying change them all, just look for one that is way off. If you do have to replace one, try getting several that are within %20-%30 of the original value and try a few until you find one that gives you the desired "sweet spot". If you don't find a bad by-pass cap, check the coupling caps on the first couple of stages to make sure they haven't drifted way off, again. if the value is too high it can cause motor-boating. Only change the one(s) that is/are bad.

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'81 Gibson SG jr
'97 Gibson SG Special
'82 Gibson Explorer
'63 Fender Strat
'00 Fender Strat
'76 Ibanez Doubleneck (Gibson copy)
'08 PRS Paul Allender
Kramer (parts box) V
Peavey Vandenburg (parts box)
Marshall 1959
Marshall VS100R
Marshall 1960/A x 2
Line 6 Flextone III
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Gibson GA-5
Danelectro Cadet Model 122