used a 5150 combo for years and since a couple of years I own an Invective 120, great amp! However my bandmate got one second hand a while ago and it doesn't sound that good at all. We did a bunch of tests (swapped all the tubes, used my preamp on his power amp and vice versa) and the issue is with his poweramp it seems, compared to mine his Invective has no balls (I know really scientific term :p); it sounds weak and scratchy and has a lot of noise, almost like a loose earth connection somewhere.
This noise also starts getting worse after it's been on for a couple of minutes, which makes me think it might be a capacitor or some part overheating?
I have both amps at my house now so I'm planning to do some measurements on both to see if I can find a difference. Currently I've only opened up his one, resoldered all the tubeconnections as they are easy to reach and measured the power tubes and some capacitors and resistors. I haven't found anything weird yet.
If someone has any suggestions where to start looking let me know
Greetings from the Netherlands,
Bram