Failing Power supply Cap Question

[ncusack]

I've been working on a Laney Protube 50 that had its power transformer primary burnt up and one of the primaries on the OT shorted. Apparently the power at the last gig it was played at was really dirty and when the amp blew the mains fuse the sound man decided to wrap it in tinfoil. In any case the PT and OT have been replaced and I've powered it on for the first time without tubes. No fuse blown but when measuring the B+ nodes Im getting 489V @ OT centertap, 480V @ screen node after the choke, 460V @ PI node and 455V @ following nodes. To me this seems strange as there should be no current without tubes installed so Id expect to see the same voltage all the way along. My question is would this likely be a faulty power supply cap or could something else cause the drop in voltage down the line?

[HotBluePlates]

... there should be no current without tubes installed so Id expect to see the same voltage all the way along. My question is would this likely be a faulty power supply cap or could something else cause the drop in voltage down the line?

Pretty astute!

I'm gonna guess (without knowing the circuit for sure) that you may be right. Try disconnecting the filter cap feeding the input stages; i.e., the one at the lowest voltage. Perhaps that will require unsoldering one side of the dropping resistor leading to that cap, depending on the layout & connections in the amp itself.

Perhaps if you disconnect filter caps, starting with the lowest-voltage one (which would have to have some current draw through its dropping resistor), you can locate which stage in the amp is drawing current. Then you can look to see if there's a legitimate circuit component drawing current (like a bleeder resistor), or if a failing filter cap is leaking current.

[Colas LeGrippa]

Hola amigo !


If the filtering caps doesn't share the same ground, you may have some kind of current developping and thus a lost of voltage . And if there is a  bleeder resistor hooked up to a cap which is separated from the next cap by a 10K resistor or so, there could be a drop.


Colas