Blowing fuses in PP amp

[vladimirwolfe]

Any suggestions for things to check for on a Quicksilver Silver 60 Mono blowing fuses? They've been running my BH Straight 8's for years with no problems and all of sudden fuse blows on startup.

TIA,

Doug

[kgoss]

It could be an output tube, but I'd ask Mike Sanders about the amp.

[Paul Birkeland]

Pull all the tubes, turn on the amp and see if the fuse blows.  If the fuse doesn't blow, then your power transformer is likely OK.

Next, put the rectifier tube in and try again.  If the fuse blows, you want to have a look at the power supply caps.

If the fuse is still in-tact, add the driver tubes and recheck.  While the driver tubes are in there, if the fuse is holding, check the grid pin of the output tubes (refer to the tube datasheet, you want G1) with your multi-meter to look for an appropriate amount of negative voltage.  If you see something like -25V on one grid and 50V on the other, you have a leaking coupling cap that needs to be replaced.

These are kind of the basic first steps.

-PB

[Doc B.]

Any other odd behavior in the times you ran the amp before the fuse blew? Like increasing hum (filter caps going south), glowing plates (bad coupling caps), crackles, pops (bad tube, dirty sockets)?

[vladimirwolfe]

Paul & Doc,

Thanks for the replies and I apologize for taking so long getting back. Life gets in the way and I do have another system so it wasn't absolutely imperative to get this running right away.

Paul, I tried removing all the tubes (rectification is SS) and the fuse still blows on startup.

Doc, I did have some crackles and pops and I cleaned and "squeezed" the power tube sockets and the problem went away. This was about 2 weeks before the problem.

I did talk with Mike Sanders at Quicksilver and he recommended checking the diode bridge and the main power supply cap...
I'm trying to avoid having to ship these heavy beasts but am nearing the limits of my skills.

 

[Paul Birkeland]

Disconnect the wire that departs the SS bridge to the first filter cap, then retest.