Power filter cap failures

[mat]

In my build (dumble ODS50 nonHRM) I had nasty crackling sound problem few months ago. It appeared to be failed spraque atom 100uF/350V. I changed both of them to F+T 100uF/450V's. The problem was solved immediatly. Now I think I have a failed the F+T('s) also.

For one thing it could be that other one of the parallel 220k resistors has gone bad. But if I remember right, I changed the resistors also when swapping the new caps in. I will buy tomorrow new pair of F+T's and 220k 3w resistors and I'm pretty sure that this will fix the crackling sound (and very low B+1 voltage) problem. Guitar sound comes trough but with really low volume level and distorted.

So my question is that what are the things that could make the main filter caps to die ?

[Tonegeek]

In my build (dumble ODS50 nonHRM) I had nasty crackling sound problem few months ago. It appeared to be failed spraque atom 100uF/350V. I changed both of them to F+T 100uF/450V's. The problem was solved immediatly. Now I think I have a failed the F+T('s) also.

For one thing it could be that other one of the parallel 220k resistors has gone bad. But if I remember right, I changed the resistors also when swapping the new caps in. I will buy tomorrow new pair of F+T's and 220k 3w resistors and I'm pretty sure that this will fix the crackling sound (and very low B+1 voltage) problem. Guitar sound comes trough but with really low volume level and distorted.

So my question is that what are the things that could make the main filter caps to die ?

I assume the input to the caps is within acceptable range and that your wall voltage is free of spikes. No lightning hits, etc. (hey Ive been hit twice). I also assume you are using the totem pole supply which doubles the voltage handling of the caps.

Are your caps on the same side of the standby switch as the B+1 (OT center tap and choke/sag resistor)? Or are caps charged up when you turn on the amp before you take it off standby? There has been some discussion that having the caps charged up unloaded could cause too high a voltage to appear across them although I can't image it would exceed the rating of the F&T caps you have now (2x450 = 900).
I suppose a shorted tube could take out a cap. Only things I can think of at the moment....

[Luthierwnc]

Tough call. I wouldn't look for two cap failures in the same place so soon. Have you checked to see if you can put any DC through them (you shouldn't be able to)? sh

[mat]

Thanks for the responces, I found the cause !

I have four turrets on the B+1 spot and have tied them together with bare wire and solder. Amazingly while the wire goes around each turret and has solder on them it did not have good connection between them..

I did put the new resistors + caps in and the problem did stay. Then I measured the connectivity between the turrets and there it was. Resoldered every turret and immediatly the voltages and the sound came to normal.

Are your caps on the same side of the standby switch as the B+1 (OT center tap and choke/sag resistor)? Or are caps charged up when you turn on the amp before you take it off standby? There has been some discussion that having the caps charged up unloaded could cause too high a voltage to appear across them although I can't image it would exceed the rating of the F&T caps you have now (2x450 = 900).
I suppose a shorted tube could take out a cap. Only things I can think of at the moment....

The standby switch is before the main filter caps. The powerboard is the old dumble style model with added caps for the built in dlator.

Tough call. I wouldn't look for two cap failures in the same place so soon. Have you checked to see if you can put any DC through them (you shouldn't be able to)? sh

Would it be ok to do the test with a single 9V battery and DMM on parallel with the cap ?

Now it comes to my mind that the sprague atoms I threw away might actually have been ok...... DOH!