New bothersome hum...

[Japhy]

Hi folks,

Just tried swapping out the 47uF and 1000uF PS caps with a different flavour last night.  Fired things up and didn't hear anything unusual the first time around, although I may not have noticed.  This morning I turned the SEX amp on and heard a soft thump in the right channel.  That speaker went quiet right after and then a slow soft buildup of hum.  Can't hear it back where I sit to listen, but ear up to the speaker definitely.  Does not change with volume position, so some type of ground hum?  I resoldered all the locations in the area i was working to no avail.  Any recommendations!?

Cheers!

 

[JC]

Did you replace them with electrolytics?  If so, are you absolutely certain of the polarity?

 

[Japhy]

Both electrolytics, JJ for 47uF and Elna for 1000uF.  Stripped (-) terminal on #12 and #32 for the 47uF and #11 and #31 for the positive terminal.  #12 and #32 for the stripped (-) terminal of the 1000uF cap with the positive sides going to #15 and #35.  ie both stripped terminals of these caps ending on #12 and #32.

 

[JC]

Just to make sure that it isn't something else that has come up coincidentally to the changing of the caps, I think I would want to rule out the usual suspects:

1)  Swap input cables to see if the hum stays in the same channel or follows the cable.

2)  Swap tubes to see if the hum  stays in the same channel or follows the tube.

3)  Make doubly sure that you are happy with the soldering of the new caps.

If the hum stays in the same channel with both swaps, and you are 100% convinced that your new caps are correctly installed, then my advice would be to first get a few hours of operation on the new caps to see if the hum diminishes as they break in.

If the hum persists, then it seems to me that you may have to try changing back to the original caps on that channel.  I would probably do them one at a time, starting with the 47uF.

How ever you proceed, I hope you will continue to post your results.

 

[Japhy]

Well call it sun spot, or solar flares....
I had done everything you mentioned already.  The one thing I did notice that one of my interconnects was wrapped around a power cable by accident.  For the third time I resoldered everything, pulled the tubes and replaces them, etc.  Finally silence!?  Not sure which invariably did the job.  I'm usually pretty careful with soldering so I'd more want to say it was something to do with one of the caps themselves.  Does this ever happen?  I was away for the weekend but fired it up last night and everything sounded just fine.

Thanks for the quick feedback for some root cause possibilities.

Cheers!

 

[JC]

But, I expect the cause will never be known.

I'll stick with the theory that one of the new caps needed some operation time to get its electrolytic dielectric formed up properly, but that is just a very wild guess, of course.

With the hum gone, do you like the new caps?

 

[Japhy]

I want a little more run time into them before I really sit down to have a good listen.  A low levels last night I liked what I heard, but with a little one (14 months) in the house, I have to get strategic about my listening time.  I've been putting off the MQ iron upgrade for a while, but would also like to do this sometime in the fall.  Generally speaking I really love this little amp.  I'm driving a set of Forte I's and it is gives my triode strapped Dynaco Mark III's that I've spent nearly 5 years with a run for their money. 

Will post more as soon as I've had a good listen.

Cheers!