Sunday, September 14, 2008

Mullard Five-Ten - out with the old.

 
Over the last week I've spent evenings removing bad components and replacing with good ones. I found a few old Dubilier 0.1uF paper in oil capacitors that looked sound and measured OK on the test meter, so I've decided to try them. They certainly look the part. I decided it best to buy new electrolytics (except for the large HT filter cap - expensive) - Rifa's from RS look good and have decent specs.

As for the output transformer I've decided to borrow one from one of my Vortexion's. It's far too big (you can see the original in the background of the photo), but it will allow me to try the amp out before spending a lot on a new one, or a rewind of the old one - I wonder how much that might be? These days it always seems cheaper to buy new rather than repair, unless you do the repair yourself. Umm, maybe I need to learn how to wind transformers.

Yesterday I switched it on for the first time, and it worked!  There was a lot of hum and some motor-boating so it was fairly obvious that the 50 year old HT smoothing capacitor wasn't up to the job.  In my junk box I happened to have an LCR 68uF 500V electrolytic that happened to be exactly the same diameter as the existing 40+40uF one, though much shorter  (I peeled off the bright blue plastic wrapper). So I've fitted this as C1 and for C2 used a 47uF 450V axial I had.  Both were bought about 12 years ago, so should be good for a few years yet.  It did the job.  There's still a little hum, though it vanishes completely when the EF86 is removed, so I suspect it's due to the wiring layout.   
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1 comment:

Michael Saunby said...

Update -

The Dubilier PIO capacitors were a bad idea. The amp worked but wouldn't give much power without distortion. The phase-splitter wasn't right, because the cap was leaking.

As for the hum, it was probably caused by a poor connection on one of the EF86 pins - which one?

So perhaps I'll try changing the smoothing caps back to the original.