Re: 475 questions


Eric
 

For the life of me I cant remember the peace of gear that cap was it but it was a filter and VERY old and I cant forget the cap cause I was shocked at the tolerance spread as well. But even in the 576 some of the filter tolerances is +100% -10% and some of the ceramics is +80% -20%

On 5/29/2020 11:23 AM, Raymond Domp Frank wrote:
On Fri, May 29, 2020 at 04:07 PM, Eric wrote:

There are 2 fundamental differences that need to be taken in to account when
replacing “vintage” caps. And I use the term vintage loosely. One is
tolerance. In the worst case I can remember a capacitors value was +150% -%100
tolerance. The means that for a 1 uF the measured value of anything between
2.5 uF to .1 uF would be considered “in spec” old radios still amaze me
they ever worked. Now a days +-20% is the normal. The service manual will tell
you the tolerance of the filters. From memory I am guessing it is going to be
+100% to -20 % so pretty wide design tolerance on the filters.

The other important spec is the voltage rating of the cap. And here vintage
caps and modern caps differ greatly. Vintage caps were very tolerant of over
voltage especially given how tube gear warms up before the tube’s comes in
to operation the B+ will spike some times as high as 200 to 250 volts higher
then when the device is operating and can hang there for about 15-30 seconds.
This is not an issue in your 475 as it is solid state. However modern caps are
completely intolerant of over voltage so if you have the physical space it is
always good to bump up the voltage rating of the cap it wont effect anything
to replace a 63V cap with a 400V cap excepta little cost in $ maybe one or 2
and physical space it will be slightly bigger then it’s modern lower voltage
counterpart. However both are usually smaller then their vintage counter part
even doubling the voltage the modern can will be smaller physically.
I don't think anyone would have accepted caps with +150/-100% tolerance, not even electrolytics...

Any information on the inability of modern caps to withstand overvoltage more than vintage caps used to? Maybe WV was just very conservatively spec'ed?
I'd rather have known the actual max.

For (wet) Al electrolytics, choosing a spec-voltage very much higher than it's ever going to experience, has a deformation effect, so is not recommended.

Raymond

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