Pye MM

[Yorkie]

Most valved gear just uses a couple of electrolytics in the power supply and one as the catohde bypass, which is usually 25uF 25V, and in my experience, invariably duff. I don't think an ESR of six Ohms in the power supply is going to be a problem - most smoothing caps rather than having a high ESR are leaky, causing hum. I guess that the best way to check a smoothing or reservoir cap to see if it's doing its job is to check the residual 100Hz hum on a scope. (Apart of course from using ones ears to see what comes out of the loudspeaker).

I may have mentioned before that Bob Parker, the Aussie ESR guru and designer of ESR meters, created a table of worst case ESR values, which can be found here:

http://www.your-book.co.uk/design/esrchart.htm

ESR meters are very useful on transistor sets such as ther Bush TR82C and Roberts R200s etc. They use several el-caps, which are often past their best, and an ESR meter can do two things: Firstly, confirm those caps which are still working fine, so can be left in situ, and secondly, to identify those that need to be replaced. Much better than just yanking all old caps out routinely, making work and destroying originality needlessly, or on the other hand, hoping for the best, and leaving dodgy caps in place on the bassi that ignorance is bliss.

Or so it seems to me.

What makes an ESR meter a useful homebrew project is that commercial ones are expensive, with their LED readouts to decimal places, but they're simple to make using a 50uA analogue meter, for under £15.00, We simply don't need to know whether a cap has an ESR of 9.875 Ohms to tell us that it's duff - anything over about five Ohms should be compared with a known good one of the same value as it's highly questionable. And el-caps in switched mode PSUs must have very low ESRs on an Ohm or less.