The PHILCO Phorum

Hi all, I’m curious as to why there is a difference between some sets as to how they obtain negative bias voltages, in this case both Philco sets. I picked 2 random sets to illustrate my question. In this first set, you can see the center tap of the power transformer develops a negative voltage going through the bias resistors 60 and 59 to chassis ground with no bypas or filter electrolytic.

[attachment=29282][attachment=29281]In the second set, center tap goes through bias resistors 33 and 34 to chassis ground but there is also a filtering or bypass capacitor of 10ufd #31. This filtering is absent in the first set! I’ve always wondered why there is that cap in one set and not the other considering the similarities of both circuits!

Ron

I will hazard the following guesses:
1. The cap circled in red needs to be there as a "bypass, due to the Output stage being single ended, while the power supply without the bypass cap has a Push Pull Output stage.
2. The cap may be elsewhere on the schematic.

Ok, so I did some more digging and looked at several more schematics and haven’t found any sets with push-pull outputs having that cap across the bias resistors. On single end output sets, it’s a toss up. Found some with but many without. Just for giggles, the next set I work on without that bypass cap I might just gator one in and see what if any difference it makes!

Ron

I suspect you will not see much difference in those that did not have it initially.

There is a practice in engineering, called "for good measure", or just in case. Put that extra capacitor, extra ferrite bead, extra stitching via, extra square inch of copper on the power pad, extra 10W of the power supply capacity or an extra Amp of a choke saturation current; extra this, extra that. What if....

Cost you some money but gives you peace of mind. Maybe this is one of those things.

Thanks guys for satisfying an old man’s curiosity!

Ron