New projects complete and today's hamfest find

My last post was a while ago and in it I mentioned that I was starting new projects. I completed these using two more east European ammunition cans, just like I used for the Fender 5E1 build. The red unit is a high voltage DC power supply which will initially be used for reconditioning electrolyitic capacitors. The grey unit is a AC volt/amp meter which I built to use with the variac. Both of these will support the further tube work I will do.

The last photo here is of a purchase my neighbor Brian just made at the Hamfest we went to today in Milford, Ohio. This is a 1952 US Navy Ocilloscope. Very compact and light weight. He paid $20 for this thing!

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Final 5E1 changes and beginning of new project

In my last blog I described my problem with audible hum. My father instructed me to lower the DC in my circuit by inserting 550 ohms prior to the first filter capacitor in the power supply. I did this and also replaced the first two 8mfd electrolytic caps with a single canister electrolytic with two 20mfd taps. Hum dropped dramatically with these two modifications. On my father-in-laws recomendation, I also changed the wiring to put the fuse on the incoming line side of the 120ac, right before the switch. All of these changes and the resulting voltages are in the schematic to the right. A view of the amp with the added can capacitor is here. Don't forget to click on the images to see enlarged versions.

A group of articles proved most useful in understanding capacitors, power supplies, and reducing noise. My father provided three articles which I will scan into PDFs and make available in the future. The other articles are linked below:

Testing Capacitors
Restoring Capacitors
Repair and Replace Electrolytics
Replacing Capacitors in Old Radios

The next project I am starting will either be a power supply for reforming electrolytics, or another amp project. I have another old TV power transformer and ammunition can, as well as a pile of other parts left over from the 5E1 project.

Revised 5E1 Schematic

This schematic reflects the changes which I made as I built my project amp. Notice the hybrid rectification method using SS diodes and the 5Y3. Fender didn't see fit to show the 6.4vac feeds to the audio tubes in their schematic, so I added them. My 60hz hum is still present. I tied a 10mfd electrolytic cap in series with the second 8mfd cap in the power supply, just after the choke, and this reduced the hum by 50%. I am now testing an old 20mfd can cap removed from one of the console TVs I scrapped. If this thing is good, I will swap it for the 8mfd that is in place now. The voltage values in red are what I actually am reading with the amp powered on. I will adjust this with resistors later. Don't forget to click on the image to see a larger version, or click on the link below to go to the actual blog.