Hello, and welcome to my blog!
The last post that I wrote about this amp project was in January. If you'd like to read that post, you can find it through the following link.
I got distracted away from this amplifier project for a while, only working on it intermittently for the past couple of months. I managed to build the input stage of the preamplifier and add a new power cord connector and power switch. I had to cut a hole in the chassis to mount the power cord connector, but that turned out to not be too difficult. Then I used the hole that the old power cord went through to mount the switch into. The fuse holder is original to the amp. Here's the back of the amp after modifying the power input and mounting a power switch.
This is the power wiring on the inside.
After checking all the wiring and connections to make sure that I had not left something undone, I decided to test the amp with power. First I tested the amp with no tubes in it to make sure that I hadn't miswired anything in the power supply. Then I plugged the tubes into their sockets and used a variac to adjust the AC voltage going into the amp. A variac is a variable voltage transformer that you plug into your house voltage, and is used to adjust the voltage going to whatever you plug into it. It's like a high powered light dimmer. With older tube amps that haven't been used in a long time, it's best to power them up slowly so that you don't overload the power supply filter capacitors. This also protects the amplifier if one of the capacitors is bad. A car analogy would be making sure you have oil pressure in the engine before you start revving it up.
The live power test with the tubes in did not go very well, I started getting erratic voltage readings from the power supply circuits before I had the voltage more than half way up to the 120 volts AC of the standard U.S. house voltage.
Because this amp is probably 60-70 years old and the power supply capacitors are original to the amp, I assumed that there was a bad capacitor, not an unusual problem with old amps. I decided to replace the capacitor can, which contains 4 capacitors, with a new one. I had a new one on another amp that was incomplete, so I used it to replace the original one in this amp. The thing in the middle of the picture with the red, yellow, and black wires going to it is the bottom of the capacitor can, where all the connections are.
Once I had removed the original capacitor can, I found that I had to enlarge the hole in the chassis to be able to mount the new capacitor. Once the chassis hole was ready, I mounted the new capacitor.
After rewiring all the power supply wires, it looks like this. I also replaced the power tube cathode resistor with a larger one for more reliability, the original one was barely adequate.
After doing this work, I tested the amplifier again. As it turned out, that did not solve the problem. When I got the voltage to the amp about half way up to the normal wall voltage, I would start hearing the output transformer start buzzing. With some voltage testing, I discovered that there was an oscillation that was being amplified, causing what is known as ringing in the output transformer. This causes big voltage spikes to the power tubes and can damage the amplifier.
I also discovered that if I removed the power amp driver tube, the ringing stopped. With that tube removed, I was able to bring the voltage up to normal with no further problems. This basically means that there is something wrong with the driver tube circuits. I'm going to have to take that circuit apart and rewire it. I may have to change that circuit to solve the problem. I will know more about it after I take it apart and check all the parts to see if one of them is bad.
That is as far as I have gotten with this amplifier project at this time.
That's all I have for this post, thanks for stopping by to check it out!