A few months ago I got an old Bogen amp from the 1950s. A regal vacuum tube amp with 30 watts of power. I just had time to connect it and verify what I suspected, it was not working. So I thought it would be a good weekend project.
The first thing I checked was the fuse and it was blown so I crossed my fingers, replaced the fuse, and turned on the amp. The new fuse blew out immediately.
To summarize the story I found out that it was the rectifier valve that goes into the power supply. I looked for the valve locally and it turns out that in this city there is practically NO ONE that sells vacuum valves ... well, after all we are in the 21st century, I must be the only one in the city looking for that valve.
Tube replacement using solid state diodes
As I was frustrated not being able to do anything for my amp, I came up with the idea to replace the valve with solid state circuitry, after all a simple silicon diode does the same thing as a rectifier valve.
The damaged valve was a 5U4G. I looked for the datasheet on the internet and after some calculations to simulate the internal resistance of the valve I opted for the following circuit:
The numbers 4, 6 and 8 are the 5U4G valve pins. I do not use the filament pins because it is not necessary.
It is not very difficult to conclude why I put those diodes there. I used 1N4007 diodes because they withstand high voltages (up to 1000V) and they are the ones I had on hand. I set the resistance to simulate the internal resistance of the valve. I had doubts whether to put a 150 ohm or a 200 ohm one, but after trial and error (measuring the voltages after the valve) I decided on the 150 ohm one. It is very important to clarify that depending on the amplifier it may be necessary to modify this value. On mine it works great with the 150 ohm resistor. Well I didn't actually get one of that value so I have a 100 ohm and a 50 ohm connected in series.
I soldered the components in the same socket valve, which must be removed carefully so as not to cut yourself on the glass.
An old amp comes to life
Let me tell you that to my own surprise the replacement worked like a charm and the amp sounds flawless! I hope this story can serve anyone in the same situation as mine.
Upgrade: I continued with the restoration of this old audio equipment. If you want to see how it finally turned out, I share the following link, it was great!
For those who want to know a little more about how amplifiers work empty tubes internally, I leave you this other article.
Note: watch out for voltages on tube amps, they can be a few hundred volts !!! If you are not completely sure what you are doing take the amplifier to a repair shop.