Knowing how to replace vacuum tube rectifiers with diodes is the key to turning that beautiful old radio gathering dust in your attic into a functioning conversation piece. Vacuum tubes are still available online but those sources are becoming fewer and fewer with each passing day. In addition, specially manufactured tubes can be quite expensive, whereas rectifier diodes are plentiful, easy to obtain and cheap. Converting a vacuum tube power supply to using silicon power diodes is an easy modification to make, but requires an understanding of power supply design and knowledge of how to select the proper size diodes.
Things You'll Need
- Radio schematic-wiring diagram or vacuum tube manual
- Digital Multimeter (DMM)
- Soldering station or 35 Watt soldering iron
- Rosin core solder
- Needle nose pliers
- 2 silicon diodes
Using the schematic-wiring diagram for the radio you are working on, identify the high-voltage (HV) secondary leads on the power transformer. Alternatively, identify the HV leads by color. The Radio Manufacturers Association (RMA) used red for the beginning and ending leads and red/yellow for the center tap lead. The Radio Corporation of America (RCA) used brown and brown/black with their older transformers, and the RMA code in newer transformers.
If the colors are no longer visible, take live voltage readings using a DMM set to the Alternating Current (AC) range to separate these leads from all the other leads. Some of these old power transformers may have a dozen or more wires coming out of them. Use caution when taking voltage readings; these HV windings averaged 375 to 800 volts--enough to kill you.
Another way to identify the leads is to use the vacuum tube manual to locate the plate pins on the tube(s) being used. The wires attached to those pins are the end leads from the HV winding. You will still need to take a voltage reading, however--you need to know the voltage that you are working with in order to select the right diodes.
Using the schematic or tube manual locate the two anode or plate leads and the two cathode leads on the vacuum tube socket(s). If the set used a duo-diode vacuum tube (two tubes in one) there will be two plate leads but only one cathode lead.
Select two diodes with a Peak Inverse Voltage (PIV) rating equal to the secondary voltage of the transformer. (The diodes current rating isn't critical because most of these old sets had a plate current of under 300 Milliamperes (ma)--any diode with a Forward Current (Fc) rating of 0.5 Amperes (A) or higher will work fine.)
Turn on the soldering station or plug in the soldering iron and allow it to heat up.
Identify the anode and cathode leads on the diodes. A solid line that runs completely around the diode's body identifies the cathode leads.
Using the needle nose pliers, make right angle bends in the diode leads.
Solder the diode's cathode leads to the vacuum tube socket's cathode pins and solder the diode's anode leads to the tube socket's anode or plate pins.
Remove and discard the old vacuum tubes.
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