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Friday, August 21, 2020

Tube of the Month : The 5R4


This month I would like to present a classic full wave rectifier tube. Meet the 5R4.

The 5R4 is a directly heated full wave rectifier which means it contains two diodes with a common filament.

The 5R4 shares the same Octal pinout which is used by most of the classic 5V rectifier tubes. The pin connections can be seen on the left. The plates are connected through pins 4 and 6 with unused pins between them and to the filament connections for better voltage insulation. The filament is operated at 5V and needs 2A. The rectified voltage can be obtained from either end of the filament or from a center tap of the heater winding on the power transformer. The 5R4 has the highest peak inverse voltage rating among the 5V rectifiers. It can handle up to 2800V of inverse voltage, which was later uprated to 3000V in the 5R4B. The data sheet recommends a maximum input capacitor of 4uF, which was uprated to 20uf for the B version. DC output voltages  of 800 V can be obtained from a single 5R4 and even in excess of 1000V with the 5R4B. It can provide up to 250mA of DC current. But this comes at the cost of higher internal voltage drop compared to other rectifier tubes. While the 5R4 was initially made in ST (shoulder type) glass, the 5R4B were made with straight sided glass bulbs. I only have 'B' versions in my collection. Let's start with 5R4GYB made by RCA:

The G in the type designation stands for glass bulb, while the Y indicates micanol base. 

For good high voltage insulation the unused pins were omitted from the base.

Next we have a RCA 5R4GB with black plastic base.

Next we have a General Electric 5R4:

5R4GYB made by PhilipsECG (formerly Sylvania):

There was a special version made for the military called 5R4WGY. Here we have one made by Cetron:

The peculiar shape gave this tube the nick name 'potato masher'.

These were made in a very rugged fashion with additional internal bracing for shock resistance.

Another potato masher made by Chatham:

If you are looking for a modern alternative to the 5R4, the ELROG ER274B can be used in any application designed for the 5R4.

Best regards



  1. Thomas,
    Thanks for the post. If your subscribers are interested, a couple years I did a 5R4 teardown, https://youtu.be/2B0QscRu7Iw . Have a great weekend.