After last months huge transmitting triode, this months tube will be at the opposite end of the size spectrum, the 6X4.
The 6X4 is an indirectly heated full wave rectifier tube for small currents. Ideally suited for preamplifiers.
The 6X4 is a miniature 7 pin tube. The base diagram is shown on the left. It has a 6.3V heater which consumes a very modest 0.6A. The maximum DC output current is specified as 77mA. Due to the limited peak inverse voltage capability the maximum reachable DC output voltage stays below 400V. As such the tube is ideally suited for situations in which only small current is needed for example preamplifiers, separate supplies for driver stages in power amps or bias supplies. With it's small size and very modest heater power it can be mounted pretty much anywhere. Even inside a chassis is possible with the proper ventilation. The tube is only 6cm high and has less than
20mm diameter. Being directly heated it provides a delayed start of the output voltage with a slow ramp. The 6X4 does not seem to be very popular any more among amplifier builders. I myself am guilty of ignoring it so I cannot share any practical experience with this tube. But there is no reason why it wouldn't perform well. If space is limited and not much heater power is available this tube can be just the ticket. NOS tubes can still be found and also finding good quality 7 pin sockets is not a problem. Since I never used 6X4s for any project, I did not stock up many of them. I only have a few ruggedised 6X4W tubes made by Raytheon. See the datasheet for all technical details. Quite remarkable is the shock resistance which is stated up to 700G(!) for the 'W' version.
The tube in it's full glory:
Views from other angles:
This tube came in a bland military box:
It has a nice internal support for secure shipping:
Some details of the tube:
Here one of the extra support rods is visible which braces the internals:
The two plates are very close to each other so that they almost seem to be a single piece:
To examine the construction in detail I opened one up:
The top mica:
The holder of the getter:
Very rugged construction indeed:
The bottom mica:
The heater wire coming out of the cathode:
The two support rods.
Now the cathode is visible between the two plates:
One of the plates:
A fine piece of vacuum tube engineering!