Medium-Mu Triode
The 30 Vacuum tube is a glass type used as a voltage
amplifier or detector in battery-operated receivers. It
was first manufactured in 1932 and requires a
four-contact socket.
The 30 is also a detector and amplifier tube of the
three-electrode type. It has a coated filament which
takes as little power as possible consistent with
satisfactory operating performance. This feature makes
the 30 particularly suitable in battery-operated radio
receivers employing the 32, 34, 31 and/or 33 where
economy of filament current drain is important.
The 30 was intended as detector or amplifier tube in portable radio receivers. It is a battery tube like the 31.
It has the same UX4 base and pinout as the 31 and also the same size bottle, ST-12. Being intended for battery
powered equipment it's oxide coated filament was designed to operate at as little power as possible. The filament
voltage is only 2V and needs a measly 60mA to operate! That's only 120 milliwatts of filament power. The 30 is
typically operated at 135V plate voltage (180V max) and at around 3mA bias current. It has a plate resistance of
10k Ohms which makes it still usable with an interstage or line out transformer if they have enough primary
inductance. It is more easily usable with LC or RC coupling. The amplification factor is 9.3.
I never used the 30 myself. It could be an ideal companion for the 31 in a nifty flea power SET amp to drive
ultra efficient speakers, or as tweeter amp in active set ups, or maybe a nice little all DHT headphone amp. The
plate curves show the typical excellent linearity of directly heated triodes.
The 30 was made in globe shape as well as ST shape. The globes were named UX-230. The most beautiful 30's are the
Sylvania VT-67's, a variant of the 30 made for the military.
These have beautiful white ceramic bases.
There had been two military variants of the 30 with different VT-numbers. Initially it was introduced as
VT-27. The VT-27 had a black phenolic base which was upgraded to low loss bases for the VT-67. The latter
could be micanol or ceramic. Here a micanol base.
Electrical Data:
Filament Voltage .......................... 2.0 DC V
Filament Current ............................ 0.06 A
Preferred Substitute ......................... VT27, VT67, 30.