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Thursday, June 28, 2018

Tube of the Month : The 1H4


I just found some small directly heated triodes in my tube stash which deserve their spot in the Tube of the Month series. Meet the 1H4:

The 1H4 is electrically equivalent to the 30 which was already covered in this series. It is basically the same tube with an Octal base instead of UX4.

The pinout is shown on the left. Since it only has 4 connections half of the pins on the base are unused. As mentioned, the 1H4 is absolutely identical to the 30 except for the base. It was also intended as detector or amplifier tube in portable receivers. Hence the classification as battery tube. It has the same ultra low filament consumption as the 30. Only 2V and a minuscule 60mA of filament current which is only enough for a very dull glow. It was designed to operate at rather low plate voltages. The RCA data sheet gives sample operating points at 90V, 135V and specifies a maximum plate voltage of 180V. The amplification factor is 9.3 at a plate resistance of a little over 10kOhms. These values are very close to those of the indirectly heated 37. As always let's compare the plate curves from the data sheet with those taken from a tube sample with the tracer:

As linear as can be expected from directly heated tubes!

The 1H4 was made with both shoulder type and straight sided glass. The shoulder type was named 1H4-G and the straight sided ones 1H4-GT. As mentioned above I only have a few 1H4 and only of the shoulder type. Above we see a 1H4G made by RCA.

The tube from different sides:

Some details of the tube:

Shiny, uncoated nickel plate.

The base:

The tube designation on the glass:

This tube came in military packaging.

A bunch of 1H4G tubes made by Sylvania:

Some close ups:

Nice getter flash which almost completely covers one side of the tube:

Some more details:

The top:

Here we get a nice view of the grid and the filament which is an inverted V shape:

Let's see the internals:

Now we get a better view to the top.

Close up to the suspension of the filament:

In different light:

The bottom part showing the extremely thin filament welded to the connection wire:

More views of the plate:

Grid and filament removed:

Here we see the two holders through which the filament was threaded:

The grid without filament:

This is the first time I have seen such supports for the filament midway between the top and bottom.

Probably to reduce the risk of the very thin filament touching the grid when used in portable equipment.

The extremely fine filament is even thinner than a human hair:

What a great little tube!

Best regards



  1. Excellent pictures! This tube is a work of art. Thanks!

  2. Planning on constructing an amplifier using 2 1h4G tubes. The circuit needs a B+ of 45 volts. The filaments require 2VDC, so I'll use two NiCd batteries in series, with a 3 ohm resistor in series, providing a 0.4 volt drop to safely power the filaments. Depending on the signal, I'm hoping a speaker with a matching transformer will produce adequate audio from an iPhone or CD player.