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Sunday, August 28, 2016

Tube of the Month : The 30

Here is another small directly heated triode which probably deserves more attention than it gets. The 30:

The 30 was intended as detector or amplifier tube in portable radio receivers. It is a battery tube like the 31 which was the tube of the month in January.

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:

Which look just as nice in real life:

Since I never used the 30 in any project, I do not have a lot of samples. It was made in globe shape as well as ST shape. Unfortunately I have no globe 30s which were named UX-230. But I do have some rather nice ST shaped tubes. Let's start with the most beautiful ones. Sylvania VT-67, a variant of the 30 made for the military.

These have beautiful white ceramic bases.

Some close up shots:

The base with leaf-logo:

The top:

The plate:

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:

A phenolic base:

The base above is from a tube made by Tung-Sol:

The top:

Side view:

Another VT-27 made by Sylvania:


Side view:

Hytron type 30 / VT-67:

Another angle:

Side view:


And of course the 30 was also made by RCA:

And as last example a 30 from National Union:

A beautiful little tube with excellent linearity.

Best regards



  1. Beautiful pictures. Were there any globe-shaped 30s, to your knowledge?

    Also liked the green leaf mark on the early Sylvanias ('silva' of course means forest in Latin).

    1. Thanks :-)
      Yes there were globe shaped versions. the tube was introduced as globe UX-230

  2. ahh.. sorry, now I notice you you did write that in the article (about the globes..)

  3. A long time ago, I planned to make a 2-stages little amplifier with 30S + 71A (or a line preamp with different output transformers).
    I have some of these tubes, and OEM transformers for that, but components still on the shelf. sigh !