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Tuesday, April 17, 2018

Tube of the Month : The 19AU4


Last month the friends of directly heated triodes got a treat with the coverage of the 2A3. This month we are looking at something cheap again, another TV Damper, the 19AU4.

Many TV tubes have been made with variants which differed in heater voltage. While the standard 6.3V heater types are intended for parallel heating, variants had been introduced which could be heated in series with other tubes in a TV set which use the same heater current.

The 19AU4 is such a variant and is equivalent to the 6AU4 except it has a heater rated at 18.9V and 600mA. For in depth details about this family of tubes refer to the 6AU4 tube of the month post. Of course the 19AU4 also shares the same octal base with the 6AU4 and can be used in the same applications if the heater supply is changed accordingly. While prices for 6.3V heater TV damper seem to go up a bit, these variants with odd heater voltages remain dirt cheap and are available in abundance. If the 18.9V are not readily available a dropping resistor from a higher voltage, say 24V is an easy solution. Readers of my blog are aware that I am very fond of TV dampers and use them as rectifiers in virtually all my amps and preamps. If you are bored by lots and lots of photos of tubes, now is the time to leave this site. If you like to look at TV dampers from each angle and to gaze at the construction details, this is the right post for you since I got carried away taking photos of my stash of 19AU4s.

Starting with RCA.

Large getter on the side:

Some details:

The top:

Heater wire entering the cathode at the bottom:

General Electric:

The top:

Close up showing the heater inside the insulation spiral within the cathode.

As is common with TV dampers only the actually used pins are present on the base:

Next we have 4 varieties of 19AU4s made by Raytheon:

Let's have a detailed look at each of them:

Some close ups:

This one has the getter applied in the top.

Although not used, pin one is also present at the base.

The next one:

Slightly different internal construction and printing on the base instead of on the glass.

Close ups:

Getter also in the top dome as on the first one.

The next one is again different.

Side getter.

Here we see the connection from the cathode to the base pin:

The last of the Raytheons:

Next we have Zenith branded 19AU4 tubes:

Another Zenith variant.

More Zeniths, slightly different:

The last Zenith:

And as last brand we have Sylvania.

These tubes look very similar to some of the Zeniths above which is not surprising since Zenith used to source their tubes from Sylvania.

A Sylvania in the more modern box:

Here we have a tube which has all the getter consumed. Either because it reached the end of it's life or it developed an air leak:

So it is a good candidate to crack open for inspection of the construction details.

The heater removed from the cathode:

The heater wire is much longer as in the 6AU4 and folded more often to achieve the 18.9V rating.

The cathode still inside the plate:

The cathode removed:

The plate:

Opened up:

A quick check of the heater current at the nominal al voltage:

And lastly some photos showing the glow of the tube in operation.

The massive bundle of heater wire provides quite a nice glow!

A really nice tube and I have to admit I never used any 19AU4 so far, only the 6AU4 variant.

Best regards



  1. Hi Thomas,

    Great job as always on these tubes. A candidate for next month's tube, how about 717A ? I'm very curious about that tube. Yamamoto uses that tube in his SE 45 power amp and it's very well reviewed amp.


    1. Thanks!
      I never used the 717A and do not have any of those tubes. So I am afraid it will not be covered


  2. Love your photos of the 19AU4 and 866A. They really take me back to childhood. (I love the smell of old valve gear as it warms up).

    I can remember dissasembling 6AL3's as a kid and marvelling at the heavily zirconium dioxide coated helix between the heater and cathode because the cathode is at such a high pulse voltage in a TV.

    The 19AU4's with the getters at the top look far more "handsome" than their "fugglier" side gettered brothers.

    Love the heater lit shots too, really getting a good look down into the cathode.

    Thanks for the 866A shots too, I remember foring up 866As at about 100mA Anode Current then removing the heater and watch the glow spread and fill the bulb before extinguishing. Probably did not do the cathode much good though!!