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Monday, October 12, 2015

Tube of the Month : The 84 / 6Z4


Over the last 4 months I presented a series of indirectly heated small signal tubes, the 27, 37, 56 and 76. I mentioned their usability for example in preamplifiers. So naturally the question comes up, which would be a suitable rectifier tube to go with these? Here it is, the 84.

Of course there would be many possibilities. But we would want something that is not over powered for such a preamp, which comes in the same shape, from a similar era and preferably with the same base.

The 84 ticks all these boxes. It was mainly made in the same ST shape as these triodes and some early versions even came in globe bottles. The current capability of 50mA is plenty to supply two triodes and this comes with a moderate heater current requirement of only 0.5A at 6.3V. It has a rather low maximum peak inverse voltage rating of 1000V and a max plate to plate rating of 350V. But again this is plenty for using it with these triodes since they operate below 200V DC. The 84 has the same 5 pin base. The pinout is shown on the left. It is indirectly heated which is perfect for using it with the indirectly heated triodes to avoid that the plate voltage comes up before the heaters are hot.

The ST version of the 84 has exactly the same dimensions, so this would also be aesthetically pleasing on the same chassis. For all technical details, see the RCA data sheet. The 84 was originally introduced with the 2 digit designation, however it never had the early three digit number like the UY227. At least I have never seen one. It was later named 6Z4 based on the RETMA naming convention. Most of them bear both designations. The 84 is available in good quantities at moderate prices. Again a tube which seems to be totally ignored by the audio crowd. I must admit that I have not used it myself yet. But I do plan to build a line stage with the 27 tube and I am considering the 84 as the rectifier. It's characteristics are similar to the 6X5 which probably is a development based on the 84/6Z4. However the 84 would be my choice to go with 27/37/56/76. I already stocked up on the tube not only for the line stage but to have some variety to show in this post. Lets start with the 84 made by Sylvania for the military:

Besides the commercial designations 84 and 6Z4 these also carry the military VT number VT-84. This is one of the few examples which have the same number in both naming systems.

Beautifully packaged:

In boxes which allow testing of the tube without removing it.

The tube in close up:

The 5 pin base:

The top:

The plate structure:

The similarity of the internal structure to that of some versions of the 6X5 is quite obvious.

The printing on the glass with the dual designation:

Here a tube made by Ken-Rad:

Again a similar plate structure as seen in 6X5 tubes.

Also with dual naming:

RCA 84:


Two different 84 made by Raytheon:

One of them with straight sided glass:

The other ST:



General Electric:

Canadian General Electric, made for the military:

Close ups:

Next another tube made in Canada, by Westinghouse:

This is a special variant called 84M

Rogers, another canadian manufacturer developed a special M-series of tubes. These consisted of equivalents of a range of types with external spray coating.

All the M-series tubes came with Octal base even if the equivalent it was based on had a different one.

The spray coating even covered the base, and only left the top clear:

Quite an interesting variant!

84 made by Sylvania this time in commercial packaging:

And another variant from this manufacturer for the military:

This was probably the last version of 84 which was produced late 70ies or beginning 80ies as indicated by the date on the box. Different plate structure from the others:

And finally a beauty, the only globe shape 84 in my stock:

Made by National Union.

Very different plate structure from the more modern variants:

As always also some photos showing the glow of the tubes, starting with the JAN Sylvania 84:

The heater is quite visible not only on the top and bottom but also between the two plates which are arranged vertically above each other.

The Ken-Rad which has the plates side by side:

Close up to the bottom part:

And finally the globe shape National Union:

A nice little rectifier, just waiting to be used!

Best regards



  1. Great Thomas,
    Interesting article, this tube I did not even know. When can we rake the with the tube 27 Line Stage.? That would be very interested. The tubes are priced very cheap.
    Best Regards

    1. Hi Peter,

      I plan to build the 27 Lifestage in January


  2. Hi Thomas, whats the reason for en external spray coated tube ? Is it to have a complete dark assembled unit ? Kind regards Michael

    1. Hi!
      It was meant for shielding. This M series was introduced by Rogers. If you google 'Rogers M series tubes' you will find more background information