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Wednesday, March 9, 2011

Tube of the Month: The 6AX4

Hi!

As you could already read in previous articles, I prefer TV damper tubes for rectification. My favorite among those is the 6AX4.


The use of TV dampers in power supplies was advocated by JC Morrison in Sound Practices magazine back in the 1990ies. He mostly uses them as a slow turn on device in conjunction with silicon rectifier bridges. TV dampers have been developed for TV service. Their purpose was to dampen oszillations in the deflection system during the fly back period of the electron beam. This requires a very tough diode which can handle very high peak inverse voltages and peak currents. These properties made them perfectly suited for mains rectification as well. However this was not promoted by tube manufactures since they wanted to continue to sell their dedicated rectifier tubes, which where more expensive. There is still a prejudice against TV dampers as rectifiers because of this. Many years of experience with TV dampers in various power supplies proofed that they are extremely reliable and perfectly usable as rectifiers. Another advantage is their extremely low cost.

The 6AX4 shares the same base diagram with other similar types like 6DE4, 6DM4, 6AU4 and many others. It is also available with different heater voltages and for series heater connection as types 12AX4, 17AX4 and 25AX4. A detailed datasheet can be found here. The 6AX4's current rating is on the low side compared to some of the other TV dampers. So why did I chose it as my favorite? The current rating is still way beyond the needs of most amps I build. More than enough to power both channels of a SE power amp. When followed by a choke input filter, a 6AX4 rectifier bridge can deliver more than 300mA continous. The advantage of the 6AX4 is it's lower heater current compared to others. It needs only 1.2A. When 4 of them are used in a bridge, the higher heater current of other types can become a difficulty. Therefor I stick with the 6AX4 for most applications. If higher current is required, I use some of the beefier TV dampers. Another big plus of TV dampers is their good insulation between heater and cathode. Please note that this voltage is quite different depending on the polarity. The cathode can be at substantial higher voltage than the heater, while they don't like the heaters to be too postive against the cathode. For the 6AX4 the cathode can be up to 900V DC above the heater potential. This means that all heaters of the 6AX4s in a Greatz rectifier bridge can be wired in parallel and fed from a secondary winding which is referenced to ground. This is safe for power supplies with choke input filters which deliver up to approximately 650VDC. Higher voltage power supplies like those for 211 amps will require at least 3 different heater windings for a rectifier bridge to ensure cathode to heater voltage is not exceeded. The schematic below illustrates how the heaters need to be supplied and biased in the case of higher voltage PSUs:


The 6AX4 was available from almost any tube manufacturer. Although production of this tube has ceased a long time ago, it is still available in large numbers. Below is a selection of 6AX4 tube boxes of different brands:


The 6AX4 was made in various forms, most with the common large Octal base, in later years it was also broduced with the flat 'coin base':


The last photo shows a 6AX4 Graetz bridge in a power supply:


Best regards

Thomas

24 comments:

  1. Hello Thomas,
    Do you have a prefered brand or type of 6AX4? I am finding GT / GTA / and GTB versions in many brands. Thx!

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  2. Hi!

    No, I am not after specific brands. Brand preferences wouldn't make a lot of sense anyways since a lot of crossbranding was done with tubes. You can never be sure if a tube really got manufactured by the company who's brand name it carries.

    GT/GTA/GTB differ in warm up time

    Best regards

    Thomas

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  3. can you please explained why you conected the heater with the cathode. As indirect heating valve, you should be able to use one single heating source for all valves. Am I right?
    thank you.

    ReplyDelete
    Replies
    1. Hi!
      It is explained in the post. For voltages up to 600-700V the heaters can be supplied from a single source which is referenced to ground. For higher voltages separate heater supplies as shown in the schematic are needed to keep heater-cathode voltage within allowed limits.

      Thomas

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    2. Thank you. I thought that the standard schematic is as shown in that figure.
      I am trying to build a full wave rectifier for only 170 V so I hope will do well, with one heater.

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  4. I'm planning to use 12AX4 to be a slow ramp up device for B+ of 600V 60mA. It will be placed after the solid state rectifier. I think I don't need to tie cathode to heater. So, from your experience, is it safe to do so? Thanks.

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    Replies
    1. Hi! It would be safer to tie the cathode to the heater. But why not use two 12AX4 with two silicon diodes in a hybrid arrangement. This will give the benefits of a tube rectifier and the slow warm up

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    2. Thanks for your advice, Thomas.

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  5. how about 6X4 series are smaller??

    ReplyDelete
    Replies
    1. The 6X4 can be used in preamps. It can handle only small currents

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  6. we are vacuum tube electrons collector from Jakarta,Indonesia
    wana saying so good for these articles...bravo...

    ReplyDelete
    Replies
    1. Thanks! I am glad you like them
      Thomas

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  7. Hi! Love your website. Your article notes that for power supplies up to about 650 volts "all heaters of the 6AX4s in a Greatz bridge can be wired in parallel and fed from a secondary winding which is referenced to ground". Would an appropriate ground reference be achieved by connecting the heater-transformer center tap to ground? Thanks!

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    Replies
    1. Hi!
      It doesn't matter if one and of the heater winding is connected to ground or the center tap. Both achieve the same result

      Thomas

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    2. Got it, thanks!

      Could the heater center tap be connected to a voltage divider somewhere else in the power supply? That would reduce the heater/kathode potential and provide a bit more flexibility (another 50 volts, perhaps).

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    3. Carefully examine the heater cathode voltages before you decide anything like that. Be sure you understand what you are doing. It al depends on your exact configuration and voltages

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    4. Is 6AX4 similar to the 6W4? I remember an old Phillips TV that used 2 6W4 as rectifier and another as damper.

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    5. Yes they are similar but different ratings

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  8. May I ask why you have chosen a Graetz bridge and not just a normal Fullwave with two diodes instead of 4 ? Only for pratical reasons or as well sound quality difference ?

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    Replies
    1. Hi!
      The bridge configuration is the better supply. You get better utilisation of the power transformer

      BR

      Thomas

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    2. Thomas, have you ever tried using a double wound center tapped power transformer with two 6AX4's in a full wave configuration verses 4 of them in full wave bridge configuration? If so, then what did you discover?

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    3. I lost interest in center tapped power transformers many years ago. Why don't you simply try out yourself and use what works best for you? If 4 6AX4 tubes are too much you can use two together with silicon diodes in a hybrid arrangement

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    4. Thomas, were you seeing specific problems with center tapped power transformers? Thanks!

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    5. No specific problems, but I think the bridge configuration is generally better

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