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Friday, March 9, 2018

The 6AH4 Line Stage


Since I mostly write about the 10Y linestages (those are also requested more often) many are not aware that I also offer a smaller, single chassis linestage using indirectly heated tubes like the 6AH4.

Depending on the gain requirements this linestage can also be built with the 6J5 or 27.

This version has a slightly different design than the 6J5 linestage.

The tubes are prominently displayed at the front with all capacitors and the power transformer mounted on the top plate. The 6BY5 rectifier resides in the left corner at the back.

Here some impressions from the internals. The top plate with sockets and parts mounted and wired:

All chokes and the line output transformers reside on a separate plate, in this particular build there are also two input transformers to offer a set of XLR inputs.

The transformer volume controls are then directly mounted on the front plate by the volume control switch. Here the preamp powered up for the first time in the workshop system:

A compact solution for those who do not have the space for a 2 chassis preamp and it comes at about half the cost compared to the 10Y line.

Best regards



  1. The 6AH4 is an excellent performer for low-level signals, it just isn't easy to find nowadays.

    In an earlier article about the 6AH4 you hinted at the possibility of using it as a power tube in a speaker amplifier. I have actually tried this type of tube in a single ended design with the 6N7 as driver (a schematic very similar to your 46 amplifer, with ultrapath, Monolith ITs and 5k Hashimoto OPTs) and the result wasn't bad, but also not convincing in the long-term. It had good midrange, but lacked transparency and bass was not well defined. Maybe I could have chosen a better amplifier design, but those are my findings so far.

    1. Hi,

      I'm going to be owner of this preamp. I know the 6ah4 if well implemented in any form can be special. After reading about the great results of this tube ( as well as other low mu tubes) online in preamp duty , it was very easy for me to order this from Thomas. He is special designer that I've long admired and his great designs speak for themselves.


  2. Hi Thomas,

    Are you able to still use full wave hybrid bridge rectifier scheme with this preamp since 6by5 is a dual diode tube? Thanks

  3. Hi ,

    Did you still find original silicon diode from landscape build to work best with this preamp even with switch to 6by5 rectifier here?

    1. I don't quite understand this question. This is my entry level preamp. Design choices are made mainly based on budget and size constraints

    2. I remember reading your 6ah4 octal build in landscape before uses 2 6ax4 dampers that's why. Here you use 1 6by5, I'm assuming the silicon diodes remain unchanged?

    3. These are rather unimportant aspects of the design not worth worrying about. Try it out yourself and use whichever works best for you

  4. Hello Thomas!

    I really like the design of your amplifiers.

    Which of your initial preamp is more interesting, in your opinion?

    27 or 6J5 or 6AH4 (here I mean the version of MK2)?

    I did not hear any of this tube. But your opinion is very interesting.

    In the future, I would like to build one of them. For my amplifier KIT AudioNote 300b .

    1. Hi!
      Thew choice depends on the gain needed. These tubes are very different in gain

    2. Tomas, if we do not take into account the level of amplification and consider the sonic qualities.
      Is it possible to say that 6ah4 has better resolution or 6J5 more musical (as is sometimes said of 2A3 with compare 300b).
      Or if the design implementation is good, these tubes will be in parity?

    3. No, you simply can not detach that. Most important is to choose the circuit and tube according to technical needs. The tube alone does nit determine the sound of an amp or preamp

  5. Hi,

    I’m the owner of this preamp and wanted to give brief update. I’ve only heard a few preamps in my life and have owned one before made by a respectable US company but I want to state here that this is probably the best preamp I’ve ever heard. What Thomas has been able to achieve with such an unknown tube is nothing short of amazing. I’m sure his 10Y may be a bit more refined but there is no doubt that in this circuit and how Thomas utilizes the tube and executes everything along with the Slagle AVC and transformer coupling that this will easily compete with the best preamps in the world. This is driving my 45/2A3 amp that Thomas also built and the sound is simply marvelous. A lifetime investment that I will never sell.