01a Preamp (Gen2)


IMG_1962The 01a preamp has a long history in my system. I played a lot with many DHTs in the past from 26 to 71a, 30sp, 112a, 46, 4P1L and 10 amongst others as preamp valves. I particularly found the CX301a to be a special one. Disregarded by some as claimed to be microphonic, I found it to be much better behaved than the famous “dancing lady” 4P1L. As a preamp valve it has its limitations. It’s definitely not a 4P1L in terms of current driving capabilities, however, it provides a delicate and warm sound that is unique in my perspective. I simply love the sound of it. I made some further impressions here if you are interested.

After so many DHT preamp/line stage builds, I decided to go back to the CX301a given I found the perfect excuse. I needed a tad of extra gain for the Starlight CD Discrete DAC player. I’m utterly happy with my 4P1L stage, but adding back the particular tone of the 01a to my system seemed the right thing to do.


I tried in the past all sorts of circuit variants for the DHT preamps. I really like two in particular: the transformer coupled and the gyrator-loaded mu-follower. The latter provides maximum gain as well as low output impedance despite the output coupling capacitor. Since my load is simple the 4P1L preamp, I decided to implement the gyrator-loaded mu-follower stage.

This circuit has been covered many times and in particular I wrote about this here.

I will explain again the circuit for those out there that they are just getting to grips with this topology. The 01a is configured as a mu-follower with an active load that is not another valve but instead is a FET. In this case is not one FET, is a cascoded pair formed by a FET and a depletion FET. The DN2540 undertakes the heavy task of the high-voltage in the circuit, whereas the 2SK170 not. The latter has lower capacitance so it provides a better HF response overall. The gyrator has a frequency response similar to an inductor, but is not one. The particular benefit is that it will set the operating voltage to be constant therefore some call it a voltage controlled CCS. The voltage is set by the CCS formed by M1 and M2 and R4. The cascoded depletion FET pair sets a precise current set by P1 which will create a stable reference voltage through R4. The pair R6 & C1 set the low-frequency response of the gyrator. The lower C1 the better so we increase the value of R6, which also helps with the HF response. D1 is there to protect J1. Trust me, you will need it in case you short the anode (as I did in my test rig). R7 is the mu-follower resistor which typically is set to 1/gm for optimal value.

The great benefit of this circuit is its PSRR and the ability to accommodate different power supplies. I use a tap from my 4P1L Siberian preamp which provides a  235V rail. You can get away with a 150V / 10mA supply very easily. I leave it to you to design that supply. It won’t need a big filtering stage thanks to the cascoded DN2540/2SK170 and LND150 pairs.

01a preamp gen2 v01.

Filament bias has been extensively discussed in the past before. I will simply stress here that the 01a has its filaments starved. This reduces microphonic noise as well as improve linearity. After several tests, I settled for the following operating point which proves to be quite good:

SX-201a triode starved op point


Starving the filaments to 200mA also brings additional benefits. The Rod Coleman boards don’t need a big heat-sink and the raw supply is easier to implement.

I use NOS Dale wire wound resistors rated at 20W I have in stock. You definitely don’t need that power rating capability in this circuit as less of 1W is dissipated across this resistor. A good quality wire-wound 3-5W should do well here.

If you need the gain that this circuit provides in your system and you also need driving capability (e.g. due to the load presented by the amplifier input) then you may want to consider the addition of a cathode follower (or source follower) to this circuit. Here is an early draft which still needs further tweaking.  Instead of the cathode follower, I’d suggest you opt for a MOSFET source follower instead. Much simpler indeed, and it sounds good.

The design of the filament supply is crucial for low noise and performance. I implemented the typical filament raw power supply I published before here which is based on some of Rod Coleman’s recommendations for his regulators. In this case I used all sorts of components I had at hand so the supply is not worth following but here is the description in case you want to use this as a guideline:

  1. A 13V split bobbin 100VA transformer (100VA)
  2. Schottky bridge and 100n/47Ω snubber network in the secondary
  3. Choke input supply based on a 300mH choke and 22mF cap. A tuning cap was added prior the choke to set voltage to desired level.
  4. 43mH CMRR choke and a final 22mF EPCOS cap stage

Regarding the HT supply, you can get away with a very simple and tiny one. This stage only requires a 150V @ 10mA raw supply, you don’t need the 235V shown in the diagram above. As said earlier, I’m sharing an HT supply with my 4P1L stage. Filtering is not crucial as the cascoded stage of FETs provides at least 100dB of PSR @ 100Hz.

As you can see, I’ve made several compromises here in this design to use parts I had in stock. I think I bought just a few parts for this preamp.


IMG_1951The most important aspect in my view of the DHT preamp build is the correct wiring of the ground and power scheme to avoid loops and interference issues leading the hum. I’ve highlighted this before many times with some tips in addition of what you can find out there. There is a very good article around grounding and interconnection schemes  here. I suggest strongly you take the time to read this to understand what could be happening in your system when you hear the annoying hum noise and immediately want to blame the design for it.


I created several blog entries during the design and construction of this preamplifier. I suggest you take the time to read through these if you are interested in some of the details and experience gained through this build:

  1. Build part 1
  2. Build part 2
  3. Build part 3
  4. Build part 4
  5. Build part 5
  6. Build finished

If you’re interested in getting a PCB for the gyrator, see here. 

The valves have a set of silent blocks to help with the microphonic noise, however I found this preamp to be extremely quiet. I can tap the copper top plate and only hear a dim feedback, compared to the 4P1L which is more microphonic by miles.


The LT and HT supplies are separated by 1m or more. The long cables are not an issue. In particular I use a shielded 4-wire cable for the LT raw supply connection.

LT supply

Here is a view of the back top plate with the supply connectors and output RCA sockets:

01a top plate back

 Performance and Listening impressions

The preamp measures really well. I still need to take my measurement equipment down to the listening room to do full tests as the measurement below is using my noise bench power supply so 50Hz noise is actually lower than the 100μV (-80dB) shown below:

01a Preamp Test 4

As you can see distortion is really low (typically 0.01%-0.02% depending the input level) and is consistent across the audio bandwidth. The main harmonic is even H2 and odds are very low. The key measured specs are:

  1. Gain: 18.55dB (or 8.4)
  2. THD < 0.02% for Vin < 400mV
  3. Output noise lower than 100μV @ 50Hz
  4. Bandwidth: 10Hz – 100kHz

Bandwidth should be slightly better that what I managed to measured with my soundcard below:

01a Preamp test 2 FR


So the preamp measures as good as we expected to be, but how does it sounds? Oh, what a question. Never liked writing about the sound as is so personal and subject to your end to end system and listening conditions. However let me share some thoughts after some weeks of listening to this stage:

  • The unique sound of the 01a is there. The fingerprint of the thoriated-tungsten filaments in this valve is beautiful.
  • The dynamic response is very good as well as the detail that the DHT brings
  • The bass and overall frequency response is great. I can notice a slightly improvement in the bass and the cymbals, brass and even top strings sound very nice.
  • I was afraid that this pre wasn’t good for rock music, but I was wrong. It does have the stones to make my 4P1L-46-816 SE amp shake without impacting the sound quality.
  • Classical or jazz and female vocals come at their best. The beauty of the 01a stage takes the Starlight Discrete DAC player to the next level.


Well, enough of preamps for me for now. I hope this article encourage some out there to build the preamp. Be aware that the NOS 01a valves are expensive and not easy to find and you may be disappointed to buy some of the stuff that is out there in the web. Just do the proper research before embarking on this project 🙂


33 Replies to “01a Preamp (Gen2)”

  1. Hi Ale, just love your blog and amp builds. I dont know if youve done this in the past or not, but for a newbe like me would love a step by step approach to a build thats of course if you have the time. Also would love a blog about your work bench the test equipment you recommend using for building valve amplifier. The one thing I find hard about this hobby is where to start, I thank you for the advice youve always given me and am pursuing them.

    1. Hi Paul, thanks for your comments. Unfortunately it takes good effort to write up a step by step build process. Generally I ended up taking a couple of pictures at the end of the process and then writing the blog entry/article when I get some spare time for it. I hope I can do something more descriptive in the future. I recommend you to read Morgan Jones’ “Building Valve Amps” latest edition as it will give you a lot of tips and ideas around testing equipment, methods and building process and techniques.

  2. That “01A journey” is very interesting! Plenty of informations and comments useful for anyone wanting to build a preamp with this triode. Great!

  3. Well, I’ve built several preamps from 26, 10Y, 4P1L, 71A and so forth. A lot of the common DHTs. I even built an 01A PP preamp. But nothing sounded as good as this in my system when Ale brought it over. It’s simply stunning – the best preamp I’ve ever heard. Obviously, I’m building one as fast as I can. It’s rather special.

  4. Hi There,
    Thank you for a wonderful blog. This O1A preamp interest me alot. I’m actually thinking of implementing this as the front end of my Hypex UCD 400 oem modules. As for it’s low drive issue, I’m thinking of combining it with a BUF03 buffer circuit. Would appreciate if
    you would kindly share your thoughts.

    Thanks again

  5. I was pretty pleased with my 30SP preamp but after hearing Ale’s 01A at Andy’s I had to convert. Now that it is running the improvement is remarkable. This is a stunning preamp, give it a try!

    Thanks Ale for sharing your hard work.



  6. Hi Ale,

    You burn 01A 3.4V, is it right ?

    I read somewhere that burning heater or filament out of range +/- 10% nominal range will short life time’s tube.

    Is Starving filament good?

  7. That’s helpful information, Ale.

    I found 10M45s and one pair of 01A on my store. Plan for this preamplifer is in near future. I intend 10M45s for current source, Road Coleman’s board for filament like what you did. I have no experiences on filament bias.

    01A and 112A have the same power of filament (5V 0.25A) and they have nearly same size of plate. I think 01A plate can run much more current and so Rp keep down.

    I will get 20 ohms resistance in the cathode also. If I supply 8V or more for filament, the tube will run more current ?



  8. Hi Ale!

    Many thanks for sharring your thoughs and experience with CX301A preamp.

    Did a trial built with minimium componants except a DN2540+LM334Z CCS, and it was so good sounding. Much better than my 801A pre with LL1668 plate choke and a TVC.

    BTW, do you’ve for sale those FET-gyrato or CCS stuff .



  9. Hello,
    I plan to build this 01A pre amp. How do I buy this loading board ?
    Please contact me via my email attached.
    Best regards,

  10. Hi Ale. Although I’m building the Gyrator version of the 26 at the moment, do you think it’s ok to use 100k shunt attenuator paralleled with 510k Resistor? I have a spare Khozmo 48 step one!

  11. , done it yet Ale, I am converting the 26, 3rd Gen into Gyrator version, I have a 100K TKD Pot, but wish to replace it with the Khozmo 100K Shunt Attenuator, so it has a fixed Resistor which goes to the Grid, but pot then increases Resistance to Ground, with Volume half way it Registers 47K in Parallel with 510K Resistor(total Resistance), so rather than reducing Resistance when increasing Volume it increases Resistance to Ground, Input to Output has 100K Fixed Resistor, to the Grid, then Pin 3 Goes to Ground with Resistance increasing to Ground as Volume increased, so rather than Decreasing Resistance going onto the Grid, it increases it after the Grid.www.khozmo.com/products_dale_shunt.html

    1. Hi Barry, you didn’t answer my question am afraid. Either way, I’m assuming you’re looking to place a shunt attenuator at the output. The purpose of the 510K output resistor is to provide a charge/discharge path to the output capacitor and avoid any “pops” when connecting it. It’s a safe resistor and its high value makes no impact to the load of the gyrator as most of the cases you have a load impedance as high as 100K. I recommend not removing it. You should be fine with the stepped attenuator.

  12. The Grid of the Valve is after Attenuator, I would have no issues with using a Ladder Type of Attenuator, but this is a Shunt Attenuator, anyway I’ll wait until I have all the Parts here. It also looks dead easy if I would wish to convert the whole thing into a 01, should I wish.

  13. Hi Ale,
    What was the value of the dummy load on your 01a Filament Circuits, also the same question for the B+, I don’t have the Valves Yet but want to test before I fit them. Thanks Barry.

  14. Just for Your readers Ale, I have finished the 01a & after modifications & changing Caps for some nicer ones, C1 is now Jantzen 220nF & C2 Mundorf SGO 680nF, this is now a truly amazing Preamp, I used a 100K TKD(top quality) Potentiometer & 510k Resistor from the Valve Grids to Ground in case of Pot failure, I also used RCA UX201a Valves, these were a matched pair, obtained from a trusted US seller, Ale is correct in stating that it isn’t very easy to obtain good ones, trust Me I found out the hard way when a was building the Gen 2, 26 Preamp, I couldn’t get the voltages right or balanced, had hum in one channel, this was totally attributed to bad Valves, they aren’t cheap either, an old adage here, buy cheap, buy twice!

  15. I’m building the gyrator with rosin FT-2 teflon caps. One on the gyrator and a couple in parallel to give 0.2uF for the output cap. Very cheap and I’ve heard nothing better. They’re rated 200v but will take more. Small enough to fit neatly on the gyrator.

    Ale now can fit the BF862 to the boards he supplies, and the R-Mu (R7 on the diagram originally) is now 470 ohms as I understand it when you use BF862 in place of 2SK170. Ale can advise.

    This is my current drive stage for my 2x4P1L outputs and loving it. I’d recommend choke input filament supplies for the ultimate smoothness. I use Hammond 159Y.

  16. Correction – should read Russian FT-2 teflon caps in above post. Not sure why “rosin” is in my head, though I am a double bass player……. spooky…..

  17. Thanks Ale. I have 100k pots, so it makes sense to use them, at least to start with.
    But my next question is, (using my type 26 PSU) what voltage would be ideal into the gyrator? With a couple of minor component changes I can get 150V-170V.

    1. Hi Jaffrie
      Yes you can, however you will have to carefully select and match a pair of 01a. This is not that simple, but doable.

      The gyrator should be the same, no changes. You will run it at 7mA, probably better to use a BF862, but 2sK170 is fine.

      The stage will have lower output impedance thanks to the higher transconductance of the FET as anode current is higher. There is no free lunch am afraid. You will have double input capacitance, but this hopefully isn’t a problem


      1. Many thanks Ale. Well I’ve got 4 tibe which are supposed to be matched. Anyway will try it out
        when the time comes & see what happens

        Thanks again

        1. You will run 2 filaments in parallel so filament current is now doubled (i.e. 400mA). To keep the same bias point you will need to halve the filament resistor which will be shared between the 01a pair. Bear in mind that you will need to change the setting resistor of the Rod Coleman regulator to achieve the 400mA current.

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