The Muffsy BSTRD - Class A Valve Preamp

An open source Class-A valve preamplifier and regulated PSU, complete with Eagle project files and detailed Bill of Materials

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The BSTRD iis a simple Class-A valve preamplifier, complete with Eagle project files in the files area of this project. The preamp has been built and tested, so it's perfectly safe to order the PCBs from the fab house of your choice.

The project is fully open source, no strings attached. Use the attached files as you like, for what you like. I'd appreciate if I were credited, but I don't require it.

The circuit is modified version of a the line amp from a preamp published in the Danish magazine Ny Elektronik:

A suitable power supply has been designed as well, and it's added to this project.

PCBs for the Muffsy BSTRD and PSU are available on Tindie:

Open Source Hardware

Eagle project files are available in the "Files" section of this project.

The Bill of Materials is maintained in this project log.

The PCBs can be bought here.


The original BASTARD circuit used no feedback at all, ran on very low voltage and the gain was controlled by the power voltage. Several people collaborated on the Danish forum HIFI4ALL, made measurements and came up with a modified version that has feedback, much lower distortion values, higher power voltage and a set gain of 2.2X:

This preamp was called the Bastard because it was a three-part hybrid:

  • A transistor phono stage
  • A valve line stage
  • An input selector with relays

As this project uses only the line stage it's no longer technically a "bastard". I decided to call it the BSTRD as a nod to the original design.

Vital specs

*) Measured using Russian 6S2S tubes:

+78V/0.7A and +6.3V/1.5A Power Supply for the BSTRD - Eagle Project Files

x-zip-compressed - 76.02 kB - 11/29/2016 at 17:05


The BSTRD Tube Pre - Eagle Project Files

x-zip-compressed - 63.07 kB - 11/29/2016 at 17:04


  • The BSTRD - Measurements

    skrodahl12/19/2016 at 08:20 0 comments

    We've already established that The BSTRD sounds pretty nice, but how does it measure?

    Keep in mind that my QA400 audio analyzer has a tendency to pick up 50 Hz mains when used with a computer connected to mains (it does better on a battery-only laptop), so this will skew the results a tiny bit.

    I'd say it's what you can expect from a tube construction, or better.

    The frequency range +0/-3 dB is 10 to 30.000 Hz. SNR is 83.5 dB (without A-weighting) and 93.2 dB (with A-weighting).

    This thing produces harmonics, with the second being prominent. 3rd harmonic and greater are near or below the noise floor. As a result, it shows THD measurements of 0.35-0.4%.

    The SNR/THD(+N) measurements have been done at 1 Vrms output. The numbers are will be even better at 2V output, which is the common output of modern equipment.

    Frequency range (10 Hz to 30 kHz):

    SNR/THD(+N), without A-weighting (1 Vrms output):

    SNR/THD(+N), with A-weighting (1 Vrms output):

  • Testing the BSTRD Some More - Sweetness

    skrodahl12/06/2016 at 22:46 0 comments

    This is quite a little performer,

    The sound is crystal clear, there's no hum, hiss or noise of any kind, the stereo imaging is wide and precise to the point where I can close my eyes and point out the instruments and vocals in the space in front of me. And the sound is very rich and warm. 

    I swapped the 15V AC on the PSU for the filament heater to 9V AC, and the heat sink (while still warm) is cool enough that there won't be any need for extra cooling.

    Besides, who can resist that little glow peeking at you? All in all, this is full tube/valve Class A sweetness :)

  • Testing the BSTRD - Success!

    skrodahl12/04/2016 at 22:52 0 comments

    Testing the BSTRD - Success... Not entirely true at first; I had used a 330 ohm resistor in the feedback loop instead of 330k. The result? No sound. At all. Not even noise. What a bummer...

    Well, replacing the resistors made all the difference. It was a real joy to see the 1 kHz sine show up on the scope, nice and clean.

    I hooked it up to a power amp and a single speaker (I tested one channel at a time), and all I can say is that this thing sounds sweeeeet. All the way from the meaty bass to the tingly treble. :)

    Note that the PSU needs two transformers, because of the 78V part using a voltage quadrupler. Two tube filaments drawing 300 mA each, is 600 mA in total. The regulator and the heat sink get really, really hot. I should probably find a way to get more cooling, it will most likely mean moving the regulator off the PSU board. UPDATE: The excessive heat was because I used way to high AC input on the 6.3V side. Replacing the 15VAC power with 9VAC made it all better.

    I measured the regulated outputs for AC, and my multimeter shows 0.00 mV on both the 78V and the 6.3V. I am quite happy with that result. :)

  • Components, Bill of Materials

    skrodahl12/01/2016 at 08:48 3 comments

    Here's the bill of materials for both the BSTRD Tube Pre and the Power Supply.

    It contains part numbers from Mouser, and other sources where Mouser can't provide the parts. It's recommended that you buy more than the specified component quantities, especially for cheaper parts like resistors.

    These are just suggestions, there are a lot of cheaper, more expensive, better, worse and higher or lower quality components available. There are also a lot of other places to get components, like,, to mention a few.

    Mouser Shopping Cart

    All components below except LED resistor, transformers, tube sockets and tubes have been placed in this Mouser shopping cart:

    The BOM and shopping cart were last updated 2019-04-03, ensuring that everything is in stock.

    Note that the 470uF/160V electrolytic capacitor has been replaced with a 200V version, and that the 100V ceramic capacitors have been replaced with 200V models.

    The BSTRD Tube Pre:

    Quantity Component Value Mouser Part #, or other source
    2 Resistor, 0.6W 1k ohms 279-LR1F1K0
    2 Resistor, 0.6W 22k ohms 603-MF0207FTE52-22K
    2 Resistor, 1/4W 100k ohms 603-MFR-25FRF52100K
    2 Resistor, 1/4W 330k ohms 603-MFR-25FBF52-330K
    2 Resistor, 1/4W 1M ohms 603-MFR-25FRF521M
    2 Electrolytic capacitors, 25V (≤10mm width, 5mm pin spacing) 47 uF 647-UKZ1E470MPM
    2 Film capacitors (7.5mm pin spacing), ≥ 100V 1 uF 505-MKS41.0/100/10
    2 Ceramic capacitors 0.1 uF 80-C320C104K1R
    1 Stereo Potentiometer, logarithmic, 16 mm 100k ohms 652-PDB182-K430K104A
    2 Sockets, for tube 8 pins Search for "octal socket pcb" on eBay, Amazon etc.
    2 Tubes, valves 6J5 or equivalent Available from various sources, such as eBay and other online stores.
    3 Screw terminals 3 ports 651-1705469

    Alternative tubes that can be used with this preamp are:

    • L63
    • 6S2S (Russian tubes, also known by the Cyrillic 6C2C)
    • 7A4
    • CV1067
    • 12J5 (needs 12V filament heater)

    The BSTRD Power Supply:

    Quantity Component Value Mouser Part #, or other source
    10 Diode 1N4004 821-1N4004
    1 Resistor, 2W 180 or 200 ohms 71-CPF2200R00FKE14
    1 Resistor, 1/4W 100 ohms 603-MFR-25FRF52100R
    1 Resistor, 1/4W 820 ohms 603-MFR-25FTE52-820R
    1 Resistor, 1/4W 3k3 ohms 603-MFR-25FRF52-3K3
    1 Resistor, 1/4W 6k2 ohms 603-MFR-25FRF52-6K2
    1 Resistor, 1/4W Suitable for LED @6.3V Calculate here
    4 Electrolytic capacitor, 100V (≤13mm width, 5mm pin spacing) 390 uF 661-EKZN101E391MK35S
    1 Electrolytic capacitor, 100V (≤13mm width, 5mm pin spacing) 47 uF 647-UHE2A470MPD
    1 Electrolytic capacitor, 200V (≤18mm width, 7.5mm pin spacing) 470 uF 647-UCY2D471MHD
    4 Electrolytic capacitor, 25V (≤13mm width, 5mm pin spacing) 3300 to 3900 uF 661-EKYB250E392MK40S
    1 Electrolytic capacitor, 16V (≤5mm with, 2.5mm pin spacing) 47 uF UKZ1E470MPM1TD
    4 Ceramic capacitor 0.1 uF 80-C320C104K1R
    1 Voltage regulator, TO-220 TL783 595-TL783CKCSE3
    1 Voltage regulator, TO-220 LT1086 (or LM317) 512-LM317T
    2 Heat sink SK104 567-637-20ABPE
    2 Heat sink mounting kit TO-220 532-4880SG
    3 Screw terminal, 5 or 5.08 mm pin spacing 2 ports 651-1711725
    1 Screw terminal, 5 or 5.08 mm pin spacing 3 ports 651-1705469
    1 Transformer, ≥ 15VA 6 V AC (or up to 12V) The print on the PCB
    says 9-12V. but 6-9V is
    better suited.
    1 Transformer, ≥ 10VA 18 V AC...
    Read more »

  • Eagle Project Files

    skrodahl11/29/2016 at 17:11 0 comments

    Eagle Project Files for both the Tube Pre and PSU are now added to the files area:

  • Power Supply Built and Tested

    skrodahl11/29/2016 at 17:10 0 comments

    The Power Supply has now been built and tested (without load).

    There are two parts to the power supply:

    • +78V, 700 mA (using a full wave voltage quadrupler and an RC low pass filter)
    • +6.3V, 1.5A for filament heater

  • Boards Received, Construction of Preamp Board

    skrodahl11/28/2016 at 20:58 0 comments

    There's been a little progress, now that the PCBs arrived today.

    The heat sinks fit nicely, and coupling two 2-port screw terminals together worked out nicely:

    Everything fits as it's supposed to, and I like having ceramic components on a PCB. I really think they look the part.

    The only thing left to do on the preamp board is fitting the tubes:

    There you have it, one preamp fully assembled. Next up is the power supply.

  • BSTRD Update

    skrodahl11/23/2016 at 23:00 0 comments

    The BSTRD has been updated. Changes in layout, component values and footprints.

  • Power Supply Updated

    skrodahl11/23/2016 at 22:55 0 comments

    Here's an update to the power supply. Same principle, different layout and correct component footprints/values.

  • From Russia with Valves

    skrodahl11/03/2016 at 08:35 0 comments

    These puppies just showed up, all the way from Smolensk in Russia. They are 6S2S (C is the Cyrillic letter for S), which is one of the models that are compatible with 6J5. Others that will also work with the BSTRD are L63, VT-94 and 38565J.

View all 11 project logs

Enjoy this project?



kagehisa wrote 05/29/2024 at 19:09 point

Hi, there is an error in the mouser shopping cart as well as in the component list.

Screw Terminal 651-1705469 is 2 pole type, should be a 3 pole type 651-1705472.

Is the mix between MKDS and SMKDS intentional?

  Are you sure? yes | no

cjmaultasch wrote 01/24/2020 at 20:45 point

Where can I buy a completed unit?

  Are you sure? yes | no

vikram Joshi wrote 08/18/2018 at 15:40 point

Hello Sir

I will be building this Preamp, this will be the first time I will build a tube preamp. Kindly guide with wiring instructions . Regards-Vikram

  Are you sure? yes | no

skrodahl wrote 08/19/2018 at 21:11 point

Hi Vikram Joshi,

I believe the wiring is quite self-explanatory. 

 - There are two AC inputs on the PSU; 9V AC and 18V AC.

 - The PSU has two DC outputs (6.3V and 78V) + ground. You'll find the same labels on the Preamp board for power input.

 - There's a stereo input, and there's a stereo output, clearly marked on the Preamp board.

  Are you sure? yes | no

skrodahl wrote 04/10/2017 at 08:49 point

Hi Hirschmensch,

The regulators get really hot IF:

 * You use the wrong AC voltage for the 6.3V filament heater

* You don't use heat sinks

Otherwise they're just warm to the touch.

As long as you get 80 VDC or more + 6.3 V AC or DC/0.6A filament power, you can use any power supply you'd like for this preamp. I wanted DC for the filament heater to avoid hum, which is why I made this power supply using a regulator.

Also, you can skip the 6.3V filament heater completely and use AC directly instead. The regulator on the 80V side only needs to provide a few mA, and it doesn't get hot at all.

  Are you sure? yes | no

Foxchild wrote 04/10/2017 at 08:42 point

At first I thought "very interesting! you are using voltage regulators so you can use whatever transformer you want". Then I read that your voltage regulators are getting really hot... :) well, in my opinion, Amplimo makes the best power transformers for everything related tubes. your power supply would bevome much simpler with the right transformer I guess.

  Are you sure? yes | no

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