The Muffsy BSTRD - Class A Valve Preamp

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):

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... 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. :)

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 http://elfadistrelec.com, http://digikey.com, http://tme.eu 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: https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=b93f29a67d

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:

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:

Transformer options are not given in the shopping cart, as there is too many choices depending on input voltage and designs. However, this one should work for 115-230V AC input and both 9V (parallel) and 18V (serial) AC output:

• Tridad Magnetics VPT18-1390 Power Transformer.

Also note, it's better to get transformers with lower output than stated on the PCB. The reason for this is that many transformers output a higher voltage than stated due to higher than nominal mains voltage.

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

https://hackaday.io/project/16944-the-muffsy-bstrd-valve-preamp#menu-files

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

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.

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

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

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.