• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

The BSTRD - Class A Tube Pre and PSU with Eagle Project Files

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
There was a strange little thing that appeared in Danish magazine Ny Elektronik (New Electronics) in 1989. A very simple preamplifier that used two L63 tubes in Class A with no feedback and it operated pretty much badly out of spec. (It was supposed to though, that was the whole angle of the article.)

It was called The Bastard, and gathered quite a following (it got the name because it was a hybrid. The phono stage used transistors for better SNR), and somebody suggested I should try it.

The Bastard was tested by some Danish guys and was found to have 6% THD+N or more. They upped the voltage and added some feedback, and it ended up having 0.185% THD+N at 2V output. It started clipping at 17V output.

I know virtually nothing about tubes, but I got a request to design a PCB for it. I also designed a suitable regulated power supply for it. I'm giving away the Eagle project files for free, and they are within the limits of the free Eagle license. Etch, order or change it to your liking: https://hackaday.io/project/16944-the-muffsy-bstrd-valve-preamp

The Pre and PSU have been built and tested, and it's sounding pretty sweet. It's got some richness/warmth to it, but it's surprisingly neutral.

31264101666_98d76c18a1_c.jpg


31443506716_51d3ca764c_c.jpg


31470149785_e66ef80c87_b.jpg


I have measured this at 1V output with Russian 6S2S tubes, and got the following numbers:

Frequency range (+0/-3 dB): 10 Hz - 30 kHz, flat from 20 Hz to >25 kHz
SNR (without A-weighting): 83.5 dB
SNR (with A-weighting): 93.2 dB
THD+N: ~0.4%

The 50 Hz peak in the graphs below is something that my QA400 seems to pick up from my computer when it's connected to mains (it's there with no input as well), it shouldn't affect the measurements too much. You'll also notice that I have a bit more feedback than what's shown in the schematic, so I have only 1 dB gain.

It's got primarily 2nd harmonics, 3rd and higher are close to or below the noise floor.

Frequency range:

31350939680_cf16d0b2ae_c.jpg


Noise/THD(+N) without A-weighting:

31350939700_35e9a9b755_c.jpg


Noise/THD(+N) with A-weighting:

31350939690_cd2dc23ed2_c.jpg
 
Last edited:
Thanks :)
I have no clue on how to calculate or measure the output impedance thought...
Connect a signal generator to the output via a 100k resistor, set it to 10Veff @ 1KHz(sine). Measure the voltage (Vrms) at the output of the preamp. Divide that voltage by 100 microamps and you have your output impedance.

This is not super accurate since the output impedance is added to the 100K resistor but for an impedance of 1k it's only 1% error. You could measure the AC current to eliminate that error but multimeters which accurately measure such low AC currents are expensive.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.