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The BSTRD - Class A Tube Pre and PSU with Eagle Project Files
The BSTRD - Class A Tube Pre and PSU with Eagle Project Files
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Old 23rd December 2016, 08:01 AM   #1
skrodahl is offline skrodahl  Sweden
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Default The BSTRD - Class A Tube Pre and PSU with Eagle Project Files

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-th...d-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.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

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:

Click the image to open in full size.

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

Click the image to open in full size.

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

Click the image to open in full size.

Last edited by skrodahl; 23rd December 2016 at 08:31 AM. Reason: Added image of the PSU
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Old 23rd December 2016, 02:10 PM   #2
Sodacose is offline Sodacose  United States
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Nice build and measurements! Did you calculate the output impedance of this?

Good to have more low voltage tube projects around (especially with measurements). Looks like a very nice pre.
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Old 23rd December 2016, 02:19 PM   #3
skrodahl is offline skrodahl  Sweden
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Thanks
I have no clue on how to calculate or measure the output impedance thought...
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Old 23rd December 2016, 03:11 PM   #4
Groucho2004 is offline Groucho2004  Spain
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Quote:
Originally Posted by Sodacose View Post
Did you calculate the output impedance of this?
I simulated the circuit using the schematic provided in the link above. Without any load, gain is ~2.5, Ia is ~1.5 ma, output impedance is about 2.0KOhm. With some more supply voltage these numbers would be much better.
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Old 23rd December 2016, 03:23 PM   #5
Groucho2004 is offline Groucho2004  Spain
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Quote:
Originally Posted by skrodahl View Post
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.
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Old 25th December 2016, 02:13 PM   #6
Sodacose is offline Sodacose  United States
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Hey skodahl, I stuck this in the /r/diytubes sub at Reddit too. Couple of people had questions there as well if you're on Reddit.
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Old 25th December 2016, 04:50 PM   #7
skrodahl is offline skrodahl  Sweden
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Thanks Sodacose, I went over there and posted an answer.
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