B-17B Preamp Circuit Built

The prototype B-17B preamp circuit has been built. This includes the power supply and preamp tube circuit, but not the VU and buffer circuit. Here it is:

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If you look closely at the picture above you can see the tube glowing.

It actually worked the first time it was turned on. There were a couple of things found in initial testing, which is not surprising.

The first issue is that the power supply is actually at about 350V, rather than the 290V predicted by simulation. One contributor to this is that the transformer is actually putting out 134V RMS instead of the 120V RMS rating on the secondary winding. The primary side of the transformer is at 119V RMS which is within spec. The maximum anode voltage for the 12AX7 is 330V so further analysis is required.

The second issue is that the first stage amplifier is adding compression earlier than expected. Below is the output of the first stage with 1V RMS 200Hz sine wave input.


As you can see the bottom part of the waveform shows some rounding, If the input voltage is reduced to 0.65V RMS then the rounding goes away as shown below.


The first stage was predicted to have more headroom than this.

The second stage appears to function more or less as predicted. Here is the output of that stage with the Gain control set about halfway up, and you can see plenty of compression on the bottom part of the waveform.


When the gain control is reduced the compression goes away. It’s hard to tell if this happens at about 1/3 of the rotation of the gain control as predicted, it seems to happen a bit before that.


So there’s still work to do to figure out how to adjust for these discrepancies. But good progress so far – it works!