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.

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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.

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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.

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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.

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

Update

The 1946 Hallicrafters S-40 radio turned out to be easy to diagnose – as is often the case with these old radios, the capacitors had seen better days. The worst offender was a canister triple that was leaking badly as you can see in the picture below. That yellow material near the bottom of the square area is capacitor guts that have leaked out.

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So rather than just replace that capacitor, I ordered up a full set and replaced all the old electrolytic and paper caps. Here is the bottom of the radio with the new caps installed.

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And here is the collection of old caps that were removed.

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The radio works great! I attached a small loop antenna and was able to pick up an incredible number of stations on shortwave last night. And it comes with that nice odor of musty old tubes when they get hot. Very enjoyable.

Back to the B-17B – now that the remainder of parts have arrived installation has begun. The main part of the power supply is built. Here is the top of the prototype.

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And the backside as it looks now. Still a lot more to build…

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B-17B Preamp Construction Begins

It’s finally time to start building a prototype of the B-17B preamp. Being that this is a prototype and we’ll need to be able to easily probe signals and swap components, I decided to build it using just a sheet of aluminum rather than a full enclosure. You’ll see what I mean below. But first here’s the layout:

Drill layout

The five holes along the bottom are for the potentiometers for Treble, Shift, Bass, Gain, and Master controls. The hole in the middle is for the 12AX7 tube socket.

This layout is designed so that it will fit in a 5-inch by 7-inch enclosure eventually. Below you can see the test fit of the components once I drilled the aluminum sheet (this is just stock aluminum that I picked up at the local hardware store).

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As you can see this won’t exactly meet UL standards, but it should work well for our prototype. Next step is to wait for the power transformer to arrive (due Wednesday) so I can drill those mounting holes before starting to add the components. But meanwhile there’s time to add a little NSE yellow…

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I also picked up another project yesterday that I hope to dig into today. It’s a 1946 Hallicrafters S-40 shortwave radio. So far the only sound it makes is buzzzzzzzzz….

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The quest for VU meters

The B-17B needs some type of indication to show when the preamp and amplifier are in the “clean” signal area and when they are cranked up past¬† the level where distortion kicks in. For this purpose you can’t beat the cool factor of the old analog VU meters. Unfortunately these have become rare in the digital age, and where they are available they tend to be expensive. It would not be hard to spend more than the entire cost of the amplifier parts on a couple of new analog VU meters.

With that in mind I considered designing some LED bar graph type VU meters for the B-17B preamp prototype. But then I recalled a piece of surplus equipment that I had purchased for just a few bucks a while back. It is – or was – a commercial-grade VHS editor. And as you can see from the photos below, it has some miniature VU meters built in. My intent when I bought this thing was to use it for surplus parts, since I don’t plan on editing any VHS tapes in the near future.

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Although my intention is still to design the B-17B to use off-the-shelf available parts, I thought it would be OK to use these meters in the preamp prototype. We can figure out later if we want to use LED meters in the final version or find a source for reasonably priced analog VU meters. So with that it was on with the teardown…

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What a mass of spaghetti in this thing! I definitely got my money’s worth just in wire and hardware.

Eventually I got to the front panel. You can see the board with the VU meters below.

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And with a little desoldering, here they are. Success!

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