Author: history_km5q8r

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.

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.

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

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.

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

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:

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).

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…

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….

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.

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…

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.

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

The parts are on order for a prototype build of the B-17B preamplifier and high voltage power supply. The latest schematics for each are below.

Notice there is a block for VU meters and a preamp output – these still need to be designed. But next step is a turret board layout for the preamp section – more to come!

Finally have some time to get back to the B-17B design after the holidays. The focus lately has been on the preamp circuit, getting the input stage right and making adjustments to the tone stack. The amplifier section has been reworked as well – simulation of the previous design showed way too much distortion without some type of global feedback. But that will be the feature of a future update. Right now here is the latest preamp circuit:

As mentioned, the amplifier has been moved to a separate diagram.

You may notice that there is a new potentiometer labeled ‘shift’. This idea came from a circuit in Designing Tube Preamps for Guitar and Bass by Merlin Blencowe. The shift control changes the center point for the bass and treble controls, and is a more flexible alternative to a midrange control. Otherwise the tone stack is a standard James design used in several guitar and bass preamps.

Here are some bode plots (phase is omitted) for the range of tone controls. Each plot shows all combinations of the bass and treble controls set to zero, 50%, and 100%, to give the total range of frequency response. Click on the individual plots to see them full size.

The first plot is with the shift control set to zero:

Here it is with shift set to 50%:

And with shift set to 100%:

The effect may appear subtle, but our ears are very sensitive to the midrange and this appears to be a good starting point to at least try in the first revision.

Next steps are to do some additional simulation of the preamp with some wav files to see how it sounds by itself. Then its time to choose exact components and get them on order. There’s no reason why the preamp can’t be built before the amp is finalized, they are really two individual circuits by design. This will allow substitution of a more powerful amplifier in the future, or a different preamp while keeping the same amplifier.

More to come…

Work continues on refining the B17-B bass amplifier. The intent is to make this a fairly lightweight portable unit that can be used at least in smaller venues to substitute for heavier equipment. Some of the features to help with this:

• Use of a switching power supply for the high current output stage
• The second stage of the tube preamp is biased to begin tube compression at higher volume levels, to give it that “tube sound”
• By continuing to increase the level beyond where tube compression kicks in, it will enter solid-state clipping if additional harmonics are desired.

Here is the latest circuit. By adjusting the value of R8 we can vary the onset of tube compression. By varying R19 we can determine how hard the output stage is driven and thus the onset of solid-state clipping.

The HV (High Voltage) supply is shown below.

The V+ Supply will be a TKD-Lambda LS15048 switching supply.

More simulation and calculation is required before component selection is confirmed, but this circuit is getting close to being a buildable prototype.

Stay tuned!

The combined tube/solid state B-17 design is progressing and here is the schematic of the updated and much simplified version:

Note that the design no longer includes a complete solid state power amp, the voltage gain is handled by the 12AX7 tube and the solid state circuit is simply an emitter follower and a unity gain output stage. The tone controls have been moved between the two triodes and this means that only a single 12AX7 tube is required.  This is also the reason that no midrange control is added, to do this right would require another tube stage.

The solid-state portion has been modified for single-ended supply operation for simplicity.

This design does not currently have a provision for a global feedback loop so the distortion figures would be closer to a vintage amplifier than a modern solid state design.

This is still a very preliminary design but you can see how the concept is progressing.
More to come!

I’m back after a long hiatus for summer. For now I’ve set aside the tone control design and am working on combining the previous B-17 preamp with a solid state amplifier design. Here is the first cut – please don’t try to build this amp as the exact component values, etc. still need work.

The power amplifier is a standard 3-stage design with an old-school quasi-complementary output. I picked the venerable 2N3055 output transistors to start.

As I mentioned earlier, this circuit still needs a lot of analysis and design work before it is ready for building. For now it is useful for LTSpice analysis and trying out concepts.
Stay tuned!

The B-17A prototype with the enhanced midrange modification was tested in the south lab and was declared to sound much better. The midrange is now sometimes too strong – ideally there would be a midrange control in addition to the bass and treble controls.Otherwise its sounding pretty good, and that is probably as far as we will go with that version.

So now we move on to initial design of the next iteration – the B-17B.

First, let’s start with the addition of a midrange control. Here is the circuit updated to include a mid adjustment.

You can see that there is now a lot of passive tone control here, and as you might guess the controls are not entirely independent – each control has some effect on the others.

This got me thinking about equalizer circuits. I found this circuit at http://gilmore2.chem.northwestern.edu/projects/equal_prj.htm and entered it into LTspice.

As you can see this is a tube equalizer. The center frequencies are 28Hz, 2Khz, and 20Khz. These are easily adjusted by changing the capacitor values, for instance C2 and C3 for the bass control.

So how does it work? Here is the effect of the treble control stepped through its range as simulated in LTspice:

As usual you can click on the image for a closer view. So far this looks pretty good. How about the mid control?

Now we are starting to see some limitations – the mid control does indeed affect the frequencies around 2 kHz, but also has some effect on all the frequencies above that.

Let’s try the bass control:

Here we see the same thing – the control does change the response at the bass frequencies but also affects all the frequencies above that. This is not exactly ideal.

So, I have some more thinking to do on tone control design.

The second aspect under consideration is the inclusion of a power amplifier in the next iteration. One possibility is to simply include a standard push-pull class A tube amplifier with an output transformer – this design is well understood. The main drawback as always with this design is the output transformer – large, heavy, expensive, and not exactly distortion-free. An example of a good transformer that gets down in the bass region and is capable of 100 watts output is the Hammond 1650RA. That transformer retails for $186 and weighs 12 pounds.

So what if we used those evil semiconductors in the output stage? 100 watts for a pair of power MOSFETs is a walk in the park. But what about the hard clipping you get when you overdrive them?

My proposal is to design the circuit so that it is impossible to overdrive the output stage – set it up so the preamp tubes will saturate and limit the signal to a level that the output stage can easily handle without clipping. That way you get that nice soft tube clipping even with the solid state output stage.

One question remaining is – how much of that “tube sound” comes from the distortion inherent in the output transformer? One way to find out would be to design a solid state output stage, build it, and see how it sounds. But I can’t believe I’m the first one to think of this, so a little more research on those who have tried this before is in order.

That’s all for today !

The B-17A prototype arrived safely at the South Lab this week. Initial musician testing reveals a need for more treble – here’s how we can accomplish that.

First, let’s go over the predicted vs measured frequency response of the prototype. Below is a table summarizing the results.

The predicted voltage and AC analysis values came from the LTspice program, and as you can see they align nicely with the measured values on the prototype. The values with the level switch open vary a little more but still are pretty good overall. So this shows that we can use the LTspice program to predict the frequency response with reasonable precision.

Here is the frequency response of the B-17A prototype as predicted by LTspice – you may need to click on the image for a larger view:

The roll-off at low frequencies prevents amplification of any stray beat frequencies – which is a good thing to do. Low B on a 5-string bass is around 31Hz so no need to go much below that. On the higher frequencies you can see that by 600Hz the preamp is down to 0dB gain (no gain) and after that starts attenuating the signal. There are definitely harmonics that go well above that, and clearly our resident bass expert would like to hear more of those.

So our first modification will be to remove C2 completely. The circuit diagram was presented in a recent post, and if you refer to that you can see that C2 goes from the signal path to ground and thus filters the higher frequencies. Without C2 in the circuit the frequency response should look like this:

Now the 0dB point has been moved out to around 3300 Hz. If this adds too much treble, we can substitute a smaller value capacitor for C2. The voltage at that point shouldn’t exceed about 16V so a high voltage cap is not needed. If there is still not enough treble, we can look at more extensive modifications.

As a reminder, here is the circuit board layout so you can see which cap is C2. Looking forward to the results!