In Restoration: Dynavox Rebuild

This Dynavox was originally a so-called widowmaker amplifier. We completely rebuilt it with all-new components, installed a power transformer, and revised the circuit to eliminate the obsolete widowmaker tubes. It is now a safe, functional 1.5 watt practice amplifier. It can be used alone for bedroom practice, or plugged into a second amp for interesting tube overdrive tones.

We originally purchased this Dynavox because it was in great condition, and we loved the simple vintage styling of the cabinet. Because it did not have a power transformer, it required modification before it could be safely used.

The chassis of the Dynavox was very small and mounted in a very odd way. Note how little space the tubes have.

We considered installing an isolation transformer and upgrading the plug to a three-prong. However, the amp was extremely noisy, and although a new transformer would make it safe, it certainly wouldn’t help it sound good. It also uses two obsolete tubes, which were chosen not for tonal quality but for their heater voltage specs.

So, we decided to completely strip the chassis of all circuit components and install a totally new amplifier. Although we have the highest appreciation for original vintage amplifiers, we proceeded with a clear conscience because the amp sounded like garbage, was extremely hazardous and honestly that swirly grill cloth deserves better.

We were immediately restricted in our design by the size and placement of the chassis. It is a rectangular piece of metal about the size of a small envelope that is mounted perpendicular to the faceplate. The tubes and the output transformer are mounted sideways between the chassis and the speaker baffle. When you remove the back, the tube pins and all of the circuit components are facing you, while the tubes themselves are on the other side.

One redeeming quality of these tubes: they’re Dynavox brand.

The original tube complement was a 12au7, a 50c5 and a 35w4. All of these tubes are very small and lightweight, so they easily fit in the small space allotted to them. For the redesign, we needed a tube complement that occupied the same footprint. That meant that we could not use a conventional power tube, because even a 6v6 was too tall to fit between the chassis and the speaker baffle. We also could not mount the power transformer to the chassis, because even if the chassis wasn't too flimsy to support one, there was physically no room for it.

For that reason, we decided to use a 12au7 wired in parallel as the output tube. Although the voltage gain of a 12au7 is smaller than a 12ax7, it provides enough current to drive a small speaker. It also fit easily in the space provided. The result would be a very low-wattage bedroom amp, which aligns with the purpose of the original Dynavox.

As for the power transformer, it became obvious that we would have to mount it somewhere at the bottom. We created a small, square chassis to house the entire power supply: transformer, rectifier diodes, and filter capacitors. We chose a solid state rectifier because we wanted very little voltage drop here, in order to give the preamp tube as much headroom as possible. And although we usually prefer to mount preamp filter capacitors near the stage that they are filtering, we had a couple of really nice Spragues leftover from another build that we wanted to use. There was no way those (or really any other filter cap over 10uf) would have fit on the main chassis.

Moving on to the main chassis, we swapped the physical locations of the power and preamp tube, because we wanted to mount the preamp tube as close to the input jacks as possible. Conventionally, the preamp tube is kept as far from the power supply as possible. This is to avoid noise, but also to maintain a logical wiring scheme because the B+ voltages are sent to the power tubes first. But because the power supply was moved way over there, at the bottom of the amp, we had the freedom to orient the remaining two tubes in any order we wanted.

The original amplifier had point-to-point wiring, a strategy that we also followed due to the size of the chassis. We opted to use modern low-noise resistors and polypropylene capacitors instead of period-correct parts such as carbon composition resistors. The original high-wattage resistors were in pristine time-capsule condition and had a very cool vintage appearance, and it was painful to clip them although they would have been worse than useless in the new build. After that, we felt there was no looking back on installing brand-new parts.

We did leave the original input jacks - not that we had a choice, because the faceplate was actually glued on top of them. They were in great shape and worked fine. (The amp itself was in great condition; all the issues were tied to the circuit and the layout.) We wired two of them as hi and lo inputs.

Because nobody needs three inputs and nobody is certainly going to plug a microphone into this thing, we wired the third input as a preamp output. This is a great feature because, if you ever find yourself needing volume and power, you can send the amp to an external amp. Because it isn’t a buffered aux output, the Dynavox’s speaker output is diminished when the preamp out is in use. However, the output signal itself is very strong and sounds great.

As for the overall tone of the circuit, it is warm and lively, with a satisfying distortion at high volumes and a comfortable bedroom tone when played quietly. It is definitely surprising how much tone a 12ax7 and a 12au7 can offer.