High Desert Sonic Institute

Mod Blog

Follow the process of converting a 1950’s Bell and Howell Filmosound 179 tube amp into a gnarly guitar amp, with pictures and detailed descriptions.

Bell & Howell Filmosound 179 Conversion

Not long ago, I came into possession of a real treat: a Filmosound! It’s a tube amplifier that began it’s life crammed into a film projector from the 1950’s, designed to provide the audio to accompany movies. This model is the 179, a model that utilizes a compliment of really exciting tubes, from a guitar perspective. So, like many before me I’m going to embark on an exciting journey of repurposing this amplifier from a soundtrack player to a solo shredder.

The Filmosound as it came to me.

The Filmosound as it came to me.

There is a whole bunch of excessive circuitry and poor wire management inside this amp, as well as unusable parts such as the ancient and oversized oil-filled capacitors, so to maximize the end product we’ll actually be gutting the entire chassis and starting from scratch. We’ll leave the original transformers and tube sockets, and will reuse most of the other hardware and some resistors, but new capacitors, wire and most importantly a new layout will provide for a hum-free amp.

filmogutz.JPG

These amps are known for excessive hum. Poor power supply filtering and a rats nest of wires will do that. But another source of hum has to do with the preamp tube: the 6J7 has a grid cap, meaning the input to the tube runs along a wire out of the chassis and onto the top of the tube. This exposes the most sensitive conductor in the amp to room hiss and hum. To prevent this, we’ll make one small but significant circuit change by replacing this tube with it’s brother the 6SJ7, an almost identical tube except it’s grid connection is on the socket, protected by the chassis.

6J7 and the grid cap. This grid wire is very susceptible to picking up hum. The tube on the left is a photo-sensitive tube that would read changes in light on a portion of the film that represented the movie’s audio.

6J7 and the grid cap. This grid wire is very susceptible to picking up hum. The tube on the left is a photo-sensitive tube that would read changes in light on a portion of the film that represented the movie’s audio.

With a pentode preamp and a 6SL7 concertina phase inverter driving a pair of cathode-biased 6V6GT’s, this baby should sing! We’ll be staying true to the Filmosound’s circuit in large part except for a few notable changes:

  • Revamp and beef up the power supply for better filtering, and remove an extra 6V6 from the bias circuit

  • Reconfigure the overly-complex tone circuit into a Tweed-style tone control, and remove global negative feedback - this will simplify the signal path and boost the gain of the output stage

  • Add a cathode bypass capacitor to the output tubes for further gain in the output section

Otherwise we’ll start by maintaining the original circuit values and tweak things in the final stages for tonal considerations. But the first step is gutting the thing!

Preamp side, totally packed.

Preamp side, totally packed.

Power section, also totally packed. They were focused on maximizing a small space more than future repair work when they built these.

Power section, also totally packed. They were focused on maximizing a small space more than future repair work when they built these.

The old filter capacitors in a monolithic block.

The old filter capacitors in a monolithic block.

And finally empty. Two of those tube sockets will also go, but this is a good stopping point for the night!

And finally empty. Two of those tube sockets will also go, but this is a good stopping point for the night!

Once the old wiring is out and the new componentry layout has been determined, we’ll begin with wiring the filament run. The filaments are the parts of the tube that glow, and serve to heat up the cathode of the tube so that electric current can flow freely from it to the anode. These filaments run on about 6 volts AC, and the wires that feed them that voltage are big hum-inducers. We can minimize the hum they can cause by twisting them tightly together and keeping them away from other sensitive wires. To that end, I will run my filaments down low against the chassis, as shown.

Like my mama always said, ‘Keep your filaments short, tight and twisted!’

Like my mama always said, ‘Keep your filaments short, tight and twisted!’

Next comes wiring up the power section and the output tubes. I’ve redesigned the power supply to have plenty of filtering. This will minimize sag at lower volume levels to provide for tight cleans, but it should more importantly minimize unwanted hum and hiss. These amps are known for being noisy due largely to the small chassis, so minimizing noise is a priority of mine in this build.

New filter cap hanging off the back - had to get creative with the placement.

New filter cap hanging off the back - had to get creative with the placement.

Here the power tubes are finished and we’re ready to move onto the phase inverter and preamp.

Here the power tubes are finished and we’re ready to move onto the phase inverter and preamp.

Next comes the preamp. I reused a small turret board from the original for the preamp tube, and other than that and two terminal strips, this puppy is all point-to-point. I wanted to minimize the chance for crosstalk on long wire leads, so keeping it point-to-point helped, but point-to-point layout can be a real challenge. Relying on components to create your current path presents many challenges, but my rule of thumb is the less wire you have in the amp, the shorter the distance your tone must travel from guitar to speaker and the better the sound! Certain tricks were utilized in the original, like having the junction between my coupling caps and grid stoppers for the power tubes land on pin 1 of the 6V6GT’s which is unconnected in the tube and therefor a fair connection point. But a certain number of terminal strips or tag boards is often necessary.

6SJ7 tube socket wired up. They have a sweet little suspension system on this socket to prevent microphonic vibrations!

6SJ7 tube socket wired up. They have a sweet little suspension system on this socket to prevent microphonic vibrations!

Next come the 6SL7 concertina phase inverter, the volume and tone controls and the jacks. This is where it begins to get really tight, and it took a couple configurations before I landed on the exact layout I wanted. I also originally had a cathode bypass cap on the PI driver stage, which I realized was excessive. I removed this and tweaked the placement on the PI tube socket and got much better results.

The initial PI wiring.

The initial PI wiring.

The final wiring. You’ll notice I’ve dropped that first resistor in the power supply, it was way too large and dropped almost 100 volts initially!

The final wiring. You’ll notice I’ve dropped that first resistor in the power supply, it was way too large and dropped almost 100 volts initially!

And here it is! With the 250 Ohm cathode resister on the power tubes I was reading 14 Watts plate dissipation! This is the absolute maximum for 6V6GT’s who are usually biased no more than 12 Watts, so I adjusted the bias with a new cathode resistor. They’re biased at just over 12 Watts plate dissipation now, ripping hot! I’m hoping these old JAN’s can handle it and fully expect them to have a decent lifespan, they tested solid.

Finishing touches.

Finishing touches.

Next is the head cab. I’ve got a nice piece of salvaged CVG fir that came from a salvage yard in LA. An ancient piece of old growth, probably cut down and hammered onto a house around the time this amp was being assembled and crammed inside a ‘state-of-the-art’ film projector. Seems appropriate that this old amp and this old wood should be reinvigorated into a new life together. Perhaps it’s not even their first encounter . . .

The Wood.

The Wood.

Half-blind dovetails provide a strong joint for the main box. In fabricating a front and back panel I chose to emphasize this wild knot in the wood, and give a viewing window for the tubes to glow through. It’ll get grill clothe to provide physical protection.

Head cab in process.

Head cab in process.

Completed head cabinet.

Completed head cabinet.

And here it is, a repurposed amp inside a house of salvaged wood. This puppy will be for sale in the shop section of our site. If you’re in the Joshua Tree area, swing by to try it out! Thanks for reading folks!

Jacob Erwin