Bell & Howell: Model 601 Amplifier Conversion Project

When I started this project, I knew some people prefer to keep these amps stock. However, running something with electrolytic capacitors from 1948 seemed like a disaster waiting to happen. If one failed, it would be a nightmare to fix. In fact, if anything failed, the cramped design would make repairs extremely difficult. On top of that, quite a few resistors were wildly out of spec after 76 years, so leaving them as-is wasn’t an option.

That said, I completely understand the hesitation to modify one of these amps. The lack of space in the chassis makes them a real challenge to work on. Every step required careful planning and a lot of patience. This wasn’t a quick project by any means.

Below is the conversion path that I've taken.  Here's a link to my original post with photos etc

1949, BELL & HOWELL GAUMONT, MODEL 601 REBUILD

The Model 601 is often compared to the Filmosound 179 - essentially the British versus the American version.  While they look similar, there are schematic differences.

For the Tech Nerds:

This amp features:

•  A 6J7 pentode preamp stage

• A cathodyne phase inverter using a ECC35

• A cathode-biased 6V6GT push-pull power stage (no grid stoppers or screen resistors)

The phase inverter and power stage are reminiscent of a Tweed amp, which makes sense given its vintage. However, the standout feature is the unique tone control—more on that later.

The next two images below are two sides of one board - in the image above, the board is jammed into the top left corner.  You can see the positive end of the caps poking out.  The negative ends were soldered directly to the aluminium frame that they're attached to.

 

Perhaps unsurprisingly, a lot of the resistors have drifted out of spec over the last 76 years.  Unfortunately many are too far out of spec to use.

STRIPPED BACK TO THE ESSENTIALS

Stripping the amp down was painstaking. With so many components crammed into tight spaces, it must have taken a meticulously planned assembly line process to build these back in the day. You’d definitely want to get it right the first time.

Here’s what it looked like stripped back to the essentials, with just a few components, transformer, and heater wiring left in place. (excuse the masking tape, that was my labelling system)

POWER SUPPLY

The first task was tackling the power supply - cause if that doesn't work...  . Here’s what I changed:

New Power Connector: I replaced the original four-pin Jones power connector with a standard IEC socket. This required some filing on the chassis but made the amp much more practical.

Modern Fuse Holder: The old fuse holder used an uncommon fuse size, and I wasn’t confident in the aging plastic. A modern replacement solved this issue.

Filter Capacitor Replacement: I swapped the original 16uF + 16uF capacitor for a new F&T 16uF + 16uF 500V cap. While I considered reforming the original cap, its size and age made me hesitant to leave it in the circuit.

OUTPUT JACK

I added a new output jack between the existing two-pin connectors for convenience, opting for the 16-ohm tap from the output transformer. The 8-ohm tap is insulated and tucked away.  Maybe I'll put them both on a switch one day.

REBUILDING THE CIRCUIT BOARDS

Many builders opt for point-to-point wiring after gutting these amps, but I chose to use tagboards with my own layout. In hindsight, this didn’t make things much easier—it was still a tight squeeze! The tagboard I used has smaller spacing than the original, allowing me to fit more components.

Component Choices:

• I replaced the out-of-spec carbon composition resistors with 1W carbon film resistors.

• The original Hunts coupling capacitors were still within spec, so I reused them where possible. For the rest, I opted for mustard caps to keep the vintage aesthetic.

The layout I used isn’t a replica of the original, as I didn’t trace the circuit in detail. My focus was on creating something functional and serviceable, not an exact restoration.

Below is the layout that I used for the boards - I have no idea how close they are to the original, as I didn't spend the time to trace the circuit - cause I'm not that crazy.

I've intentionally left some of the wiring off the layout to hopefully discourage anyone that isn't sure about what they're doing attempting this.  It's not easy, I went slow and it was still a little frustrating to work on.

I did relocate the bias resistor on the preamp cathode to the valve socket, as there was room, and this makes it easier to measure bias or change at a later date.  Same could be done for the cathode resistor shared by the two 6V6.

Note the dire sounding safety warning on the layout - this should be taken seriously if you are opening up any amp to poke around in.  

TONAL TWEAKS?

A few people have messaged or asked on socials what have I done to the circuit, or what circuit did I use.  It's almost the stock circuit, as I wanted to do as little as possible to the amp.

• Simplified the preamp by removing the feedback circuit - it's a straight 33k grid stop with a 1meg grid leak.  Pretty standard.

• I added a very low-value bright cap on the volume pot - 68pf.  It just adds a little bit of air to the top end, without changing the character of the amp.  

• Increased filter cap values, but not excessively so by modern standards.  4uf up to 10uf, 8uf up to 16uf etc. 

SPEAKING OF TONE

The tone control is one of the most intriguing aspects of this amp. It’s surprisingly complex for a film projector amplifier and operates quite differently from most guitar amps.

Here’s what’s happening (or what I think is happening):

• Bass Cut and Gain Interaction: Turning the tone control up cuts bass, making the amp cleaner, while turning it down adds bass and makes the amp dirtier.

• Treble Subtleties: Treble changes slightly across the sweep, but not as dramatically as the bass.

• Feedback and Presence: The tone control is tied to negative feedback from the output transformer, acting somewhat like a presence control.

• Cathode Bias Interaction: The tone pot is in parallel with the cathode resistor, so bias etc changes with frequency

The result is an interactive tone control that introduces harmonics and a dynamic range of tones, depending on its position. It’s a quirky one.

FINAL THOUGHTS ON THE AMP

It's a really interesting sounding amp - I like the weirdness of the tone control, and the feel and sound of the amp in general.  

This is also my first amp with a pentode preamp.  The pentode is a bit microphonic at higher levels, so that's a bit of a downside, but otherwise this is a cool looking and sounding amp.  Sitting directly in front of it with the old alnico goodmans pointed at my head, I can really hear this being a great recording amp.

Did I mention that it was difficult to do?  I would not recommend this project to anyone lacking patience and/or experience.  If you are lacking these attributes and you see one of these going cheap, maybe buy it and put it on a shelf until you are ready to tackle it. 

THE CAB: VINTAGE CHARM & A UNIQUE VOICE

I haven’t talked much about the cab until now, but it definitely deserves a mention. The 12" Goodmans speaker has its own distinct character. It delivers that classic alnico compression and a rich, mid-focused tone that adds plenty of warmth. However, when you push it, you can start to hear its limitations—which, to be fair is part of its charm. It’s the kind of sound that would sit beautifully in a mix, but I would not want to push it too hard.   

Visually, the cab is a stunner—a true piece of vintage gear that will look just as good around the house as it would in a studio. It’s bound to catch attention, and I imagine plenty of people will want to try it out in a recording setup.

That said, I wouldn’t want to haul this thing around for gigs. It’s more at home in a cozy space than on the road, but for the right setting, it’s an absolute gem.