Bell & Howell: Model 601 Amplifier Conversion Project

While some people choose to run these stock, I didn’t really like the idea of running something with electrolytic capacitors from 1948 - if one failed, it would be an absolute nightmare to fix.  Actually if anything failed it would be a nightmare.  Apart from that, quite a few resistors were way out of spec.  

But after getting to the end of the process, I completely understand anyone’s reluctance to rip into one of these.  It’s not easy to work on due to the lack of room in the chassis.  This was slow work that required a lot of planning and patience.

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

BELL & HOWELL GAUMONT, MODEL 601 REBUILD

I will be using as many components as I can scavenge from the amp, and I'll keeping the schematic similar to the original (minus the weird bits).

The 601 is often said to be the same as the Filmosound 179 - British vs American; they certainly look the same, but there are differences in the schematic.  

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

This is what it looks like clean with a few components and some transformer and heater wiring left in place.  Excuse the dodgy paper masking tape - that's my labelling system for leads.

I can't imagine what it must have been like on the production line making these - so many parts were tucked away in tight locations, and there had to have been a very specific build order to get all the parts in.  You'd really want to get it right the first time.

POWER SUPPLY

Given the first thing that needs testing is the power supply, this was first on the list to finish.  I replaced the strange four pin Jones power connector with a regular IEC.  A little filing on the chassis was required to make it fit, with a couple of new holes drilled.  

The fuse holder will stay for the moment, and perhaps permanently.

The gigantic filter capacitor from 1948 has been replaced with a new F&T 16uf + 16uf 500v capacitor.  I was considering keeping the original in and trying to reform it, but I decided against this, largely due to the size.  It just took up too much space, and I was not confident about leaving it in the power supply, given its age.

REBUILDING THE CIRCUIT BOARDS

A lot of people go the point to point route after they strip these down, I've decided to rebuild on tagboard using my own layout, as I think this will be easier to do (famous last words).  In retrospect, I’m not sure the board made it any easier - it was still difficult.  The style of tagboard that I'm using has smaller spacing than the original, so more components can fit.  

1w carbon film resistors replace the old out of spec carbon composites that looked like a mix of 1/4 watt and 1w based on their size.  I've managed to keep a few of the original Hunts coupling capacitors in the mix, as they are bang on spec, and still test like a new capacitor. 

This 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.

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.

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.  

I still need to mount the output jacks and tidy up a wire or two, but it's pretty much good to go.  It's less of a rats nest than the original, but this was still slow going for me.

BALMORAL ELECTRIC: Set of Tone Benders (MKI, MKII, MKIII)

Just to offset the amplifier posts for the pedal folks - here’s a set of three Tone Benders that I just finished.  By a set I mean a MKI, MKII and a MKIII - and no I did not include a MK1.5.    

The layouts and info on how to build these can all be found on the blog.  Standard MKI on phenolic, Sola Sound MKII and the MKIII uses the D*A*M Fuzz Sound layout with stock values.  

Off-board wiring for positive ground with the battery draw and LED can be found here and if you want to get a really fancy, tips on cable lacing can be found here.

The enclosure is a Hammond Eddystone available from RS.  Instructions for how to do a battery cut-out using simple hand tools can be found here 

PROJECTOR: Bell & Howell Gaumont, Model 601

I know, I know—this is starting to drift even further from pedal circuits with each post, but stick with me for a moment.

These old Bell and Howell projector amps have quite the following, so I’ve been keeping an eye out for one for a while. The catch is, they don’t pop up on the secondhand market very often—at least not the older valve-based models.

So when this one showed up on Marketplace at a reasonable price, I couldn't resist. It came from an older couple on the Sunshine Coast, which is about an hour from my place (Queensland, Australia). According to them, the amp originally belonged to a great uncle who bought it new in the early '50s. It had been in storage for about 20 years, and no one in the family was interested in keeping it. Apparently, it was still working when it was put away.

Basically the amplifier is hidden away under the projector (which is built like a tank by the way).   

BELL & HOWELL GAUMONT, MODEL 601

These amps are often called "Bernie" amps, named after a guy in Canada who’s believed to have been one of the first to modify them for guitar use. Bernie himself is something of a mystery—nobody seems to have ever met him or been able to track him down, and almost every amp he worked on has its own unique character.

Beyond their vintage charm and distinctive components, these amps are known for their unusual tone control. The tone circuit varies between different models, but this particular one is wired into the negative feedback loop and the cathode of one of the valves. When you adjust the tone knob, it feels like a lot of subtle things are happening at once.

The amp itself has;

• 2 x 6V6GT  cathode biased, push pull, power amplifier
• 1 x 6SL7GT (ECC35) dual triode for the phase inverter
• 1 x 6J7GT (EF37A) pentode for the preamp
• 1 x 5Z4 full-wave rectifier (Model 179 lists a 5Y3GT on the schematic).  

No little mini 9 pin bases here - all octal bases.  There's an additional 6V6GT as part of the projector system, and some other random valve that has no place on a guitar amplifier.

I really like the little grill cloth protector panel. 

This is actually the back, despite looking a bit like a speaker grill.

Unfortunately the rubber had hardened and has become so fragile that the leads can't be used.  I am saving the metal connectors, as they still seem good.  The cable is so bad, I can't even strip the outer layer to save the wires inside.  It's all gone.

The projector and speaker both came in really well made canvas cases.  Given that these were made in the late forties, perhaps someone with a bit of experience making kit for the military put these together?  The leather carry handle poked out the top - there's nothing left of it, apart from the wire frame.  Surprisingly the leather around the base is still in great shape.

While it doesn't have a label, these are usually marked as B.A.F.  Type T60 (British Audio Film Limited) and the general consensus is that they were made by Goodmans, and is similar if not the same as the Audiom series.   I have another Ampro projector speaker cab with a Goodmans in it that sounds great.  This one is not as nice as the Ampro, but it’s still pretty good (the Ampro has a physically larger cab, and a different model speaker) I was happy to see this speaker in such good condition.

And thanks for checking the speaker GB - in April 1949...  

And if you've seen photos of the projector and wondering how difficult it is to get the amp out - it's not.  It drops out the base after removing four screws.

These amps are notoriously tricky to work on—there seems to be two main schools of thought: strip the circuit out and start from scratch, or, leave them almost untouched and run with it as is.

Since this one’s from the late '40s, I’m not too confident about the state of the capacitors.  That said, there are people who using this model of a similar vintage without mods or repairs, so it’s not entirely out of the question. Maybe there’s hope!

BELL & HOWELL, MODEL 621 SCHEMATIC

I hope you don't mind the post John - so far your schematic is the only one that I can read (unlike the one above), that matches the 601

From the Hammond 1750 output transformer series, this is listed as a drop-in replacement for a Fender Vibrolux Tweed, 5F11  -  they're supposed to look similar to the original Fender transformer where possible.  That being said - this sticks out like dog’s balls when mounted on a crusty old amp. 

Materials etc are also supposed to be similar to get as close as they can (within reason I suppose).

It was a bargain, at $76 +GST (Australian dollars) from Mouser, including free delivery.  Like all things Hammond, it feels well made.  

• 8,000 ohms plate to plate
• 16, 8 & 4ohm secondary windings (these options would not have been on the original)
• rated at 15w
• bobbin wound

Turn Ratios

4 ohm turn ratio     44.77  (2004.35)

8 ohm turn ratio     31.58  (997.29)

16 ohm turn ratio   22.38  (500.86)

Frequency response

75hz to 15khz  

Inductance

19.66H

HAMMOND 1751M OUTPUT TRANSFORMER DATASHEET

THIRD MAN: Fuzz A Tron

 If you're in the USA, for $75 you may as well buy the kit.   

THIRD MAN FUZZ A TRON VERO LAYOUT

Spoiler alert - it's a modded Fuzzrite.  330k resistors instead of 470k, and it's missing a resistor that formed a high-pass filter on the output of Q2.