BELL & HOWELL: Model 601 amplifier

Picked up another Bell & Howell Gaumont today, with the speaker cab - a Model 601 from 1950 (based on the capacitor dates).  Nice to see the original speaker still in the cab - I gave a quick test, sounds fine.  

The 601 is the same amp as the 621 (full details here).  The 601 and 621 amplifiers share the same part number (202000); I believe the model relates to the projector more so than the amp in some cases.

It came as the complete package with the projector and what appears to be a really exciting Australian training film about bandages from 1975.

Not sure where I will go with a conversion just yet.  I did quite like the last one that I did using an early Gibson circuit - so that's probably top of the list at the moment.  

BELL & HOWELL PROJECTOR AMPLIFIER - MODEL 601

A few minor differences with this one - transformer ends are painted, and there are a couple of large wirewound resistors that I've not seen in these before - quite good quality by the look of them.  

OUTPUT TRANSFORMER MEASUREMENTS

~8.4K with 16 and 8 ohm loads

Primary DCR 470 Ω 

10.35 VAC across the primary and measured two secondary taps:

• Tap A: 0.454 VAC

  • Turns ratio (Vp/Vs): 22.80:1

  • Impedance ratio (N²): 519.7

  • 0.454 V with 16Ω → 8.32 kΩ p-p

• Tap B: 0.319 VAC

  • Turns ratio (Vp/Vs): 32.45:1

  • Impedance ratio (N²): 1052.7

  • 0.319 V with 8Ω → 8.42 kΩ p-p

So far, on all of these that I've had, the plastic on the fuse holder breaks when I try to check the fuse.  Not great, as this leaves an uninsulated 110v conductor sticking out the back.  Bit of a death trap, but it will be replaced with a new one.  

I like that, back in the day, someone clearly thought about protecting the speaker, adding this tea-cosy-style cover.   These are normally branded as B.A.F Type 60 (British Acoustic Film) - which look just like a Goodmans Axiom from the same time period. 

The valves mostly tested OK - one of the 6V6 is a bit average, but not from the output pair, so that's good.  There is a mix of English Mullards, Australian Miniwatts.

Bell & Howell Part Number 202000

BE: Fuzz Face, SZ-3 (Sziklai pair)

Just one of my little side projects - it's a pretty hot day here, so not one for playing with germanium transistors in a big black box with a tin roof (also known as my garage / workshop).  The leakage goes nuts, and they sound like balls...  

While playing around with a bunch of old low-gain silicon transistors with moderate success, I remembered the Sziklai pair.   A minute later, and it was silicon fuzz heaven, so I had to come up with a layout to build one, hence the post.

BALMORAL ELECTRIC, SZ-3 FUZZ FACE

There are a few random values used that obviously aren't for everyone.  They just happen to be what's on my breadboard.   But hey, the post isn't for everyone; it's mainly for me to come back to later.

Transistor types are not listed, in part cause I can't recall what is on the breadboard.  They are all very low gain silicon transistors - I think about 50hfe or thereabouts.  I'll update the post with details later.

The Szik collector measured about 4.3v on a 9v supply.

SZIKLAI PAIR

What's a Sziklai pair?  It's a bit like a Darlington pair, but it uses an NPN and PNP transistor to create a single high-gain transistor out of the two low-gain devices.   They tend to act more like a single transistor than a Darlington does, which makes it handy for a fuzz face.

Before you get too excited, this works best with silicon; it's not a magic bullet for not so great germanium transistors, as leakage and noise are also multiplied.  

FURTHER READING

Small Bear has an article on Darlingtons and the Szik

D*A*M: Maggot Brain MB-71

Just a little fuzz face variant from D*AM - the Maggot Brain from 2019.  There are some more recent variations on this as well.   Could not see the cap and pot values, so just a partial trace.  Being a fuzz face, you kind of know what they might be.  I mostly breadboard before building a fuzz face cause they are so small, so I'm really just taking advantage of the form factor of the layouts at times.    

There's also a version using BC109 transistors, with a different bias set-up.  33k on Q1-C and 5k9 on Q2-C.  Some Maggot brains have a 15n output cap.  

D*A*M MAGGOT BRAIN - VERO LAYOUT

BIXONIC: Expandora

OK - so this is what happens when you agree to make a pedal, before knowing what someone might ask for.   I'm not sure that this is 100% my kind of thing, but here it is.

Yes it's a huge layout, but it is going in a 1590XX enclosure, so why not.  I started this layout using the Dirtbox Layouts Expandora vero layout (and basically made it wider), as the first version that I did had a few too many rows, for the space available in the enclosure - and I will probably only be making one of these.  So credit where credit is due...  

I may still add the guitar / bass switch - see how it sounds.  There is room on the board for it.  Not hard to work out if you want to add it.  

If you are unfamiliar with the signal flow on the Expandora - it's kind of a Rat with an extra gain stage up front, and an unusual compressor like gain control that's always on, which is combined with a couple of switches to help set the overall gain of that stage.

The fun doesn't start until "forbidden mode" is engaged - which opens the gain right up.  Talking big ugly square waves.  The Expandora moves from a fairly innocuous drive or mild distortion, to a full blown fuzz monster.     

BIXONIC EXPANDORA

This is it on the bench for some testing - just a bit of rough / temporary wiring to confirm all works as expected, before I wire it up properly.    I think the tin can LM308 opamps look cool, and they were all I had.

It's been a while since I worked on small vero, or should I say normal size.   It took some adjustment, and I did have a tiny little copper trace left on a cut that took a decent amount of time to track down.  It took a magnifying glass and a multimeter to track it down - the damn thing was about the size of a bee's dick (which is a unit of measurement in Australia). 

ENCLOSURE: Wiring rig

This was long overdue - I finally got around to making a half-decent wiring rig.  I’d been using cardboard from pizza boxes (yes really), which worked, but it was pretty ordinary to say the least.

Once the parts were laid out and the enclosure marked up, I drilled the pilot holes in the chassis, then cut and drilled a sheet of lightweight galvanised iron to match.  It turned out to be far easier than I expected, and I honestly can’t believe I waited so long to do something like this.

This one in particualr is for a Foxx Tone Machine, but as a generic layout for a 3 knob circuit in a 1590XX this will be fine.  

I quite often do most of my wiring in the enclosure, this makes things a little easier.  I'll have to make a few more, for different types of enclosures.

GIBSON: GA-30 Schematic

GIBSON GA-30 GUITAR AMPLIFIER SCHEMATICS

This is one of the GA-30 amps - there were a few by the sounds of it.

While they share a similar topology, there are clearly some differences from the one above.

Apart from changing the circuit, their appearance also varies over time - I'm not going to try and document them all, and I'm by no means an expert on the topic.  I'm just using these as a reference for a Bell & Howell conversion.  

Text below is for the first schematic 

6SJ7 MIC PRE

Grid leak 1m

Grid stopper (none)

Ra 470k

Rk 2k2 20u bypass 

Screen 2m2, 50n bypass

10n coupling cap to 500k volume pot.  100k mixing resistor to PI

6SJ7 INST PRE

Grid leak 470k

Grid stopper 47k (forms a voltage divider with the grid leak)

Ra 220k

Rk 2k2 (not bypassed) 

Screen 2m2, 50n bypass

20n coupling cap to 500k volume pot.  100k mixing resistor to PI

470k NFB from OT secondaries, 700pf in series with bypass switch 

6SN7 PHASE INVERTER

Ra 100k on each plate

Rk 1k shared (not bypassed)

50n coupling to 6V6

220k / 4k7 pair to PI grid

Tone 2m pot, mixing 250pf and 50n to grid of PI input 

6V6

220k grid leak

Rk 250 ohms

No grid stoppers or screen resistors