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.  

The really large tin can capacitor was not great - it was heating up at relatively low voltage, and did not want to reform.   So time to go.  

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

BELL & HOWELL: Filmosound, Model 156

This one turned out to be slightly different to the others I’ve worked on  -  it's a Bell & Howell Filmosound, Model 156 Projector Amplifier (Part No. 13817), field-coil speaker version.   Apart from field-coil speakers, there are clear differences between the 1576 vs the more common Filmosound 621 & 179 models.  The tone control and phase inverter are not the same, and the heaters are not elevated via the 6V6 cathode.  

It also came with the original speaker cab, but the speaker had been replaced with a regular alnico speaker at some stage.  Still a nice little package.

Pulling it apart was a bit of a battle.  That huge capacitor block down in the bottom right is a nightmare to get out unless you remove the power transformer first. I really didn’t want to go that far, so I wrestled it out with the PT still in place — doable, but not fun.

BELL & HOWELL FILMOSOUND, MODEL 156 PROJECTOR AMPLIFIER

Part Number 13817

Likely to be from the late forties, maybe fifties.  

115V AC

Valve Line-up:  All RCA

3 x 6V6

1 x 6SL7

1 x 6J7

1 x 5Y3

Quite a few caps are Suzuki oil caps - made in Japan, which seems a little unusual for the time.  Maybe some work was done on the amp at a later date?  There are certainly a few areas where construction looks to be lacking, compared to other Bell & Howell amps that I've worked on.

There is the occasional Cornell-Dubilier capacitor in there and a standard mustard cap.  

Bell & Howell Model 156 projector amplifier.

You need to be careful with older models that use cloth-insulated wiring, as the insulation is often hard and brittle. I try not to disturb these wires at all. When movement is unavoidable, I gently warm them with a heat gun first to soften the insulation, then reposition them once into their final location. Where necessary, I’ll add heat-shrink or replace the original cloth insulation with fresh spaghetti tubing.

An easy way to spot this model 156 - look for the square speaker connectors with four pins.

STANCOR OUTPUT TRANSFORMER

A1218 / 138535

Primary (plate-to-plate)

• Applied test signal: 10.0 VAC (primary, plate-to-plate)

• DC resistance (plate-to-plate): 407 Ω

Secondary measurements (with 10.0 VAC applied primary)

Tap A looks great - 8 ohm speaker reflects about 8k on the primary.   Tap B is not useful for modern speakers. 

• Tap A: 0.310 VAC

  • Turns ratio (Pri:Sec): 10.0 / 0.310 = 32.26 : 1

  • Impedance ratio (N²): 32.26² ≈ 1041

  • Reflected primary impedance:

    • ≈ 4.16 kΩ @ 4 Ω load

    • ≈ 8.33 kΩ @ 8 Ω load

    • ≈ 16.65 kΩ @ 16 Ω load

• Tap B: 0.128 VAC

  • Turns ratio (Pri:Sec): 10.0 / 0.128 = 78.13 : 1

  • Impedance ratio (N²): 78.13² ≈ 6104

  • Reflected primary impedance:

    • ≈ 24.4 kΩ @ 4 Ω load

    • ≈ 48.8 kΩ @ 8 Ω load

    • ≈ 97.7 kΩ @ 16 Ω load

FILMOSOUND MODEL 156 SCHEMATIC

There are several variations on the 156.  You can find schematics here - start on page 162 for 156 models.

The schematic is not for exactly the same model, but apparently, the amplifier section is the same across the 156 V series.   Quite handy that they list the colour of the wires and resistances for transformers - the colours can be difficult to read after all these years.  

BELL & HOWELL 156 - CONVERSION TO GUITAR AMPLIFIER

I usually convert Bell & Howell Filmosound amplifiers into a single-channel 5E3, or leave them largely stock if they’re in good condition. This example was not. Leaky capacitors and some questionable previous work meant the chassis was stripped completely and rebuilt from the ground up. The original pair of RCA metal-can 6V6 valves were well on the way out and needed replacement.

The goal with this build was still very much a tweed-era circuit, but with a pentode front end retained rather than converting to a triode-based preamp. While looking for similar designs, I noticed that several classic Gibson amplifiers share a near-identical valve lineup.  The GA-30 and BR-6F both use a similar pentode input stage, with the GA-40 following related design ideas in the phase inverter and tone control.

Given those similarities, the instrument channel of the Gibson GA-30 was used as the starting point. The microphone channel has a different flavour, and while it sounds great, it offers very little clean headroom and moves into overdrive almost immediately — particularly with high-output pickups such as a P-90. Cool, but very much a one-trick pony.

 As with Fender, Gibson seems to have had a few circuit variations over time; unlike Fender, these don’t seem to be as well documented — at least not that I’ve been able to find.  So in some cases, I've taken a few liberties, but have stuck with the overall feel.   

The preamp uses a 6J7 pentode with conventional cathode bias, a high plate load, and a heavily decoupled screen supply. This keeps the gain under control while retaining the dynamic behaviour typical of early pentode designs.

The phase inverter is a floating paraphase, a topology commonly found in early-1950s tweed amplifiers and still used by some modern builders. It provides adequate drive for the output stage while remaining simple and stable.

The tone control is a Vox-style top-cut network placed after the phase inverter. Gibson sometimes implemented this control directly across the phase inverter plates. Although there is very little DC potential between the plates in practice, relocating the control avoids placing high DC voltages across a potentiometer while achieving a similar high-frequency roll-off.

The power stage is a fairly conventional push-pull 6V6 arrangement.  Interestingly, some Gibson designs also use DC-elevated heaters, which are common on many Filmosound amplifiers (though not on this particular example). It’s a feature I like, as it helps keep hum under control.

BELL & HOWELL: Model 385 Amplifier Schematic

The original schematic for the 385 was killing me - just too many part numbers listed, so I cleaned it up a bit.  I also removed the projector circuits.

I think there are two versions of this amp - this is the 6V6 version, while the other uses EL84s (which is nearly impossible to find a schematic for).

BELL& HOWELL - FILMOSOUND MODEL 385 SCHEMATIC

After the clean-up, I redrew the schematic in LTspice.  Seems to work as expected, so hopefully no mistakes...

The tone control is unique, as it uses a dual pot for the James EQ instead of separate pots for bass and treble.  Bass is cut while treble is turned up, and vice versa - or you could wire it so both bass and treble go up and down at the same time, leaving the midrange alone...  interesting.  

The dual 3meg pot is not easily found these days - in theory, resistor and capacitor values can be scaled to work with a 1meg pot, however, after spending some time on this, I can't get the response to match.   It's roughly the same, but there are obvious differences.  

The preamp could be simplified a little, and you would most certainly want to get rid of the 1meg resistor on the input.  

Hard not to notice all the little caps rolling off some highs from plates of the 12AX7 gain stages.  Something to experiment with and tune to taste.  

LTspice sim of the eq range

Original pages below

 

UNCLE DOUG CONVERSION VIDEOS

BELL & HOWELL: Model 601 Projector Amplifier

Just documenting my latest Bell & Howell conversion. - Model 601 amplifier, with 1950 capacitor dates.   

 

BELL & HOWELL: 622 Projector Amplifier, 5E3 Tweed Deluxe Conversion

Documenting another Bell & Howell amplifier conversion.

While it has a different model number and a shiny red faceplate, it's the same amp as the 621 and others in the series.  Quite often, variations of the model number has more to do with the projector than the amp. 

BELL & HOWELL 622 PROJECTOR AMPLIFIER

Stock - the infamous rats' nest.

Now with the unnecessary parts removed - although there is still more work to do.

CONVERSION NOTES

Plan A was to strip it back and rework the preamp, but there were too many out-of-spec parts and it wasn’t heading in the right direction. When I powered it up, it ran really poorly and there was a fair bit of resistor noise.

So I moved to Plan B – gutted it and built a 5E3-style amp (Fender Tweed Deluxe). It came together really well.   There’s a lot more space in the chassis once the old boards are gone.  I added a few tag strips for mounting parts – one for the first filter cap, and a couple more for the phase inverter and preamp.

Preamp

I used an ECC35 / 6SL7 for the preamp.  A single triode provides more than enough gain for a Deluxe Tweed–style amp, as the 6SL7 has higher gain than the 12AY7 originally used.  The earlier Deluxes used 6SC7 ocatal tubes for the preamps, which is almost identical to the 6SL7 when biased similarly, although the earlier models were mostly grid-leak biased.  

I added a SPDT on-off-on switch on the cathode to allow for full bypass using a 22u cap, partial with a 680n, and no bypass cap for lower gain.  At one stage, I tried this on the gain stage in the phase inverter - similar result, but I didn't like the way it was laid out in the chassis.  

Phase Inverter

The Deluxe uses a 12AX7 in a cathodyne inverter.  I kept the 6SL7 from the projector here — it's lower gain balances the extra gain in the preamp.  A 6SN7, which has even less gain, is also a reasonable option if you want something even cleaner.  Similar to the preamp, older Deluxes used a combination of 6SC7 and 6SL7 tubes here.

I recently came across the Dumble mods for the 5E3 - one of which involves adding feedback to the phase inverter.  I gave it a try and liked it.   

I used the spare valve socket next to the preamp to mount the preamp bypass caps, and the existing hole in the front panel for the SPDT switch.  

Rear Panel

I swapped the unusual power connector for a standard IEC.  A few minutes with a file and one extra hole was all it took to make it fit.  This model already had a full-size fuse holder, so that was left as is, after confirming that it was the right value fuse.

The speaker outputs were replaced with cliff jacks, as they just manage to cover the holes.  

Grounding & heaters

I used the unused 6V6 socket as the star ground point, leaving the heater wires connected there since it’s the first stop from the transformer.

The elevated heater voltage was also left as is - in a chassis this size, it doesn't hurt to have a bit of extra protection from heater hum (heater centre tap connected to the cathode of a 6V6).

 

VALVE DATA: 12AY7, 12AX7, 6SL7, 6SN7

Parameter	12AY7	12AX7	6SL7	6SN7
Amplification Factor (µ)	40	100	70	20
Plate Resistance (rp)	≈ 22.8 kΩ	≈ 62.5 kΩ	≈ 44 kΩ	≈ 7.7 kΩ
Transconductance (gm)	≈ 1750 µmhos	≈ 1600 µmhos	≈ 1600 µmhos	≈ 2600 µmhos
Typical Operating Point (per triode)	Va≈ 250 V, Vg≈ –2 V → Ia≈ 3 mA	Va≈ 250 V, Vg≈ –2 V → Ia≈ 1.2 mA	Va≈ 250 V, Vg≈ –2 V → Ia≈ 2.3 mA	Va≈ 250 V, Vg≈ –8 V → Ia≈ 9 mA
Max Plate Voltage	300 V	330 V	300 V	450 V
Max Plate Dissipation (per triode)	1.5 W	1.2 W	1.2 W	2.5 W
Heater Current	0.3 A @ 6.3 V 0.15 A @ 12.6 V	0.3 A @ 6.3 V 0.15 A @ 12.6 V	0.3 A @ 6.3 V	0.6 A @ 6.3 V
Typical Bias for ~1 mA	≈ –2 V to –4 V	≈ –1.5 V to –2 V	≈ –1.2 V to –1.6 V* see note	≈ –0.8 V to –1.2 V* see note

Notes: Values are per triode section. “Typical” figures are representative datasheet values; exact bias depends on load, B+, and chosen operating point. For 6SN7, designers commonly use ~9 mA at around –8 V (Va≈250 V); a 1 mA bias is atypical and only shown to preserve the row alignment across tubes.

Sources: RCA Receiving Tube Manual RC-30 (1975): data pages for 12AX7A/7025, 12AY7, 6SL7GT, 6SN7GT/GTB. See also Radiotron Designer’s Handbook, 4th ed., triode data summaries; and Morgan Jones, Valve Amplifiers, 4th ed., Appendix “Valve Data”.

BELL & HOWELL: Model 621 Projector Amplifier, 1951

Despite saying never again - I now have another Bell & Howell projector amp.   This is a 621 model from 1951, which as far as the amp is concerned, looks almost identical to my 601 from 1949.  I believe the main differences were with the actual projector itself - the 621 being a later and improved model.

Unlike the last amp, where I rebuilt everything on tag boards following the original schematic, apart from removing non-audio related components, I did as little as possible to this one.  Surprisingly, it didn’t take as long as I expected.  Many people gut these and start fresh, often going down the tweed fender path.

It's in the 12w - 15w output range, using a pair of cathode biased 6V6.   Controls are simple: volume and tone.  The tone control is best described as a bass cut, with a few tricks to it.

POWER

110v on the primary side, I'm in Australia, so it needs a stepdown transformer.  There's also a weird 91v tap, which I think has somehting to do with the projector.  It also had the infamous death cap across it, which I removed.

6.3v heaters with centre tap, elevated via the 6V6 cathodes 

5v heaters for rectifier

B+ is around 350V

OUTPUT TRANSFORMER

It looks a little small for an output transformer, but it doesn't seem to be lacking in any areas.  

Primary - Yellow, Black (CT), Red.  420 Ω plate to plate 

Secondary - Black (GND), green 8Ω, blue 16Ω

• 16 Ω tap → voltage ratio ≈ 23 : 1 (15.55 V ÷ 0.675 V)
• 8 Ω tap → voltage ratio ≈ 33 : 1 (15.55 V ÷ 0.474 V)
• Reflected primary impedance comes out at ≈ 8.5 kΩ from either tap

621 TONE CONTROL

Probably the most interesting part of the amp is the tone control.  It's quite different to the better known model 385 Filmosound.  Not even a close comparison, as the 385 is a James style equaliser with a very hard to find 3meg dual-gang pot.

Instead of the familiar FMV or James/Bax stack that sits between two stages, this amp has its entire tone circuit right on the cathode of the ECC35 triode - it's primarily a bass cut.  A single 50 kΩ pot and three capacitors juggle the amount of local feedback inside that valve and the amount of global negative feedback (NFB) returned from the output transformer. 

Keep in mind that this is from LTspice, so it's not going to be spot-on, but it will give an idea of what it does.  All three capacitors interact with one another and the negative feedback.

Decreasing the value of C27 shifts the frequency where the bass starts rolling-off down.  I changed mine from 50n down to 33n, as the 50n cap in this amp was dead, and why not...  If you make it really small - like 1n, you actually get a mid-range boost as part of the sweep (according to LTspice).   I'd say don't take my word for it, clip some caps in and see what they sound like.

C26 affects the overall bass response, though it operates in a slightly unusual way. Best to leave it as is.

C28 is a bit harder to describe. It also influences the bass roll-off, particularly around a dip in the response—admittedly, that’s not the best explanation, but it’s a tricky one to summarise.

 

VALVE LINE-UP

6J7 pentode for the preamp - RCA

ECC35 for a gain stage and phase inverter - Mullard

6V6 for the power amp - Radiotron Australia

5Z4 rectifier - Mullard

Before I got started - how about we just call it "an efficient use of space" and leave it at that.  This is why a lot of people just gut them and start fresh.  You literally can't fit a soldering iron down in some areas.

Clean-out underway – removing all non-audio components and doing a bit of rewiring to make things easier to work on.

It came with a friend (on the right) — a widow-maker without a power transformer.  It's useful for parts, but not really suitable as a guitar amp.  One of the electrolytic caps had exploded at some point, so the internals were a bit of a mess.

BELL & HOWELL MODEL 621 SCHEMATIC & LAYOUT

Full user manual can be found here.

BELL& HOWELL 621 AMP SCHEMATIC - TRIMMED DOWN

Trimmed back to the essentials.  I removed some of the weird feedback around the preamp pentode, as it doesn't really add anything useful.  I tried to match the numbering to the original schematic as much as possible.

6J7 PENTODE INPUT STAGE

If you only make one modification, shield the wire running to the cap of the 6J7.

The cap connects to the grid — effectively the input of the pentode, so any noise it picks up gets amplified straight away.   Keep in mind the whole amp was originally housed inside a cast iron projector chassis, fully shielded.  They were never meant to be pulled out and run standalone.

  

My notes are pretty bad - but you can kind of see what I chopped out.  The only addition I made was the 1meg resistor on the input to ground.  

Components shaded in red have been removed.