BELL & HOWELL: Filmosound, Model 156 Projector Amplifier

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.