BELL & HOWELL: Model 156 Projector Amplifier

This one turned out to be a slightly different Bell & Howell to the others I’ve looked at. The 156 is set up for field-coil speakers, which already puts it in a different camp.  Apart from that, there are clear differences between the 1576 vs the 621 / 179 models.  The tone control, phase inverter are not the same, and the heaters are not elevated via the 6V6 cathode.

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 MODEL 156 PROJECTOR AMPLIFIER

Bell & Howell Part Number 13817

115V AC

RCA Valves

3 x 6V6

1 x 6SL7

1 x 6J7

1 x 5Y3

All of the couling caps are Suzuki oil caps - made in Japan.  There is the occasional Cornell-Dubilier in there as well, and a standard mustard cap.  

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.

MODEL 156 SCHEMATIC

There are quite a few 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.  

CONVERSION TO GUITAR AMPLIFIER

I usually turn these into a single-channel 5E3, or sometimes leave them mostly stock if they’re in decent condition. This one definitely wasn’t - leaky caps and some fairly interesting workmanship - so it’s been stripped right back and I’ll be starting fresh.

I’m aiming for a tweed-era vibe, but I think I’ll try a pentode preamp this time around.  The gain just needs to be scaled back to match a single triode stage, which isn’t hard to do.  Keeping the gain down also helps reduce the chances of issues with a microphonic pentode.

That said, slightly microphonic valves have never really bothered me, especially when the amp is separate from the speaker cab.   I'd never replace a great sounding valve, just cause it makes a noise when you hit it with something - just stop hitting it...  

My notes for gain of about 30 - 32 for a 6J7, roughly the same as the 12AY7 5E3:

Grid leak: 1 MΩ 

Grid stopper: 33kΩ 

Plate load (Ra): 68 kΩ

Cathode resistor (Rk): 1.5 kΩ, no bypass cap (bypassed jumps up to a bit over 100)

Screen feed resistor: 470 kΩ 

Screen bypass cap: 100 nF from screen to cathode