Thursday, September 29, 2016

BELL & HOWELL Filmosound 285

Bell & Howell FILMOSOUND 285, modified for guitar

This Bell & Howell FILMOSOUND 285 film projector amp was brought in for modification for use as a guitar amp. New speaker output jack, new AC jack w. chassis ground, new output tubes, redundant circuitry removed, modified input stage, pot cleaning, additional shielding and some external polishing and here you go. It might not be a BERNIE but it is a start!

It is a nice little head now and can be used with any 8ohm cabinet or speaker. Here it is being tested through a tiny Auratone Sound Cube. It has some noise issues due to the exposed lead on the preamp tube (which carries the input signal to the grid) catching noise and feeding it to the amp. It would have originally been inside the projector housing and therefore more shielded. I reused part of the socket for the "exciter bulb" connection to make a nice little hat for the preamp tube and this helps a lot. There is also good ol' Johnson noise from aged (pre 1955) resistors and hum from old capacitors (this unit has all large caps inside 2 sealed tanks attached to the chassis). A bad 6V6 was straining the power supply and pulling the voltage down as well. Otherwise it was in great shape considering it is 61 years old! It is still using the original full wave rectifier, preamp pentode and driver/phase splitter and everything else.

These amps could be obtained very cheaply in the past as educational or military surplus. Their dated connections and sometimes unusual tube combinations combined with a small chassis stuffed with additional unnecessary elements (used only by the absent film projector) made them less than ideal as guitar project amps. They were cheap though and powerful enough to be useful!

They were commonly gutted and redone (it seems from various forum threads) using only the original transformers and tube sockets and there is a legacy shrouded in mystery surrounding one builder named Bernie from Hamilton. These "BERNIE" amps were Bell & Howell amps modified to some degree for use with guitars. From descriptions of his amps it sounds to me that he left the bulk of the original circuit in place. The original chassis contains a transformer, two tubes and their associated caps and resistors, a tuneable coil and various other bits and pieces that can all be removed to leave a more manageable and recognizable circuit without compromising the audio path in any way. BERNIE amps are often described as "rat's nests" and from this I am guessing they were simple (but effective) mods to the extant circuit rather than rebuilds. There is no practical way to rebuild the amp without removing the extra parts I mentioned due to internal density and order of assembly.

Anyway, that is speculation. Any tube amp can be tweaked in many ways. I simply changed the input series resistance and removed negative feedback in the preamp. There is global negative feedback still in place and everything in the tone stack etc is original. New signal-path resistors and power supply and coupling capacitors would be in order if the customer deems the noise floor too high.

Portion of Filmosound 285 Amp schematic showing input stage with optical tube (top left)
gain stage and phase splitter (top right) and unused oscillator circuit (as well as filter caps).


In 2016 it is possible to download the schematic for any B&H amp for free and this demystifies the process somewhat. At first glance this amp is a tiny nightmare!