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.

622 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.

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.

Phase Inverter

The Deluxe uses a 12AX7 in a cathodyne inverter.  I kept the 6SL7 from the projector here — its 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.  

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.   

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.  

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.

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.  Best to keep that part untouched.

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).