Despite saying never again - I now have another Bell & Howell (Filmosound) amp on the bench. This is a 621 model from 1951, which as far as the amp is concerned, looks nearly 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 the amplifier on tag boards following the original schematic—I'm aiming to retain as much of the existing amp as I can this time. It’s slow going, but I’ve got a better feel for it after the last project. That said, plans may still change.
Rear panel - unusual power connector to be removed and replaced with an IEC. Odd speaker connectors will also go. I could leave it original, but who wants to lose a weird connector and be left with an amp that doesn't work?
The valve on the far left will end up in the bin - not an audio valve. The rest look pretty nice. They have 3 x 6V6, only two are used for audio, so I have a spare. Being a cathode biased amp, matching is not critical.
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 type without a power transformer. Useful for parts, but not a viable option for a guitar amp.
BELL & HOWELL MODEL 621 SCHEMATIC & LAYOUT
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.
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. 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. The result is a surprisingly flexible, low-loss, one-knob tone control.
MODIFIED SCHEMATIC
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.
My notes are pretty bad - but you can kind of see what I chopped out.
No comments:
Post a Comment