Documenting another Bell & Howell amplifier conversion - this time I'm going to try replacing the EF37 pentode preamp, with a 6SL7 octal dual triode, with a Fender style one knob tone control.
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, the model number has more to do with the projector than the amp.
There are a few versions of these floating around - I went for the 1971 9V version, simply as 9v is more convenient, and I was thinking about something different to use one of the large gapco enclosures for.
You could drop the volume pot if you wanted something a little more vintage correct, but given the very high output level, most people want this.
Just a Fuzz Face layout that I used recently and want to make note of - it's based on the one knob fuzz line, which is a handy little layout, but didn't quite suit my needs. With some minor adjustments, it can be used for most fuzz face variants - component table here.
I've had a couple of questions about these recently, and out of curiosity I decided to take a closer look. In 2024, Sola Sound released a few versions of a hybrid MKI, built by Dave Main of D*A*M.
So what is it? It’s essentially a standard MKI circuit, but with the first two germanium transistors replaced by silicon transistors. Because silicon devices don’t leak like germanium, a few extra resistors are needed to get things working properly.
As this is an incomplete trace and there are circuit variants - use the info below as a guide only.
That being said, I have breadboarded this with success - transistors matter.... It sounded a lot like the video demo below, and it was easy to tune playing with input and coupling capacitor values.
I know right - a regular vero layout for a change. While it's called a fuzz, it has quite a bit of range. Clean(ish) to an overloaded squishy fuzz Schematic is here
