Recap Job (maybe?) on Bell & Howell Filmosound 385 Amplifier

[Johnny_March]

Hi Everyone,

I recently found out about how people have been messing with Bell & Howell Filmosound projectors and taking the tube amplifier section out of it and converting it to guitar use. I just acquired a very clean one for pretty cheap and I decided to drink the punch and make it into a guitar amp.

I don't have much of an idea how to date my amplifier, but based on the looks of the cabinet, I'd say it is maybe a mid to late 1960's model. The inside of the amp is extremely clean; it appears as though the amp has been very seldom if ever used. The filter caps are shiny and the leads appear to have little to no wear. They are two of the old Mallory silver can caps that have the multiple capacitors coming out of one large housing.

I know people generally like to clip out the old filter caps when an amp is really old to preserve the life of the overall circuit. I agree with doing so, but I don't really know much about the large multi-section capacitors that are in my Filmosound Amp. I don't know if they're constructed the same way as the single electrolytic capacitors. If the Mallory cans physically appear to be in good shape, should I just take them out because they're old, or would it be best to leave them in? I do see other discrete electrolytic capacitors that do need swapping out, but I'm not sure about the big Mallory cans.

I'd like to hear some of your thoughts and experiences. Thank you.

[maxkracht]

I just did a rebuild of an older filmsound amp, kind of a rusty nightmare… Glad yours is looking a bit cleaner.

You should treat the multi section cans the same as other old electrolytics. If you keep them in, keep in mind they could fail at any time, or they could last another 10 years. It’s easy enough replace them so you don’t have to worry about it. I usually leave the cans in place for appearance, and take them out of circuit. Modern radial caps are usually small enough to fit inside the chassis. Those amps are tight, but have projector related circuitry you can remove to make space.

[Johnny_March]

I just did a rebuild of an older filmsound amp, kind of a rusty nightmare… Glad yours is looking a bit cleaner.

You should treat the multi section cans the same as other old electrolytics. If you keep them in, keep in mind they could fail at any time, or they could last another 10 years. It’s easy enough replace them so you don’t have to worry about it. I usually leave the cans in place for appearance, and take them out of circuit. Modern radial caps are usually small enough to fit inside the chassis. Those amps are tight, but have projector related circuitry you can remove to make space.

Are the sections that I annotated on this schematic the ones that can be removed from the circuit without affecting the guitar signal?

Tech 385_ANNOTATED_diagram.PDF

[Johnny_March]

Please disregard the champ schematic I mistakenly uploaded. It's for a different post

[maxkracht]

Yes, get rid of the oscillator coil. Not sure what you are doing on the top left. Pin one is the input, looks like you have that grounded. Part of the chunk you are removing is some kind of local feedback loop that you could leave in, unless you want to redesign the preamp. The 34k grid resistor is good, you might want to add a 1M or so resistor from tip to ground on your input as well.

[Johnny_March]

Yes, get rid of the oscillator coil. Not sure what you are doing on the top left. Pin one is the input, looks like you have that grounded. Part of the chunk you are removing is some kind of local feedback loop that you could leave in, unless you want to redesign the preamp. The 34k grid resistor is good, you might want to add a 1M or so resistor from tip to ground on your input as well.

hmm, I see about removing the circuitry on the left. The plug marked 17087 is an RCA plug which takes an input from the projector. I think it should be taken out since it will be unused in a guitar circuit. Pin 9 is the suppressor grid of the 5879 which has an RC network attached to it. There's quite a bit of filtering there that would take some research before I fully understand how the circuit should be modified to exclude it so it doesn't become a source of noise.

And yes, absolutely right that there should be a 1M resistor between a 34K resistance at the instrument input and ground at the control grid of V1. That would make it a typical Fender-style input.

[maxkracht]

pin 9 is tied to the cathode, 2k2 and a 50uf bypass cap. Cut the cap and the 10M resistor going from the rca. I bet redrawing the schematic would be a useful exercise, it's pretty hard to follow as is. I think the rc network you are referring to is a local negative feedback loop. I was working on an older filmsound unit, so this thing isn't familiar to me.

[Johnny_March]

pin 9 is tied to the cathode, 2k2 and a 50uf bypass cap. Cut the cap and the 10M resistor going from the rca. I bet redrawing the schematic would be a useful exercise, it's pretty hard to follow as is. I think the rc network you are referring to is a local negative feedback loop. I was working on an older filmsound unit, so this thing isn't familiar to me.

I'm comparing this part of the circuit to a typical 12ax7 stage from a champ. It looks like the 50uF cap would be a cathode bypass resistor which would affect your bass response coming from the first stage. Cutting out the 10M makes perfect sense since it is tied to the rca jack and the grid of the 5879. Shouldn't we cut the .0002uF capacitor instead of the 50uF? It looks like the .0002uF is an input coupling capacitor.

Cutting the 50uF would probably not do any harm, but it would just make the amp overall a bit brighter and maybe tinnier? That is of course if my comparison to a Fender Champ input stage is correct.

[thetragichero]

my father-in-law is really into the clapton crossroads music festivals. if you look behind the folks on stage there are a TON of those filmosound amps. i got a kick out of it, he wasn't so impressed

[maxkracht]

Shouldn't we cut the .0002uF capacitor instead of the 50uF? It looks like the .0002uF is an input coupling capacitor.

Yes. I may have been ambiguous.

pin 9 is tied to the cathode, 2k2 and a 50uf bypass cap.

I meant, the rc network you are seeing is not connected to that node. It's just the 2k2 and 50uf cap to ground.

Cut the cap and the 10M resistor going from the rca.

I was referring the .0002 cap.

Having no bypass cap doesn't necessarily mean less bass, relatively. Bigger cap compared to smaller cap will definitely change the amount of bass that goes through. I could be somewhat mistaken in semantics.

[Johnny_March]

Hi. The conversion seems to be going well. It’s drawing a little too much current and I’m going to swap a couple more components out, but nothing seems to be running away in this amp.

There’s a component in this circuit that I don’t understand what it is; it seems to be some sort of optical component. On the schematic, it is directly above the 5Y3 tube and it says NE-2 with part number 27777. It is wrapped in vinyl tubing and I’ve seen it glow orange when the amp is turned on.

Would anyone by chance know what this component is? I’ve searched online for that part number but I get nothing.

[sluckey]

https://www.bulbtown.com/A1A_NE_2_Neon_ ... _p/a1a.htm

[Johnny_March]

Thanks, Sluckey. Now I definitely know that it’s a light bulb variant. Do you happen to know what it is for in this circuit? Is it for the projector and not the audio section?

[maxkracht]

Projector part. Clip it.

[Johnny_March]

Awesome, thank you!