Filmosound 385 Filter Caps and Awful Detune

[soundabout]

Hello, I'm new here and was hoping to find some feedback discussion to a few issues with my newly acquired filmosound projector amp model 385 (model believed anyway). I've played thru several different filmosound amps before that were altered either extremely or just a little and have been vary impressed with the sound of them ...they have a tone that wont let you turn it off and go do something else. Ok here's mine: mad humming with this unit from the first time i powered it up and horribly odd harmonics to the point of making my perfectly in-tune guitar sound completely out of tune (??). i have a av transformer but it's tucked away stored someplace and i was just not into the process of bringing it up slowly in stages because i deduced preserving the original caps might really only extend the inevitable back a little longer... i.e. gonna have to replace them soon anyway because they are about 50 years in service. I have a tone of other questions swirling around in my head about how to make this amp into a peach of a home fun amp or barroom blues cracker, but first things first, i would like to get idea's about what others have done when dealing with replacing the power supply caps such as slight or radical mods to the values in filter cap train, re situating the physical positioning of the cap(s) and what brands and types would suite it and will this alone (complete cap restoration) help with the grossly over stated odd harmonics issue?or is it something else perhaps in the preamp circuit?? btw: I've changed the tubes out and still have the same issues. I'm keeping in mind probable circuit changes of cutting down what seem to be overages in voltages (in-terms of guitar amp needs that is) that are inherent in the design, as well as from trimming out some of the unneeded circuitry like the photo lamp, oscillator and 5879 preamp tube which all 3 I've already done for the most part and still have signal passage to output. so, all that being said what are some good ways to reconfigure the PS cap situation keeping in mind it will be a guitar amp from here on out? and any light shedding theories or knowledge about the odd harmonics and does and don't s within mods and repairs that lend accommodations further refinements to the amp's circuitry. thanks!

[ReadyTeddy]

Plan on replacing all electrolytics. Think about replacing any paper-dielectric caps as well as they can become leaky over time. If your amp is quite old, and the paper caps look like waxy paper cylinders, just replace them. If they are encased in molded plastic, they might be OK. Old carbon-composition resistors can be out of spec, so check those as well (you may have to unsolder one end of some resistors to get an accurate reading. It would also be good to check DC voltages at all cathodes, plates, and grids to see if anything looks amiss, and if you have access to an oscilloscope, look at AC voltages, as well. If you want to increase the size of filter caps, be sure to check the max cap size recommended for the particular rectifier tube your amp uses--this spec applies to the cap that is connected directly to the rectifier. I would recommend getting the amp basically working before modding it. That way you'll know if problems with the amp are mistakes in the mods, or some defective component(s).

[soundabout]

Cool! I hesitate a little to check voltages because the electrolytic's measure extremely High …so they are bad. Is my thinking accurate on that?? i.e. could i get reliable readings from the the tube electrodes you mentioned while the electrolytic caps are reading super high and probably bad? possibly throwing the b+ off spec along with ac signal passage? or start at replacing the E caps and move forward from there? Paper caps, there are only 2 paper in oil waxed covered caps in the whole amp and yeah i will yank them completely out in time cause they are part of the photo lamp & oscillator circuit which are not needed for guitar amp but I'm going to leave it along for now until i know what to do with the absence of load to a once needed supply of current and voltage if anything is needed at all. thanks for the quick response and it helps to get focused on a place to start. still would like to here from someone who's actually recapped one of these particular amps to learn a tried and true method of things like what brands , where to get them, can the exact multi can electrolytic( be found? or can i use a train of single axial types etc?? there are 2 can type multi elytic caps for the power supply, cap #1: 1x 40uf @ 400v, 2x 20uf @ 400v, 50uf@50v. cap #2: 3x 15uf @ 350v. if anyone out there can direct me or put together a cap cluster that would be a suitable replacement… i'll buy it from them. thanks!

[olddawg]

These filmosound amps have a lot of "extra" circuitry because of their intended use that is not necessary for any guitar amp. It gives you a lot of options as far a heater current when choosing an amp design. If it were mine I would gut it to the chassis and transformers and rebuild it using a turret board and new ceramic tube sockets, pots, etc. Might save you a lot of headaches in the long run and you have to buy many new components anyway.

[ReadyTeddy]

Cool! I hesitate a little to check voltages because the electrolytic's measure extremely High …so they are bad. Is my thinking accurate on that?? i.e. could i get reliable readings from the the tube electrodes you mentioned while the electrolytic caps are reading super high and probably bad? possibly throwing the b+ off spec along with ac signal passage? or start at replacing the E caps and move forward from there? Paper caps, there are only 2 paper in oil waxed covered caps in the whole amp and yeah i will yank them completely out in time cause they are part of the photo lamp & oscillator circuit which are not needed for guitar amp but I'm going to leave it along for now until i know what to do with the absence of load to a once needed supply of current and voltage if anything is needed at all. thanks for the quick response and it helps to get focused on a place to start. still would like to here from someone who's actually recapped one of these particular amps to learn a tried and true method of things like what brands , where to get them, can the exact multi can electrolytic( be found? or can i use a train of single axial types etc?? there are 2 can type multi elytic caps for the power supply, cap #1: 1x 40uf @ 400v, 2x 20uf @ 400v, 50uf@50v. cap #2: 3x 15uf @ 350v. if anyone out there can direct me or put together a cap cluster that would be a suitable replacement… i'll buy it from them. thanks!

I'm not sure what you mean when you say the electrolytic caps are reading super high. The power-supply caps will have the highest voltages in the amp--that's normal. Antique Electronics tubesandmore.com has a good selection of electrolytic caps in various styles. The multivalue can types are generally the most expensive. Get a schematic for your amp if you don't have one. If you can't get one you'll have to either trace the circuit out by hand or perhaps take olddawg's advice and strip the chassis down.

[sgelectric]

The schematic can be found here on pages 215 and 216

http://www.film-tech.com/warehouse/manuals/AMPS.pdf

Hope this is useful.

SG

[frankjc]

how's this project coming? i have a filmosound 385 i just pulled out of a projector. i'll be looking for help soon.

[soundabout]

Hey thanks for the responses… I haven't gotten too much further with the project due to things getting so busy around the holidays but I'm still committed as ever to rewire this thing into a usable guitar amp. My ole pops has offered to build a combo cabinet for me that should accommodate the amp and a 12"-15" speaker and I'm stoked about that because he's a for-real woods craftsman. All that said i believe that i'll need to began by restoring the filter cap section so that i have a good clean & stable power supply platform to work from. I have confidence in my hardware abilities as i have a lot of experience in fastening electronic components and soldering and proper insulation where its needed but I'm not very knowledgeable about electronics in the technical side of things. Readyteady: what i meant about the caps measuring high is the uf value (when discharged obviously) when taken with a capacitor tester. ..and thanks for the link to buy the caps i'll check that out i hope they have what i need. I contacted a fairly well known filmosound 385 conversion expert about doing mine but he ask for 500 bucks to do the conversion for me??!! are you serious!! i have to say that after hearing his demos on youtube I'm quite impressed and really wish to get mine sounding as good or close to as good as he gets them to sound but 500$ ..no bleeping way can i sink that kinda cash into something like this. anyway, I sure appreciate any and all feedback as i work slowly towards the tone I'm looking for with this amp and sorry to frankjc for not having anything new yet to report but i can offer a few suggestions if you haven't already figured this much out. 1. make sure it powers on and is capable of producing audio and hope fully any kind of signal passage from front to end(speaker output) and if it does then at least you know its a keeper iron and good block to work with then as i was talking about above might be a good idea to make sure the power supply (filter caps) capacitors are functioning properly and if the amp sound quiet with little or no 60cycle hum when amp is powered on and passing some kind of signal it's a good sign that the caps are still good but it still might behoove the over all end result to go ahead and replace them first anyway due to their age as they might be on the verge of going bad and/or ready to pop! which i don't recommend finding out about, And always be certain the caps are discharged of any static currents because they are capable of releasing enough static charge to shock the heck out someone even if the amp is powered off and has been unpowered for sometime! anyway i'll keep posting my progress and pics of the cabinet when i get it. thanks and hap belated new years!

[Enzo]

professionals and people with considerable skill and experience do not work for $5 an hour. I charge $60 an hour for example. if you send someone an amp to convert, and he takes 8 hours to do it, there is your $500 right there.


if you have not already, get rid of that optic tube. it is turning all the various fluorescent lighting and any other light in your shop into noise.

[soundabout]

thanks Enzo… actually i removed the entire circuit for the optic tube and bypassed the 5879 rewiring the input + straight to the 1st stage of the first 12ax7 with a 68k in series and it does exactly the same thing it did before i did any changes to it at all. I heard someone refer to a similar problem on the home built deluxe 5e3 as having a "ring modulator" sound when playing thru it… i wonder if that description is of the same problem I'm having with mine where there's several out of tune harmonics happening when i play thru mine?? about the 500 bucks thing: I respect that totally. whether someone does it full time or moonlighting the burden must be offset with equity and if someone knows their stuff they should get what they ask for though i personally don't have the budget to pay for a conversion more than a 6-8hrs at $25 per hr. if i were having an nice tube mic pre worked on i would expect it to be in the range that you charge and would have to eat it with a smile thanks!