filmosound hum/buzz

[plutek]

greetings!

i've got a bell & howell filmosound specialist 285 which i want to use as a guitar amp. while still in the projector, the amp worked excellently with guitar plugged into the microphone input and the " 8" & 12" " output feeding a 15" cab that i have here. unfortunately, when i took the amp out of the projector, it has a constant hum/buzz -- still gets signal from the guitar amd amplifies it fine, but the hum and buzz is always there too. i don't believe i disturbed anything, other than unplugging the film sound input and connecting a new power cord directly to the power input on the back.

is there something that normally needs to be done when these are pulled out of the projector? does the film sound input need to be terminated somehow?

thanks!

[plutek]

uh-oh.... i'm talking to myself here. oh well....

so i grounded the case (i.e. 3-prong power cord, instead of 2-prong as before, with the ground connected to the amp case) and the hum/buzz is gone.

two remaining issues:

1. it's still somewhat noisy. is there an easy way to take the film sound input out of the circuit, so it's not contributing to the background noise? anything else to look at here?

2. when i turn the volume up past about 2/3, i get some strange squelching/squealing noises. anybody know what that's about, and if it can be easily solved?

yes, i know these may be somewhat complex issues, and i'm a novice. but any help you can offer would be most welcome!

thanks in advance!

cheers.

[Brad1]

You may have to tweak some circuitry on that thing to accept a proper guitar-level signal. Did it always squeal past 2/3, even before you dismounted it?

Brad1

[plutek]

hmmm.... sorry, not sure about that.

[Enzo]

Pull the photo-tube out of the corner socket and see how it sounds. It detects the light in the room and appies it to the signal path. You don't want that.

[plutek]

yeah, i've done that... helps some. also, i've noticed that this amp is quite sensitive to field effects -- proximity to bodies, sources of magnetic field, etc.

perhaps putting it all in a well-ventilated ferrous case would help (sheilding)?

[Enzo]

Does that condition remain when something is connected to the input? An input jack lacking grounding contact for the tip will leave the inout unterminated.

[stu1200]

Hi all,one of the most effective ways of eliminating hum is to shield the large red valve behind the optical pickup valve,I cut the top off a Can of Cascade Pale Ale(local Tasmanian beer) and carefully placed it over the top of this valve(optical valve long since removed).This seems to shield it from much of the hum generated by the output tranny,and everything else,and yes the grounding helps too.
Ps. I too get the squeeling noise when turned up past 2/3 rds.
Regards Stu 1200.
PPs I have another of these amps that requires a new Capacitor(the huge one 16-16-8 uf at 450 volts)
any Ideas would be greatly appreciated.

[Fred2202]

I have modified a few of these filmosounds and they can turn out pretty nice. Working on them in that cramped chassis is like trying to build a ship in a bottle though. I would get rid of the oscillator and "Eye" tube and it's related circuitry. I redid the heater wires and twisted them tightly. I also usually added an isolating input jack and grounded it with the rest of the stuff from the preamp. Adding somekind of homemade shields for the preamp tubes also helps. The amp will be noticeably quieter when installed in a shielded casing of some kind. Amps usually hum more when they are out on the bench. If the electrolytic caps are original (in which case they are probably 30 - 50 yrs old) they should be replaced..this will help with noise also.

[Fred2202]

Maybe I got a little ahead of myself here with my last post....but that amp was made in the 1950's. The components inside the chassis are at least 50 years old. I would not plug a guitar into that thing until a proper 3 wire AC cord was installed and all the electrolytic caps were replaced in the amp, and the internal wiring was checked out. I would have to check that particular schematic but it probably included a "death cap" on the hot AC side of the power transformer also. Please do not go poking around inside the chassis unless you fully understand how to work safely with tube amplifiers. That cap on top of the metal tube with the wire lead on the right hand side of the amp (I think it is a 6J7 tube) is probably the connection for the plate and carries some lethal voltage both with the amp on and with the amp off.
These little Filmosounds can make some nice little guitar amps but they are not the best choice for a novice to try to modify or fix. The chassis are extremely crowded and a lot of the components are very difficult to get at.

[Enzo]

I think that is a grid cap on the preamp tube, and that is why it likes a shield over the cap.

[Marc]

I have the same amp on my bench right now.
The cap is for the grid on the 6J7. the amp has a 3K3 resistor right at the cap. About half of the bumble bee caps are leaky-the PI caps were the worst, so far... Next I have to find the leaker in the tone circuit-there are from 3 to 6 volts on the tone pot depending on the position.

The filter caps appear to be sealed oil-in-can types (can you say PCB?)
The 6V6 next to the 5Y3 is not in the audio path.

I killed the squealing with a 51pf cap from the volume pot wiper to ground, not elegant or a real solution, but it killed the problem. I'm considering stripping out all of the unneeded circuitry to give a little wiggle room inside.

Has anyone seen a schematic? There's not much to the circuit but it's so jammed in there I'd rather not try to trace it out...

Marc

[Fred2202]

Marc if you send me your e-mail address I will send you a pdf file with all the Filmosound projector schematics including incremental circuit revisions. It is on my work computer. The file is on the large side or else I would post it.
I have a 285 chassis stored on a shelf for a future project. It does have two sealed cap cans buried inside the chassis. It is filled with the old mica domino type caps also but this one does not have any bumble bee caps that I can see.
With these I usually ripped out the oscillator tube, eye tube and related circuitry just to try and make some room in the chassis. Also the less unneeded stuff the better as far as hiss and hum goes.

[Fred2202]

I found the link to the filmosound schematics site..here it is.

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

[bluesman60]

i have a cople filmosonds that i really like and have fun tinkering with but a situation has arisen with one.
the amp takes forever,maybe 5 full min,to warm up enough to get sound but then works great.....any ideas?thanks

[Fred2202]

I would check the rectifier tube...probably a 5Y3GT.

[bluesman60]

thank you for your response.i did check the rect tube and the others as well,they all tested out good,and even put one from another amp in just in case but...nope.im going to change all the big caps and resistors,maybe that will do it

[toober]

Hi,
I see others have run into the problem with this amp squealing. I don't know if this will work for anyone else but this is what I did. I bought a 285 off of Ebay. I read on this forum that there was a photoelectric eye type of tube that could effect sound. The first thing I did was remove that tube. When I got my speakers and guitar connected I powered it on and when I turned the volume up past 5 it would just squeal something awful. I tried putting the eye tube back in but the squeal remained. I thought about it for a while and on a hunch I decided that the eye tube was designed to be a part of this amp's circuit, so taking it out or tearing out that whole tube's circuit would also probably effect the amp. (cap/resistor values were setup to incorporate this "eye" so pulling it might throw values of other components off) I decided to try just "blinding" it by wrapping electrical tape over the window in the black cover that's over it and the other small preamp tube. Presto! I could play full volume with only the slightest hum and no squeal. I was concerned about heat being lock in by covering the opening in that cover with 2 tubes in there. Again I thought, this cover was designed to be over these tubes with the window uncovered so let's try something else. I removed the tape from the cover and put it over the window in the eye tube itself and put the cover back on with the window open. Again Presto! full volume and minimal hum. So for me the answer was to simply blind that light reactive tube. Hope this might help someone else.
JC

[Enzo]

That will work, but it does it by attenuating the amp. The photo tube is there to pick up the optical soundtrack from the film.

[Fred2202]

Maybe that photo eye tube is providing some kind of negative feedback that kills the squeal in that particular amp. The Filmosounds that I have had I just pulled the eye tube out and clipped out the wires going to the eye tube socket and they worked fine. Of course I had to spend hours replacing all the 50 year old caps to get rid of the hum. And then replace a lot of the old carbon comp resistors to get rid of the crackling and hiss.

[bluesman60]

since i first posted my question i have since finished eliminating all the extra projector dedicated stuff,a power and preamp tube plus the photoeye,recapped and changed a ton of resistore...it's stone quiet and is a great little harp amp...
i have since started on a 285 model but not as nice as the 385....
thanks for the help here

[dextervonbakata]

excellent info!!! thanks for everything. i arrived here doing an investigation before buying a "professional" filmosound at a flea market. here in colombia sa, two weeks after i first saw it and thought what a nice guitar amp i could make out of it or all the tubes an trannyes i`d get , even if the projector was busted. that day another man had already bought it i arrived at end of the transaction . the recent new owner offered it to me willing to make a quick gain. i examined it closer and found out it was the transistorized version . i was glad i didn`t buy it. as an echange and having prayed to the lord of tubes a Victor- kalart appeared a few weeks latter, a better amp i deem, same input 6j7, as wellas two 6l6s,a 6sj7 a 5u4 and an unrecognized single triode, looking like one half of a 12ax7 in a 7 pin envelope. my first 6l6 amp.had a bunch of black beauties gone and lots of shifted or damaged resistors, electrolitics seem alright.though, i`ll change those 16 mfds, to maybe 22 or even 40 mfd wddaya think?? will it improve performance?? thanks for sharing your knowledge online, some rookies with only a couple years into tubeland hail you as makers of many navegation maps.. !!!

[lowell]

Originally Posted by Marc

I have the same amp on my bench right now.
The cap is for the grid on the 6J7. the amp has a 3K3 resistor right at the cap. About half of the bumble bee caps are leaky-the PI caps were the worst, so far... Next I have to find the leaker in the tone circuit-there are from 3 to 6 volts on the tone pot depending on the position.

The filter caps appear to be sealed oil-in-can types (can you say PCB?)
The 6V6 next to the 5Y3 is not in the audio path.

I killed the squealing with a 51pf cap from the volume pot wiper to ground, not elegant or a real solution, but it killed the problem. I'm considering stripping out all of the unneeded circuitry to give a little wiggle room inside.

Has anyone seen a schematic? There's not much to the circuit but it's so jammed in there I'd rather not try to trace it out...

Marc

I have this amp and I made a little aluminum box to go over the 6J7 and it cured the squealing. FYI.

[dextervonbakata]

with its 6j7 aluminum box to ground removed , i also made a little hat just covering the upper part and it also works and looks better, i`ve seen some strange red tubes with some original hats as these. a 6k7 works just as well in this position.