filmosound hum/buzz

[Fred2202]

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

[bluesman60]

rectifier

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]

Filmosound 285 squealing

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]

filmobuzz

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]

got an animatophone instead of a filmosound

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]

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]

my kalart squeals the same

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.