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]

Re: filmosound hum/buzz

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]

Re:filmosound hum/buzz

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]

Re:filmosound hum/buzz

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]

Re: filmosound hum/buzz

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

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

[bluesman60]

filmosound question

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