Refurbishing a Filmosound amp. Advice requested.

[platonicbomb]

I have several filmosound amps: 15W, 6V6 configurations. They are from projector models 179, 285 and 302.

I understand some of the members here have a lot of experience with these pieces. I love the sound of the Filmosounds but I don't have any of them functioning right now.

I understand I need to unplug the optical tube.

I believe I should put a three-prong cord, and I'd appreciate instructions on how to do this.

I believe I need to re-cap these amps. I have never recapped an amp, so I need some instruction/help/advice about how to do this safely.

I'll also need to install 1/4" input and speaker jacks.

These three procedures should get me pretty close to having a functioning amp.

I think it's possible to attach photos and schematics and I'll work on that.

Is anyone up for walking me through some procedures? Thank you!

[atmars]

Why, in your opinion, don't they work?

[platonicbomb]

Thank you for your reply.

SOUND:

Typically the volume of noise (hum etc) is higher than the volume of guitar.

On the amp that worked best in the beginning, there was always some hum. A couple of days after beginning to play through it, the guitar volume dropped considerably and a very loud hum took over. A hum like a train horn.

I believe this means that the caps, which held out for a couple of days, were not stable and lost their charge. I haven't turned it on since because I understand that using an amp with bad caps can damage tubes and possibly other components. So I believe I need to replace the caps.

The other amps sound similar if they sound at all.

Possibly some of the humming comes from having ungrounded power cords.

INPUTS:
Not all the amps have 1/4" inputs. None have 1/4" speaker jacks.

[atmars]

If you look on the back of the amp you are talking about, there will be a 5 digit number on the back of the chassis near the fuse. use this number to find the right schematic so you have a reference point. then you can get to replaceing the speaker jacks, filter caps and identifing which tubes are which.

Post the schematic.

[Enzo]

You say you understand you NEED to unplug the optical tube, but have you done so? It doesn't seem to me that your filter caps would work OK for a little bit then suddenly fail altogether. On the other hand, allowing the room lighting to fall on the optical tube can cause the sounds you are hearing. Just yank it out of there.

Over in the build your own section, there have been several threads on such conversions. I suggest you look them up and read through them.

[platonicbomb]

OK, finally got my power cord back so I could try these out.

The optical tube has been removed. The 6J7 tube remains... seems that's necessary.

On all amps there is squealing past about 3/4 volume. I recall there are some posts here about that and I'll check them out.

All tubes in use have been tested.

I'll name the amps by the color of the power transformer: Black, Silver, Gray.

On Silver, there's a squeal or whistle coming from the amp itself even after speakers are unplugged (and while they're connected too).

Gray makes some ungodly loud humming/hacking noise through the speakers that is louder than the guitar signal.

Black has a squeal/whistle but through the amp regardless of volume. It gets much louder past 3/4.

Good news is the guitar sounds great if you look past all the extra noises.

[atmars]

On Silver, there's a squeal or whistle coming from the amp itself even after speakers are unplugged (and while they're connected too).

I don't know what this sound would be, but in case you are unaware, it's generally a bad idea to run a tube amp without a speaker connected, or to plug or unplug a speaker when the amp is running. Doing so usually falls some where between risky and catastrophic.

i think i would start with the easiest sounding one first. Grey seems like a good place to start

Gray makes some ungodly loud humming/hacking noise through the speakers that is louder than the guitar signal.

The place i would start on grey is by replacing the filter caps. This is something that can cause hum, and the filter caps are fifty years old - beyond their expected life span. It is also possible that a ground wire or some other connection is loose or missing. After the cap job this can be determined (possibly) by close inspection or "chopsticking".

Look on the back of grey for the five digit number that tells you what model it is. Then the schematic can be found and it will be easier to start the rehab process.

Posting photos is also a good idea.

The optical tube has been removed. The 6J7 tube remains... seems that's necessary.

6J7 is a preamp tube and can not be removed. The optical tube and be removed as can the oscillator tube. This is the third 6V6 that can be identified by finding it on the schematic.

[Rayme]

I have worked on a few filmosound 179 amps and they are the hardest so far. there is just no room to work with in these units, very tight. I found you can improve the sound by changing the 2-pin connectors to 1/4" jacks and I isolate the jack from the chassis. The 16 ohm side always seems to play louder than the 8ohm. You are right, these do put out a nice sound and are well built but tough to work on. Good luck with your project!

[baddog]

I am currently working on a filmosound amp and i found that if a signal whire is running to close to the output transformers windings it affects the amp. I had the signal wire running real close to transformer amd the tone control made a squeal over half way . Very odd.

BD

[Enzo]

The signal voltages and currents are highest in the output transformer in any amp, and they vary with the signal. Put a wire next to that and they will induce that signal into it. if it happens to go to an input stage, you have made an oscillator.

[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. I didn't even add the 3 prong power cord yet and it's silent at idle and minimal hum cranked! So for me the answer was to simply blind that light reactive tube. Hope this might help someone else.
JC

[Rayme]

This is interesting and worth a try. Thanks for the info JC, Rayme