Filmosound L-1 6V6 Volume pot issue

[atmars]

I am working on a filmosound conversion. So far I have removed the oscillator circuit and the negative feedback circuit, in large part to make room in the chassis.

When I turn the volume pot past three quarters a crazy static type distortion/feedback type sound occurs, but not squealing. This phenomenon existed before I removed the negative feedback loop.

I have tried the following to remedy this situation:

Bad pot? I replaced the pot with a new 500K pot. No change.

Pot value too large? Original pot is supposed to be 500k but is really closer to 350k. I reduced the value to 300k with a resistor. No change.

Bad preamp tube? Tried a different 6SL7. No change.

Layout? Tried moving wires. No change.

Also changed wire to grid to a shielded cable. No change.

Any ideas on what might be causing this effect? Another post had a similar question about a filmosound, but the i don't believe the problem was ever solved.

I have only been working on tube amps for about a year, so my knowledge is pretty limited. Any ideas are greatly appreciated, no matter how obvious they may seem to the more educated.

Also, any other potential changes the anyone notices, that would be an improvement, I'd like to hear about.

thanks in advance.

here's the link to the schematic:

http://s294.photobucket.com/albums/m...oundL-1Mod.jpg

[atmars]

Better link?

http://i294.photobucket.com/albums/m...oundL-1Mod.jpg

[platonicbomb]

filmosound craziness

I have about 6 Filmosounds--4 of the early '50s versions, 2 from the model 302 projector. All should be amazing guitar amps, but i've had them sitting around doing nothing because I've realized they're much more complex than any of my techie friends or I imagined.

You seem to have some experience with these things. I have read a lot of posts here about them, but what I can't find is a clear explanation of what steps one needs to take to make them viable guitar amps.

Is this something you'd consider laying out for us?

[Marc]

Atmars,
You didn't mention if you check or changed out the coupling caps. I've rebuilt one of these filmosound amps into a spitfire styled circuit with good success. The pentode front end in your schematic is very much like the 1959 AC-15.

You may want to try a couple of high pass caps at the input grids, say 50 to 100 pf. You could also try 250 to 1000 pf caps accross the plate resistors in the preamp. Too large of either will slice off the top end sizzle, just enough cap help control the noise/oscillation that you have described.
In the amp I rebuilt I had to strip out all of the extraneous parts in the oscilator and motor control circuits to get the circuit to behave resonably.
Then I replaced every one of the coupling caps and a few of the resistors. Carbon comp resistors with much more than 10% drift have a tendency to also have more noise issues.
I replaced the first filter caps too, but left the oil filled caps for the preamp section.

Marc

[atmars]

You seem to have some experience with these things. I have read a lot of posts here about them, but what I can't find is a clear explanation of what steps one needs to take to make them viable guitar amps.

I have worked on a few of these so far.

I don't know that I have any extra expertise than anyone else, but a good place to start is here:

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


Find out what model you have and what the schematic is like. Some of the mid period models are tranformerless, which makes them dangerous and makes for a whole host of problems that are unique to this kind of amp and makes it hard to figure out solutions by searching the internet. This is where I accidentally started.

Some have reported that the filmosounds they have acquired are plug and play - which they generally are in my experience. However, what most have done is to get rid of all the stuff in the circuit that is not needed. After that replace the filter caps and any other caps that have filed or are likely to fail. Than you can head towards maximizing the guitar potential by changing coupling cap values and cathode cap values. All of these things are mentioned in the last post.

But ultimately, I think you will find it's a rats nest in there. There amp were designed to be a small part of a larger portable device so stuff is just jammed in them. And, they are not designed to be a guitar amp. i have found in some cases it just makes sense to gut all the electronics and build something else that is simpler and more useful in it's place taking advantage of the existing iron, tubes, tube sockets and whatever else can be salvaged (not much).

Also, if you want to sell some of them - let me know!

[Enzo]

Just unplug and remove the optical tube and the oscillator tube and you are most of the way there.

[atmars]

Just unplug and remove the optical tube and the oscillator tube and you are most of the way there.

Precise and elegant.

[platonicbomb]

Thanks, I will try that.

[platonicbomb]

I am open to selling a few of these. Ultimately I would like to have a couple of them working. THey really do sound amazing, as I know from having one work for a couple of days before a hum kicked in that was louder than the guitar sound. Would you be open to walking me through a few things, maybe a webcam meeting of the minds? I think what is getting in my way is my inexperience with things like removing capacitors, replacing ungrounded power cords with grounded, basic safety-first concerns.

[atmars]

I would suggest that you start your own thread with specific questions, photos, and posted schematics and let everyone that uses the forum provide input. There are some incredibly smart and knowledgeable people floating around out there. It would be shame to waste a resource like that. Plus it allows others to learn as you learn. i would have been lost without the help of these people and others on similar forums. Pick one of the amps you have and start posting.

[Enzo]

Also use the search function here. We hqave discussed some of the film o sounds already, and in some detail if I recall.

[brilliantblue]

Did any of you have luck finding the 302L schematic?

That AMPS.pdf in the earlier link skips the amp model I have: from 302 its model 09431. Anyone out there have a schematic for that one that they could put online? Film-tech offers schems for it and the 8302 but both are zoomed in too far (stuff trailing off the page) and don't have the models differentiated on them.

Thanks for any help!