Filmosound 302 conversion feasibility

[Rhodesplyr]

An existing customer brought me a Filmosound 302 projector, and he's interested in using it as a guitar amp since he's heard the hype. He says he used it briefly when he first obtained it, but then "it stopped making sound."

I didn't know about the Filmosound enthusiasm, so I've been doing a little background reading. This particular model does not have the power transformer on the amp chassis; it's connected via an umbilical harness.

When I first got the bottom cover off the amp, it was quite a surprise. In >20 years of working on tube amps, I think it's the most crammed, crowded chassis I've ever seen. Just replacing the electrolytic cans would take major disassembly to get at them, and there's no room for discrete caps under the chassis.

Finally, I think the owner is under the impression that converting this amp for guitar will be simple, just a matter of swapping a few wires. I'm getting the impression that these conversions are generally DIY projects because they tend to be time and labor intensive.

I'm thinking I may have to hand it back to him and tell him that a professional conversion is going to cost more than he probably wants to spend. Is there anyone who does conversions like this professionally? If so, what do they charge? I appreciate the reality check.

[g1]

Look into tex amps.
I think pro conversions get fairly pricey, but not sure about any numbers.

[mozz]

I have one of these i just picked up, don't know the model number off hand but it has 5y3, 6j7, 6sl7, 6v6's, other models for conversion have 6bq5's. No doubt they are really cramped, i just plan on gutting most of the useless stuff and using the original tube lineup. There is a 13 page thread on these on the gear page website. Somewhere on a film projector website, there is a service manual that covers "all" the models.

[Rhodesplyr]

Included in that gearpage thread is a guy who says that he put 16 hours of work into his conversion. If you think of that in terms of bench hours, hiring someone to reengineer a film projector into a guitar amp -- and one that would be safe and reliable -- starts to look very expensive.

As g1 said above, it looks like the builder at Tex Amps is building copies of the audio circuits of these amps, which could turn out to be make more financial sense to someone who can't do it himself.

[Justin Thomas]

I bought a projector for the tubes... wish I'd checked prices before totally dismantling it. Coulda yanked the tubes AND sold the projector for a chunk... And yes, tightest chassis I ever saw - ZERO room for anything. But, 2xEL84, 2xEF86, 1xECC83, 2xECC82, and some other oddball, all Mullard & Telefunken & Seimens, all for $20. No duds. And $20 for scrap aluminum. Lots of screws, priceless.

Justin.

[olddawg]

In general with these conversions... imho your best bet is gut the damn things except for the iron and basically start with a new build based on a known circuit. I don't even use tube sockets since new ceramics are so inexpensive. Nowadays, unless you are into it for a challenging project, most times you can get a better production amp for the money and time involved. As was said before. The old tubes may improve a new amp.

[mozz]

Well, building a first one with no documentation would be lots of hours. You have to start somewhere. I'll post this and schematic when done but, for now here's some pictures of the cramped chassis. Watch out for the old cadmium plating, this model 179 is from 1947.

[Rhodesplyr]

If you're building a new amp based on the iron . . . forgive me if I'm missing something, but why start with projector iron and a service-unfriendly chassis layout? On this 302, the PT isn't even mounted on the amp subchassis, nor is there room for it.

I was under the impression that there must be something about the Filmosound circuit that was good for guitar; otherwise, why mess with it?

I have seen an RCA projector amp that had high-quality Stancor iron on it, potted transformers, but the Filmosound transformers look like run of the mill stuff to me, no better than the iron on any of the other 6 or so abandoned chassis I have lying around the shop.

[Justin Thomas]

If you're building a new amp based on the iron . . . forgive me if I'm missing something, but why start with projector iron and a service-unfriendly chassis layout?

Because it's cool to freak people out with a really messed-up looking amp and then have GLORIOUS LOUD CRUNCHY RIGHTEOUS GUITAR TONE COME OUT! Like the guys who put Tweed Fenders in Gorillas. I am still wanting to build an amp in an old O-scope case...

Justin

[mozz]

5879. 12ax7, 12ax7, 6aq5 6aq5? You can rip out the other 5879 and 6aq5 and 930 tube, oscillator coil and a bunch of other stuff. Give him a real high estimate or tell him 1 year in your spare time!
http://www.film-tech.com/warehouse/w...443&category=2

It's on about page 205.

Collin Cripps, Blake Mills, James Valentine and Maroon 5.
Converted by Austen Hooks, he just (few years ago) copyrighted or patented the name "projector amp". Buy 'um up now.
Austen Hooks

[mozz]

If you're building a new amp based on the iron . . . forgive me if I'm missing something, but why start with projector iron and a service-unfriendly chassis layout? On this 302, the PT isn't even mounted on the amp subchassis, nor is there room for it.
I was under the impression that there must be something about the Filmosound circuit that was good for guitar; otherwise, why mess with it?
I have seen an RCA projector amp that had high-quality Stancor iron on it, potted transformers, but the Filmosound transformers look like run of the mill stuff to me, no better than the iron on any of the other 6 or so abandoned chassis I have lying around the shop.

You can't say quality iron is going to sound better than some of the cheaper and lesser known makers. This is a guitar amp. This watchmaster is going to be made into a guitar amp, maybe this summer.

[Rhodesplyr]

5879. 12ax7, 12ax7, 6aq5 6aq5? You can rip out the other 5879 and 6aq5 and 930 tube, oscillator coil and a bunch of other stuff. Give him a real high estimate or tell him 1 year in your spare time!

The 302 has a different amp design from the 385. It's roughly the same topology, but it has a LOT of ceramic disc caps in it performing various EQ functions, including Treble and Bass controls.

Spare time? LOL I have at least six chassis in the storage closet earmarked for guitar amp conversions -- if I ever have the time with all the gear I have to get fixed for customers. And those chassis are actually pretty mod-friendly with enough space to work in. That's one of the reasons I'm hesitant to go ahead with this job. The purpose of this horrible layout was to make the thing small to fit inside a projector case. There's no other reason for it. I'm not doing it for the fun or the challenge.

[Tom Phillips]

It has been my experience that conversions of old audio equipment for use as guitar amps do not sound any better than a good, well maintained classic amp or clone or even one of several COTS amps. They can sound "as good" after an appropriate amount of custom work. However, that custom work can be expensive unless the owner is donating the time and doing it for personal reasons. There is the cool factor of saving an obsolete item from the dump and making it perform a useful function. The problem is that customers who bring such pieces of equipment to a commercial repair / restoration company often think that the work will cost a reasonable fraction of what they paid to acquire the item. I'm willing to wager that I am not the only one who took on such a project and was then living the nightmare of part way done with no end in sight and the personal feeling of "got to finish and do a good job." Then there is the risk of the perceived magic ingredient, such as the output transformer, failing.

It takes a combination of several factors to make such a project succeed. Pick a minimum of three of the following:
1) A special commitment of a skilled electronics craftsman
2) An uncommonly understanding and patient customer
3) Many donated working hours
4) A substantial shop bill

I realize that there are stories of easy conversions that sounded amazing. People talk about them because they imagine getting a great sounding amp for nearly free. However, for long term use, they are as rare as finding a classic car in grandpa's garage that just needed a battery and aired up tires to be roadworthy.

I encourage people to do these type of projects for personal satisfaction. I also love to read postings and see before and after photos of such projects. They are great to post on the shop walls if you want people to bring you more of the same type of work. However, I recommend extreme caution if you are considering taking on the work as a paid job.

Cheers,
Tom

[RandallM]

Ok I obviously don't know anything about amp conversion but these amps have a mic input which when you plug a guitar in sounds awesome! I'm thinking about projecting while playing! What does it mean to say that they need to be converted? Converted to what?

[Enzo]

If you like the sound, that is all that matters.

But in general, projectors are more like PA systems than guitar amps. By converting, someone takes the basic projector amp, leaves the power amp stage intact (usually), and takes advantage of the fact the power and output transformers are there, plus the tube sockets are already wired for heaters (again, usually). Then they modify the preamp part to be more guitar friendly. A guitar amp preamp adds a certain character - note that a Marshall and a Fender sound nothing alike.

Also the audio part of the projector takes its signal from a photo system - light shines through the film audio strip onto a sensor. Those parts serve no purpose in a guitar amp, and can actually interfere.