Filmosound 385 Conversion Schematic

[timmyjoe]

I've got a Filmosound 385 that I'm hoping to convert to guitar use. Anyone have a schematic of a conversion that keeps the original usable circuit? Or at least the original schematic in a readable format? All the originals I've found online are too blurry to be of much help.

Or maybe the Bernie schematic?

From watching this video from Uncle Doug

, it seems the main problem of the amp is to get rid of the negative feedback loop that kills the high and low end of the amp. Also the unneeded parts of the circuit. But I can't get started on this without a schematic. 

[sluckey]

https://el34world.com/Forum/index.php?topic=22043.msg234645#msg234645

[valcotone]

hi timmyjoe,

There are certainly a few ways to go with modding a 385.

I found this redrawn schematic online (maybe here) and wish I could remember who posted it. Thanks to the mystery helper there! This shows the main amplifier components and ignores the extra bits that supported the projector. It matched up very well to the original circuit in the 385 I had.



Here's my final schematic with the list of my changes on the top-left. Basically, I removed all the non-guitar-amp components and wiring and made a few changes to dial it in to what I wanted to hear... I'm happy to help with any questions if you think I can help.



And a video demo of the results...

Good luck!
cheers!

[tubeswell]

Hi valcotone, Nice sounding amp! There's a lot of plate bypass caps in that schematic. Did you keep these in the final build? And if so, what values did you use?

[valcotone]

hi tubeswell,

Yes, I left the original plate bypass caps in. I no longer have the amp, so I can't check the values. The amp had plenty of brightness and presence for me, so I didn't feel the need to clip them out or experiment with different values.

The other thread sluckey linked to has the original (hard to read) schem that shows the cap values.

https://el34world.com/Forum/index.php?action=dlattach;topic=22043.0;attach=65550;image

[valcotone]

Some additional thoughts:

- It was a satisfying project to work on, and I have no idea if the changes I made align with those of high-level guys like Austen Hooks, Bill Krinard, the Bernie amps, etc.... but I was happy with the results, for my ears. YMMV.

- This is a very tight chassis to work in! Some finger acrobatics are needed to reach some components and solder joints.

- I found the stock circuit (with the 5879) had too much gain for guitar. I think some modders bypass/remove that 5879 input stage, but my goal was to keep as much of the original signal flow and make incremental changes to improve the sound/performance for my use. This is why I removed the 2nd gain stage.

- The adjustable negative feedback control has a HUGE impact and control over the overall volume and tight/loose feel. Definitely recommended.

- I had a hum that I narrowed down to the tone stack, and I decided to rewire it with a simpler treble control. (I had removed the switched tone pot anyway).

- The next logical steps would be to replace the filter caps, but I ended up selling it before proceeding there.

- The person who first matched up the Filmosound chassis and surplus ammo box is a genius! They are well-matched.

- I found Uncle Doug's videos on his Filmosound mods to be helpful as an initial orientation on the project. Thanks Uncle Doug!

[Lectroid]

timmyjoe,

Sorry I keep ghosting you--notiification emails been going into my junk folder.  Attaching B&H schematics for your 385.  Also schematic for my build.  It works as drawn but I've added two things that aren't on the schematic: 1) I connected the input ground bus to the speaker sockets ground bus to kill a nasty hum (probably my own fault), and 2) I added a DeluxeReverb-style negative feedback loop to the cathode of the first tube in the dual triode (your 12AX7).  But as I said, it works okay as drawn.

Answers to your other questions:
50uf cap is cathode bypass resistor for the 6V6s.  It can be 25V or 50V if you're real conservative.
Transformer:  Red is 660V p-p AC that connects to pins 4 and 6 on the 5Y3.  Yellow is 5VAC filament for 5Y3, they connect to pins 2 and 8 of 5Y3.  HT or B+ comes off pin 8 (by convention).  Green are 6.3 filament heaters.  Black and white are the primary windings that you connect the mains 120VAC to.

Hope this helps.

As you'll see, I just gutted mine and used the old iron to make a Deluxe 5E3 clone with.  It's pretty loud, and the NFB I added cleaned up the sound quite a bit.  Msg me back if you have more questions.
 

[PRR]

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[shooter]

 
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