Mozz's Filmosound 179 Conversion Plus Tweaks

[willard]

I have started to sketch the layout and begun a more detailed review. These comments about the schematic are for posterity's sake, in case anyone in the future decides to use the schematic.

1) 390K dropping resistor in the rectifier heater/B+ circuit - it's in the original (Mozz's) but it only make sense to me if its 390R (maybe dropping about 60v for the missing 6v6 and the CE-1C tube ?). Probably best to omit and measure voltage first before deciding to drop voltage since there is an added a 6SL7.

edit: its correct, see next reply for explanation.

[mozz]

The 390k is just a bleeder resistor. I don't know if i mentioned it, there is not a lot of room in those chassis. You would be best off to build it first before you do any mods.

[willard]

The 390k is just a bleeder resistor. I don't know if i mentioned it, there is not a lot of room in those chassis. You would be best off to build it first before you do any mods.

I am building but I like to know what things are for. So a dropping resistor would be in series and a bleeder is in parallel, right?

[willard]

Chassis question:



I've made a new socket on the top left side so all preamp tubes are together and away from rectifier and PT. There will only be 2 6V6s in the center. The stock chassis has a metal wall between the rectifier+PT and the signal+power tube side of the chassis. I have cut apart one of the old metal multisection caps to act as another wall between power tubes and preamps. Is there any benefit to adding this second wall this in terms of noise reduction?

Also, I have removed the coating on the chassis underside to ensure that if chassis grounding is required, there will be no uneveness in contact/resistance.


For posterity/lurkers: this is what I'm going with ...

The resonance control proposed previously is an attempt to have control over bass. Instead, now there is a straightforward bass tone control but not a Fender tone stack because I wanted bass and (treble) tones to be at different locations to optimize their influence on distortion shaping (an assumption based on internet posts). To hedge against the "location error", I will wire them to be relocatable with toggles (bass 4p2t, treble 2p2t) . These aren't intended to flipped during playing.

To increase the versatility of the amp and for testing configurations without resoldering, I'm adding 2 effects loop. Hopefully not only is there basic effects send and receive with variation based on location, there is simultaneous FX loop operation, preamp out only use (1 preamp tube with choice or both), first preamp bypass, second preamp bypass, a "0" preamp/super clean, lower output power tube configuration, etc . Relocatable C1 exist just to utilize the gain pot or not.

The choice of effects loop location has been adapted from some R. Robinette posts (C2) and his 5E3 FX loop mod (C1) on his website.

All these tweaks, so little time

[willard]

[mozz]

I hope that isn't cat5 wire I see in there.

[willard]

No it's 23 AWG Cat 6e and it's just for the mini switches. None of it was connected except for 1 pair from V1 to the volume pot for testing because I was lazy. It's for easy switch testing which I haven't gotten around to yet. If there's no problem in sound or function, I'll use it for the relocation switches.

The main point of my reply to a long forgotten post is to let people/lurkers/googlers know that the extra DCC is a workable mod. Nobody is going to care what wire I use to test the switches with.

You should try the DCC mod. Add a cathode capacitor to it though. I left it out on the schematics but I think it really improves the sound.

[Helmholtz]

What is a DCC?
Do you mean a DCCF, i.e. a DC coupled cathode follower?

[willard]

Yes. DCCF. I didn't think anybody still cared about this to read this thread

There seems to be a general bias against the Filmosound 156/179 in favour of the Filmosound 385. I think the added DCCF would go along way to counter the bias. I'm sure the person who gave me his schematics with the DCCF would agree. From what I can gather he has attempted to commercialize his design.

[mozz]

At the most I only see 2 switches, which could be taken care of with 3 wires. The reason i said is that CAT5 wire, it's only rated for 150v so don't run HV through it.

[willard]

Did you look at the picture of the front? How many switches do you see? Three in the front are wired with the CAT6 (and not yet tested). The two down the middle towards the back are wired with 22AWG 600v. Likewise the 2 in the back, which are the ones you see, are the two NFB switches.

You're trying too hard!

[mozz]

I was going by the last schematic. Only 2 switches. I just wanted to make sure you weren't running HV through the cat 5 wiring.

I actually picked up 2 more projectors, not sure of the model numbers. I'm sure I'll be doing conversions for those.

[willard]

Here's the latest/final version of the schematic. It's working 100%. The relocation switches have been discarded; they were causing problems with the DCCF. I have added extra resistance selectors as suggested by Neumann. Note that in Blencowe's "bootstrap" version of the DCCF, he suggests that the cathode bypass cap is largely dispensible, I found the caps to still have a large effect on gain and overdrive.

Please feel free to contact about schematic details. Note that DCCF implementation details are adapted from John Luchsinger's privately supplied schematic. You might want to google him and contact. I will be of no use.

Thanks to everyone who provided helpful feedback, especially Helmholtz.

Sorry to hear about Enzo.

[mozz]

Not a filmosound based amp anymore, original chassis is small, vol, tone. Looks overly complex and switches will make it pop. Oh well, you got to use the transformers.

[willard]

Some voicing notes:

1) I found the amp to be low/mid heavy so dropping the cap on the bass tone control to 130 pf helps a lot. Even 100pf is ok if you want more impact.

2) I wanted a bit more output tube overdrive, sustain, and compression. Changing the screen resistors on the 6V6's to 1K from 470 and the grid stoppers to 1.5K from 1K does this (for me -- YMMV