Filmosound M-1 2x6V6 mods?

[Dexter.Sinister]

Hello,

I have Filmosound M-1 (I think) which is 2x6V6, cathode biased. This is a movie projector amp. Needs a cap job and a couple new tubes, but thinking I will need to clear out part of circuit with oscillator etc. Funny amp! Has following tubes:

V1 5879
V2 12AX7
V3 12AX7
V4 6V6
V5 6V6

Another 6V6 for oscillator

5Y3 rectifier

I would liie to make a small guitar head. Maybe "Bernie" amp (except 6V6). Any ideas or previous experience, welcome.

Thank you, DS

[Dexter.Sinister]

B+ 335 VDC on the plates of 6V6s in power amp section, 90 VDC on the 5879 at the front end of preamp, 190-200 VDC on V2 12AX7 in preamp and 235 VDC on 12 AX7 in PI. Two taps on OT, 16 ohm and 8 ohm. Don't understand the oscillator and associated projector-specific stuff. Pretty heavily filtered for a little amp with two odd filter cap cans (one is 15/15/15 MFD and the other is 50/40/20/20). Thinking of pulling the cap cans and using F&T electrolytics (small).

The 5879 is a mini pentode in a mini 9-pin socket. It is in a shield and is soldered into the shield/socket. I haven't taken it apart, but I see a rubber grommet in there to tame microphonics like on the click channel pentode in /13 amp. No additional rubber pad between socket and chassis...might remount with rubber pad if I keep tube. Only 90 VDC on it and the tube data I read claims I could jack it to 300 VDC. Anybody work with this tube before? Should I dump it or find a replacement for V1? Could claim the octal socket for the oscillator 6V6 and wire it to be an octal pre like a 6SL7 (love that in my 5A3 Deluxe).

Advice solicted by any who care.

Thank you.

DS

[SatelliteAmps]

If it was mine, I would rip out the circuit and start over. Trying to modify those amps is easy, but the results are usually not amazing without a lot of reworking. If you are going to rework the whole thing, it is easier, in my opinion, to just start over from scratch and design a great sounding amp.

[soldersucker]

Some good info in this thread and a pic of Bernies guts:worried
http://www.thegearpage.net/board/showthread.php?t=15679

[Dexter.Sinister]

If it was mine, I would rip out the circuit and start over. Trying to modify those amps is easy, but the results are usually not amazing without a lot of reworking. If you are going to rework the whole thing, it is easier, in my opinion, to just start over from scratch and design a great sounding amp.

You are probably right. I was wondering if there is anything in the circuit that might be uniquely useful. May keep the pentode front end.

Thank you.

DS

[SatelliteAmps]

If you want to make an interesting project, I think your idea of having a switchable front end might give the best experience. Most of the individual parts I wouldn't trust to much though. I would keep the chassis, the sockets, the transformers, the knobs, and maybe the pilot light assembly. The rest I would chuck.

To get the most versatility out of this, I would end up drilling a few holes to add some extras. I would have a switchable rectifier, to use either a tube or a solid state rectifier. I would set the power tubes so I can run them as fixed or cathode biased. I would probably use a good long tail PI design. I would then try to set up the very first stage to be switchable between four options, an octal triode (6SL7), a nine pin dual triode (12AX7), a pentode (EF86), and probably set up the 5879 as a fourth option as an experiment. I would probably also figure out a massive switching idea to be able to cascade of those tubes into each other before hitting the PI, but then I would probably scrap the idea as being overly complicated.

Big project, but could be really cool in the end.

[Dexter.Sinister]

Here an update:

So I have stripped out circuit and am rebuilding as mutant brown Deluxe with higher gain, more upper harmonic content, verge of instability, and good clean up from guitar volume with Strat into 2x12 with mid 60s G12m silver and blue alnicos (The Goals). I need to draw up schematic for look at you all, but here are some features emerging:

Might still keep 5869 pentode as separate input, but focusing on 12AX7 right now because I "get" this tube more, you know?

so:

1) 33K to v1a grid
2) v1 12AX7 with either 2,7K/680n on cathode and 100K on plate (expressy/Komety) or 1,5 K/25u on cathode and 220K on plate (brown Deluxey).
3) Coupling cap/volume/tone like Brown Deluxe: 0,02u to split send to 1M pot for vol and 1M pot for tone (with 0,01 to ground/0,0005 to vol wiper if normal OR 0,02 to ground/.0005 to wiper if bright). Hmm. Probably hard to understand?
4) Then, from Kometey design maybe?, 220K drop resistor to v1b grid with 1.5K/25 cathode and either 82K/22K plate split like Komet or 100K/15K split like brown Deluxe
5) 100n coupling cap to 220K/330K split load to V2 grid
6) v2 has 10K cathode/100K plate
7) 22n coupling cap, 220K dropping resistor to PI
8) Am in quandry here: either long tail Komet like PI with one whole 12AX7 (need to cannabilize socket for 5879) or use other half of v2 for paraphase PI like Brown Deluxe. I tend toward latter, because, while some drive lost, nice "assymmetry?" makes for interesting texture and I hope to make something "newish?"
9) Then cathode biased 2x6V6 push/pull with either 22n coupling caps like Komet or 0,1 like Brown Deluxe.
10) Probably will pick screen resistors depending on how tubes are responding...somewhere between no resistor (Brown Deluxe) to 1K/5W (like Komet).

I hope that is clear enough? A bit of mish and mash, but I love tehse two amps and wish for something with vibe inbetween them if plossible. Good for our band (early electric Miles + Psychedelic + Punk), I think!

All advice, comments welcome. I am not trying to make something like either Komet or Brown Deluxe because the iron for this and the small chassis places some constraints...but many sockets available and I hope to make something useful for my gig and fun to play. Yes? If you help and I keep it, you get a piece of tape with your name on it on circuit! And if our recordings make good reviews, I mention your help! How that? Share love.

Thank you.

DS

[Swarty]

FWIW, the 5879 was used as the MIC input in some mid '50s Gibsons including I believe the GA40 and GA20. I always prefered it to the INSTRUMENT inputs on the same amp that used a 12AX7 or 12AY7.

[hasserl]

FWIW, the 5879 was used as the MIC input in some mid '50s Gibsons including I believe the GA40 and GA20. I always prefered it to the INSTRUMENT inputs on the same amp that used a 12AX7 or 12AY7.

I've got '54 GA20, no 5879 in it.

[Dexter.Sinister]

FWIW, the 5879 was used as the MIC input in some mid '50s Gibsons including I believe the GA40 and GA20. I always prefered it to the INSTRUMENT inputs on the same amp that used a 12AX7 or 12AY7.


Hmmm. Old power supply had only 90 VDC on 5879 plate. Pretty low! If kept that, would want to boost it for more ehadroom, or do you think low voltage on thsi tube sounds good?

DS

[soldersucker]

I recently made a massive tube gear score and this was one of the amps in the deal ,it sounds pretty darn good,humms a bit much and is vioced for harp at the moment.
The other amp is a sweet old Stromberg Carlson converted for harp.
http://i163.photobucket.com/albums/t289/stn23/2008_0626tort30282.jpg
http://i163.photobucket.com/albums/t289/stn23/2008_0626tort30283.jpg
http://i163.photobucket.com/albums/t289/stn23/2008_0626tort30224.jpg