Mozz's Filmosound 179 Conversion Plus Tweaks

[willard]

Hello everybody,

I recently acquired a Filmsound 156-V which is very similar to the 179 model and intend to gut and rebuild. I have taken mozz's Filmosound 179 conversion schematic and made a few changes. The main changes:

1) added Pre PI Master Volume
2) cathode biased instead of variable fixed bias

Can someone please comment on master volume usefulness/implementation and the general correctness of the schematic? I wanted the master volume to 1) make the amp more "bedroom" friendly and 2) control double frequency distortion so the amp takes overdrive pedals better.

Thanks,
Den

[Helmholtz]

Your input wiring shorts the signal to ground.
The 1M grid leak resistor should go from input hot to ground (as shown), but the 33k grid stopper should connect before the 1M.

[willard]

Thanks Helmholtz. Corrected below:

[g1]

I'm not sure about your master vol. Here is the Mozz schematic:

[willard]

Thanks g1. I used that exact schematic but made the graphics changes on one that was easier to modified clearly with my available drawing software (thicker lines to start with so I could just change it "ransom note" style).

The master volume tweak is adapted in principle from the pre PI master volume example on Rob Robinette's site for a Fender 5E3 Deluxe. There he states "You simply replace the phase inverter's 1M grid leak resistor with a 1MA pot. "
I adapted this example to the Filmosound's 270K grid leak resistor. I have no idea whether I can generalize from Rob's example to the Filmosound but his mod is suppose to be generic for cathodyne phase inverters cases.

[g1]

The master volume tweak is adapted in principle from the pre PI master volume example on Rob Robinette's site for a Fender 5E3 Deluxe. There he states "You simply replace the phase inverter's 1M grid leak resistor with a 1MA pot. "

It's easier for me looking at the 5E3 schematic. You should be fine then. Both are cathodyne as you say.

[Helmholtz]

Corrected below:

Yes, that's what I meant.

Don't see a problem with the MV.

[mozz]

Memory loss. I guess i used the 270k so i didn't overdrive the phase splitter? I often go back and look at things i made years back and wonder why i did some things. Why 410v on the plates and screens, i know i would have used JJ's to handle that. Probably left the voltages high and used fixed bias so it had some balls for being a small amp. I just counted 38 guitars and didn't count the amps yet, might be 20 of them. I have to thin the herd.

[willard]

Thanks Mozz for the background. btw your Filmo 179 build thread still exists on the wayback site in case you're interested in getting a copy.

Some of the other conversion schematics using the 6SL7 that I've seen also use a 270k resistor, fwiw.

Thanks again to everyone for your help on the MV mod and the general checks. I'll be back in about a month or two to report on the results.

[mozz]

Not sure how much range you will get with the 270k pot but easy enough to tweak. I do have a habit of stealing small parts of schematics of similar tubes, input tube values were also from something old, maybe a Valco or maybe a 6sj7 which is the same tube without the grid cap.

[Helmholtz]

With this wiring the 270k MV is bootstrapped, so signal loading is insignificant.

https://www.valvewizard.co.uk/accf.html

[willard]

Helmhotz: Does insignificant signal loading indicate that the MV is ok (that it avoids a specific problem or condition) or problematic in some regard? I have little experience in electric circuit analysis.

That being said, I have been reading on the valve wizard site and become interested in some of the ideas there. Shortly I plan to upload another schematic with further modifications for comment and review. Some extension are trivial (NFB) but I'd like to add a DC cathode follower after the 6J7 and before the 6SL7 for extra gain.

For the DC cathode follower addition, is it possible to use either a 12AX7 or 6SL7 for this? I know someone has used a 6SN7 with the Filmosound 179 circuit but contact with the person has not been possible so I can't ask him.
Is there a reason to choose between the two? (I can easily install a 9 pin socket and the chassis already has a spare octal to use).

Thanks in advance for all comments and criticisms.

[mozz]

I've had a few microphonic 6sn7's in guitar amp experiments but maybe they won't do that being used as a cathode follower?

[The Dude]

I have had the same experience with 6SN7's being microphonic. The last time I needed one, I bought a lot of 10. It took several tries before I found one that was not microphonic.

[Enzo]

How much gain are you expecting from a cathode follower?