Filmosound 179 Questions

[misterc57]

Looking to do a few tweaks on a Filmo 179.

Any advice on shielding or replacing the first input tube; 6J7? I see a bunch of folks kept the 6J7 in their guitar amp conversions.

A lot of room noise being picked up by the grid wire and cap that connects to the top of the tube. If I move my hand around the tube I can create a shield that quiets it down.

I read about converting to a 6SJ7 tube so all the wiring is in the chassis. I would have to move most of the wires on the socket for the change over.

Suggestions?

Also curious why there are 3 input wires on the power transformer primary?

Thank you, MarkO

[Helmholtz]

Grid wire should be shielded.

[misterc57]

Grid wire should be shielded.

Can I just wrap the wire in a shielded cable sleeve and would any of the shield need to be grounded to the chassis? Or replace the grid wire completely with a shielded wire?

Thank you

[Helmholtz]

Yes, (any) shield needs to be grounded inside the chassis, e.g. connected to socket pin 1 (which I suppose to be connected to ground in order to ground the metal shell of the tube).

There will still be the exposed/non-shielded top contact. I remember there were shielded metal top caps also.

But if your cabinet provides shielding, it should be ok.

[misterc57]

Yes, (any) shield needs to be grounded inside the chassis, e.g. connected to socket pin 1 (which I suppose to be connected to ground in order to ground the metal shell of the tube).

There will still be the exposed/non-shielded top contact. I remember there were shielded metal top caps also.

But if your cabinet provides shielding, it should be ok.

No cabinet. Thank you!

[misterc57]

Need advice on the tone control. Turning the tone pot does not really seem to change the tone, nothing noticeable to my ears.

There are two pots, volume and tone. Tone pot is on the right side of the picture below.

It is suppose to be a 50K pot. Full rotation swings from 2 ohms to 4.3K ohms. I am thinking that resistor sitting across the wiper and outer leg of the pot may have something to do with this. If so is this why I do not notice tone changes?

I could remove this resistor but I am not sure of its purpose in the circuit at the pot and downstream from the pot. Parts of that schematic is pictured below also.

Thank you! MarkO