Mozz's Filmosound 179 Conversion Plus Tweaks

[Helmholtz]

I understand what you mean by the switch now though. It's the NFB switch that needs the 470k resistor.

Yes, I meant the NFB wiring.

You cannot use a cap in the DCCF circuit as that would block the cathode DC current.

[willard]

Here's the latest schematic which includes a resonance adjustment as well. It's a direct copy of Rob Robinette's mods. The NFB pot was removed because there's a lot of opinion on the web suggesting it's too fine a control.

As always, any response is encouraged and appreciated.

[Enzo]

The NFB pot was removed because there's a lot of opinion on the web suggesting it's too fine a control.

Removed? Did you try it? and did it seem too fine to you? Internet be damned, it only matters what you like in your amp.

[willard]

Removed? Did you try it? and did it seem too fine to you? Internet be damned, it only matters what you like in your amp.

You flatter me! As a player, I'm not gifted with "golden ears" and not especially musical. For me, I think basic musical things like interval recognition take precedence. First things first. Eric Johnson will be nowhere near my amp.

As you can tell, I'm already getting too deep into tweaks and should really just build the amp.

[Enzo]

Well if you can't hear a difference, why pursue it? Like cooking, once the soup tastes good, stop dumping spices in it.

I bought an Eric Johnson record once. I was inspired. So I took the strings off my guitar and used it as a canoe paddle.

[willard]

Well if you can't hear a difference, why pursue it? Like cooking, once the soup tastes good, stop dumping spices in it.

I bought an Eric Johnson record once. I was inspired. So I took the strings off my guitar and used it as a canoe paddle.

Well, I can hear some differences, not all, but I do need to form some expectations as a starting point. I know my ears. Robinette does not use a pot but instead uses a switch. The gearpage website has a thread where most guys favor the switch over the pot. Even though it's the internet, not all advice is bad otherwise I would not be here. I could use a pot now to cover the "unexpected" but room inside the Filmosound chassis is limited (I haven't mentioned/included trivial things like the Humdinger pot)

Interesting thing about Eric Johnson. In later interviews he admits the "tone" chase is a bit of a rabbit hole, maybe not the best thing. I think there is a common perception that his perfectionism has hindered his career by limiting output.

[Helmholtz]

Instead of combining parts of different designs (which might not give desired results because of unsuitable circuit impedances) or things thought out by amateurs, I'd rather start with building a proven design.
E.g.: https://schematicheaven.net/fenderam...5e8a_schem.pdf

If you think you need to modify it, only try and evaluate one mod at a time.

[willard]

Instead of combining parts of different designs (which might not give desired results because of unsuitable circuit impedances), I'd rather start with building a proven design.
E.g.: https://schematicheaven.net/fenderam...5e8a_schem.pdf

If you think you need to modify it, only try and evaluate one mod at a time.

Yes. This is sound advice. I can physically prepare the chassis with a resonance pot and NFB Switch and just jumper them for the time being.

As an aside do you think there are any benefits with going to a 6SJ7 (glass) for the 6J7? Someone close by has some of them available.

Likewise, 5V4GA for the 5Y3GT? I understand the 5V4GA does not dump voltage on the B+ at startup so this will prolong tube life for the other tubes.

[Helmholtz]

Sorry, not familiar with those oddball US tubes (maybe post datasheets).
My favorite rectifier is the GZ34, which provides delayed ramp-up and has low voltage drop (not sure if that's desirable here).
Check if it's pin-compatible

[willard]

Sorry, not familiar with those oddball US tubes (maybe post datasheets).
My favorite rectifier is the GZ34, which provides delayed ramp-up and has low voltage drop (not sure if that's desirable here).
Check if it's pin-compatible

Here are the data sheets for the 6J7 and the 6SJ7. Other Filmosound conversion have substituted the 6J7 on the grounds of reduced RF exposure . I think it's only a matter of rearranging the pinout correctly. The grid cap in the 6J7 is relocated to pin 4 in the 6SJ7. I was wondering whether a glass tube would be better than metal in a preamp tube.

6J7.pdf 6SJ7.pdf

The 5V4GA is pin compatible. It drops more voltage than the GZ34 and has more sag as well which I understand is something characteristic, sometimes valued, in a guitar tube amps.

[Helmholtz]

I'd use what you already have and only consider a change if there's a problem.
Input pentodes may be microphonic and sometimes need to be selected.
So availability and cost should be considered.

[willard]

Per Helmholtz's suggestion, I have revise the included NFB loop to start from a known and proven design, in this case the Fender Twin schematic he referenced. I have made changes to this design by

1) (trivial) changing the NFB resistor to the value in the original Filmosound (Model 156 page H12) schematics (56K to 27K). Note: the NFB on the Twin comes from a 4 ohm tap, the Filmosound a 16 ohm.

2) (trivial) included a switch to select a lighter degree of NFB and an on-off choice

3) a resonance control (high pass filter). As I understand this (please correct if mistaken) the pot functions by varying the reintroduction of the frequencies blocked by the capacitor, thereby altering the pure capacitor attenuation. The pot will have 0 ohms at the lowest setting effectively bypassing the capacitor so the full feedback signal will negate the bass frequencies in the output as well. At the max pot setting (full clockwise), no blocked lower frequencies are reintroduced. The feedback signal is relatively richer in high frequency, which negates the corresponding higher frequency in the output, thereby accentuating the bass frequencies in the output.

The schematic:

[Helmholtz]

Some remarks:
1) I don't think you need the "resonance" control. All it does is attenuate HF, so counteracts the presence. None of my amps ever had a resonance control and I didn't miss it.
Also be prepared that increasing the NFB ratio might cause instability/HF oscillation. I hope you have a scope and know how to use it.
Most guitar amps provide some overall treble boost at typical tone settings. Your tone control can only attenuate treble, so only the presence adds some treble.
Consider adding the Fender tone stack.
2) Both your switches will likely pop when switched hot.
A better solution for the compression selector would be to hardwire the max. resistor (e.g.100k) and lower resistance by paralleling other resistors.

But most of all, build it and see what you like or miss.
You can't ultimately design sound on paper.

[willard]

Some remarks:
1) I don't think you need the "resonance" control. All it does is attenuate HF, so counteracts the presence. None of my amps ever had a resonance control and I didn't miss it.
Most guitar amps provide some overall treble boost at typical tone settings. Your tone control can only attenuate treble, so only the presence adds some treble.
Consider adding the Fender tone stack.

Yes. I intend to build asap. But one last question so I can think about what direction to mod afterwards. Do guitar amp tone controls (incl. presence) interact with the amp differently than an external EQ pedal?
Am I going to miss out on any "tube overdrive" goodness if I use an external EQ pedal?

Thanks for the help so far!

[Helmholtz]

But one last question so I can think about what direction to mod afterwards. Do guitar amp tone controls (incl. presence) interact with the amp differently than an external EQ pedal?
Am I going to miss out on any "tube overdrive" goodness if I use an external EQ pedal?

Yes, it makes a substantial difference if a filter (tone control/EQ) is before or after a distorting stage/overdrive circuit..