Help with modifying Circuit please

[DaveP]

Looks like it was just deleted, this one.......

http://www.valvewizard.co.uk/dccf.html

DaveP

 

[sonolink]

The DC coupled cathode follower is crucial to the best sounding amps as that article will explain....do not change it!

DaveP

Sorry I don't understand. Doesn't the modified circuit has a cathode follower just before the tone stack?

Cheers
Sono

 

[sonolink]

I have shielded all the signal path (well I tried anyway) The shielded cables are the purple and blue ones. They are connected to GND only on one end.

https://postimg.cc/nMBRGbm7

Any thoughts on this please? Is it worth shielding the whole thing as in the pic (purple and blue cables) or just using shielded cable between the input and the first tube's screen as done on amps would be enough?

Thanks!
Sono

 

[DaveP]

Screening is not usually necessary when the signal is over a volt or two, unless it is very near a strong magnetic field like a PT or choke IMHO.

Your biggest danger will be from earth loops, make sure you place the signal earth carefully to minimise loops.

DaveP

 

[sonolink]

Thanks for your reply DaveP.
That means that I should be ok screening between the input and the first stage only. But should I worry about the inductor in the PSU?

Cheers
Sono

 

[DaveP]

But should I worry about the inductor in the PSU?

That depends.

It should be placed well away from the input stage as part of normal practice,  if that is not possible, then a steel shield may help.  It is often possible to turn these components sideways to orientate the magnetic field in another plane, you can experiment with that later.

The earthing of centre taps and the power section bridge and cap earth should all be kept to the shortest dimensions and the signal earth including the power supply earth should only contact the chassis at the input socket.  The power cord earth should go straight to the chassis where it enters, this will minimise hum.

DaveP

 

[sonolink]

You do realize this is supposed to be built inside a pedal don't you? The enclosure is about 140x120mm....the PSU is a switched mode thingy on veroboard...hence my first idea of shielding everything...
Cheers
Sono

 

[DaveP]

My mistake : :-[

DaveP

 

[sonolink]

I guess I can build it the simple way, with standard screening and see what happens

 

[abbey road d enfer]

But should I worry about the inductor in the PSU?

You'll probably have to find the orientation where noise is minimized. Make sure you have the possibility to move it around before attaching it definitely.

 

[sonolink]

Well it's a little too late for that now...if the inductor is a problem and has to be moved I'll have to redesign the veroboard.

I think I'll build it with shielded cable between input and the first stage and see what happens...

Cheers
Sono

 

[sonolink]

Hello again,

So I built this thing finally and here's what it looks like:



I ended up shielding all the signal path (grey cables on pic, purple and blue cables on layout diagram)




I checked voltages as follows:

V1:
p1:140v
p2:0v
p3:1v
p4-5:0v
p6:158v
p7:0v
p8:1.6v
p9:6.5v

V2:
p1:88v
p2:0v
p3:0.6v
p4-5:0v
p6:158v
p7:87v
p8:88v
p9:6.5v

I applied my scope with a 10x probe between jack input and GND and then jack output and gnd and I got this:

Jack input no jack


Jack Output no jack


Jack Output jack plugged in


Any thoughts?

Cheers
Sono

 

[DaveP]

50Hz looks like a hum loop,  the high frequency might be pick-up from fluorescent lighting, the unplugged noise looks around normal.

Nice job by the way.

DaveP

 

[sonolink]

Thanks

the high frequency might be pick-up from fluorescent lighting, the unplugged noise looks around normal.

There's no fluorescent around though. Maybe coming from the inductor?

I guess the loop comes from the shielding so I'll check that first.

Cheers
Sono

 

[sonolink]

Btw, any way to locate the loop without unwiring the whole thing? Just hoping....

Cheers
Sono

 

[sonolink]

Ignore my previous post. I'm a retard and forgot to connect GND to the enclosure. Once that done hum is GONE

Unfortunately, I've got intermittent sound and after some chopsticking I've detected some bad solder joints.

So I'm gonna go through this little mofo again and I'll report back

Cheers
Sono

 

[sonolink]

Hi again,

Sorry for not posting more about this. To make a long story short, I finally went through all the circuit checking everything and got it to work. Quiet as a mouse

BUT

I don't really like it very much
The sound is a bit fizzy and not very good. I think all the switches and combinations "denatured" the circuit (or something like that anyway).
So I decided to go back to square 1 and built a Fender AB763. This one sounded great and was quiet too. Great sound and dynamic range, and organic response...

BUT

I missed some distortion options. I does have distortion but only with the gain all the way up.

So, I built this: https://groupdiy.com/index.php?topic=74809.msg950841#msg950841

I just wanted to close this thread and thank again all the input and advice, especially Abbey Road d Enfer

Cheers
Sono