C12a mod to remove cathode follower?

[Spencerleehorton]

HI All,

Wondered whether anyone could shed any light on removing the cathode follower from the C12a and making it more plate pickup design?

 

[Khron]

Why?

 

[k brown]

HI All,

Wondered whether anyone could shed any light on removing the cathode follower from the C12a and making it more plate pickup design?

That's about as sacrilegious around these parts as putting a Schoeps FET circuit in a U47 . . .

 

[Spencerleehorton]

Apparently there is a mod which makes it better, higher gain and lower noise and involves removing the cathode follower?

 

[Spencerleehorton]

Why?

Because I’m having hum issues and want to work out where it’s coming from.
If I can change it to not having a cathode follower and it sounds quieter and has more gain, then surely that’s better?

 

[Khron]

That's a biiiiiiiiig "if". Is it an original C12A, or a DIY version?

 

[Spencerleehorton]

It’s a diy version, hum starts as soon as I plug it in and hum is greatly reduced once valve kicks in, also polar patterns don’t really seem to do much on headphones

 

[Spencerleehorton]

I’m going to move the toroidals away from the psu pcb to see if the hum reduces but also open to other ideas to test it’s all as it should be

 

[Khron]

It’s a diy version, hum starts as soon as I plug it in and hum is greatly reduced once valve kicks in, also polar patterns don’t really seem to do much on headphones

And it can only possibly be the fault of the type of circuit, not grounding or wiring or power supply filtering, or anything else..

 

[rock soderstrom]

also polar patterns don’t really seem to do much on headphones

A well-known phenomenon, has something to do with sound transmission in the head, check the search function here, this has already been well explained.

 

[Spencerleehorton]

And it can only possibly be the fault of the type of circuit, not grounding or wiring or power supply filtering, or anything else..

I have followed the original schematic faithfully so I’m assuming that the filtering and the gnd scheme is correct.
My chassis gnd is connected to neg diode bridge on B+ side

 

[Spencerleehorton]

A well-known phenomenon, has something to do with sound transmission in the head, check the search function here, this has already been well explained.

Ah ok so this is a know problem when monitoring on headphones, no problem.
As when I test it everything tests as it’s all working!!

 

[rock soderstrom]

My chassis gnd is connected to neg diode bridge on B+ side

We've been through this before, don't do it, don't ground your PSU at this point because it's too dirty. Take the ground from the "output" of your PSU...

 

[Spencerleehorton]

We've been through this before, don't do it, don't ground your PSU at this point because it's too dirty. Take the ground from the "output" of your PSU...

You mean at the xlr chassis tab right?

 

[rock soderstrom]

You mean at the xlr chassis tab right?

There are probably several opinions on this, but I consider this point to be very suitable in a mic PSU.

 

[Spencerleehorton]

Ok I did try this but forgot to remove the other gnd so it had no effect!! Doh!
Got a stinking cold so heads a bit fuzzy.
Thanks again guys, merry Xmas

 

[rock soderstrom]

Got a stinking cold so heads a bit fuzzy.

Get well soon!

 

[Recording Engineer]

HI All,

Wondered whether anyone could shed any light on removing the cathode follower from the C12a and making it more plate pickup design?

Might as well be a C12 then? Or to keep things as tight and small as a C12a, maybe look at the Lomo 19a13 for some ideas?

 

[MagnetoSound]

Once you get this circuit working properly you will love it.

 

[Spencerleehorton]

well ive taken off the gnd from neg diode bridge on B+ and now i have IEC gnd to chassis and XLR socket Tab connected to pin1 and a connection from pin1 to the gnd at the 1uf/125v cap C11.
Hum is still there, so im going to try moving the toroidals and see if hum changes!