For your amusement: WE 141-A preamp clone

[dustbro]

I'm trying to select an input transformer for this project.
http://www.cinemag.biz/mic_input/mic_input.html
Would the CMMI-10PCA (150: 15K) be a good choice? I'm not really sure what a "mic to grid" transformer is.
Any suggestions on an output transformer? I'm assuming that this circuit is unbalanced at the moment.

 

[lassoharp]

http://www.audiosharing.com/archive/western/we_amp/pdf/No.141-A.pdf


***contains relevant notes for heater wiring.  Elevated cathodes . .

 

[lassoharp]

I'm trying to select an input transformer for this project.
http://www.cinemag.biz/mic_input/mic_input.html
Would the CMMI-10PCA (150: 15K) be a good choice? I'm not really sure what a "mic to grid" transformer is.
Any suggestions on an output transformer? I'm assuming that this circuit is unbalanced at the moment.

Cinemag should work fine.  The 10C looks to have a little better level handling capability.

http://www.cinemag.biz/mic_input/CMMI-10C.pdf


I had considered the 141 when tossing around plans for a tube console.  One of the big ++ was the lack of OT - the sonic differences in the bottom end, and of course cost.  It looks like a terminated 600:600 should work well if the amp is strapped for 600ohm output drive.

 

[dustbro]

thanks for the tips on the xformer. I'll give that 10C a try.
So when I look at this schematic I'm reading the connections on the right side as being:
5: + output
6: 0V (ground)
4: + V input
7+8: tube heaters.

or am I messed up? is 6 the - output? if so, then where is 0V?
And does pin 1 of the 6J7 go to ground?

Thanks again
Dan

 

[lassoharp]

thanks for the tips on the xformer. I'll give that 10C a try.
So when I look at this schematic I'm reading the connections on the right side as being:
5: + output
6: 0V (ground)
4: + V input
7+8: tube heaters.

or am I messed up? is 6 the - output? if so, then where is 0V?
And does pin 1 of the 6J7 go to ground?

Thanks again
Dan

The output is taken from, as you stated,  5 = hi side  & 6 = lo side

6J7 Pin 1= tube shield - goes to ground

Good luck on the build.

 

[keefaz]

I wanna give this circuit a try. Can you guys look over my layout for any errors? (sorry, I'm not a P2P guy  :-[)

Doesn't the clearance between tracks look a bit small for 250VDC voltage ?

 

[ruffrecords]

Doesn't the clearance between tracks look a bit small for 250VDC voltage ?

Yes, it looks much too small.

Cheers

Ian

 

[dustbro]

The output is taken from, as you stated,  5 = hi side  & 6 = lo side

6J7 Pin 1= tube shield - goes to ground

Good luck on the build.

Interesting... so all of these components connect to the LO side of the circuit instead of what I would consider to be 0V?
Is it safe to assume that the only points that will be connected to my chassis star ground are:
Pin 1 of the 6J7
Pin 1 of my input and output xlr
and any point labeled as 0V on my (yet to find) power supply schematic

 

[dustbro]

Doesn't the clearance between tracks look a bit small for 250VDC voltage ?

Yes, it looks much too small.

At the moment, I'm using the default spacing in PcbArtist. This spacing seems to work for my LA-1 and MILA-1 layout.
What size do you guys recommend? I'm sure that could be corrected with a push of a button.

 

[emrr]

6 is ground and 0V.

This is the perfect chance to become a PtoP guy.  I personally don't see a compelling reason to put this on a circuit board.   There's not much there.    You are hereby fined $5 for even thinking about it.   ;D

If you do put it on a circuit board, power the filaments with DC, or for AC wire them by hand, well twisted and moved away from everything else.  

 

[dustbro]

If you do put it on a circuit board, power the filaments with DC, or for AC wire them by hand, well twisted and moved away from everything else.  

Crap. I was hoping to put them on the top layer of my pcb.
But after looking at the note on the filaments, maybe i should jam 15v dc into them

 

[lassoharp]

Interesting... so all of these components connect to the LO side of the circuit instead of what I would consider to be 0V?
Is it safe to assume that the only points that will be connected to my chassis star ground are:
Pin 1 of the 6J7
Pin 1 of my input and output xlr
and any point labeled as 0V on my (yet to find) power supply schematic

By "Lo Side"  I meant your circuit ground = 0V.

For pin 1 of input XLR I normally just tie that to the ground tab on the connector.  It's often cited as recommended practice and it's worked well for me on p to p builds.

 

[dustbro]

and just so I can ease my pea sized brain, point 5 goes to the + side of my output, point 6 goes to the - and ground of my output? which would make this an unbalanced output, right?
thanks!
Dan

 

[lassoharp]

Yep.

 

[dustbro]

Can someone suggest a power transformer and circuit that can handle the B+ and +/- DC for the heaters?
Haven't played with DC heaters yet. Is it +V on one side and -V on the other side of the heaters, or is it + or - to one side and Ground on the other?

 

[lassoharp]

What they are actually recommending is a standard AC supply with the CT biased with a DC voltage above ground.

This would be something like the Pultec EQP1A heater supply arrangement. 

 

[dustbro]

Thanks so much!

 

[fazer]

Just located 2 6J7 Tubes , 2 6SN7's ,  and 2 UTC 0-16,  200 to 50k grid input transformers.  I'm going to breadboard a PTP version and see how it sounds. 

Is this the same preamp used for the Motown recordings?  I always loved that topend sound.  I'm sure most of it was the great singers however.

 

[ruffrecords]

I would love to know what the comment on r15 says I'm guessing some type of gain setting. As for 7&8 of the 6sn7 thats just some fancy-ass heater artwork.

R15 alters the dc conditions of the cathode follower final stage. With R15 in circuit the dc bias to the grid is higher so the stage standing current will be higher.

The pin 7,8 stuff I reckon is just a way of showing that thjose pins supply the heaters of both triodes.

Cheers

Ian

 

[emrr]

Is this the same preamp used for the Motown recordings?  I always loved that topend sound.  I'm sure most of it was the great singers however.

These modules were meant as add-ons for power amps.  They would have virtually never been used in recording systems. 

Has anyone ever verified that Motown actually used any Western Electric amps, or is that still just a rumor spread further by Fletcher?  If they had anything, it would have probably been a broadcast console, and those all had multiple preamps built onto one chassis.