For your amusement: WE 141-A preamp clone

[lassoharp]

rumor spread further by Fletcher?

I noticed that the recent reprint of his old Mix article omitted all the assertions of Motown/WE.  I still recall the gushings of the original.    ;D 

 

[emrr]

he retracted the statement somewhere, but never clarified. 

Bob Olhsson covered some details in his TapeOp interview, way back when.    Tube studio and SS studio, etc. 

 

[fazer]

Just read the Bob Olhsson article in Tape Op.  It was a different time.  I didn't know where I heard the WE preamp thing but Fletcher was most likely where. 

Back to the WE preamp being a front end, I guess that's the basis for all preamps.
I have the parts so what the hell.  My experience with the ouncer transformers (UTC 0-16) were in DI boxes in old days.  Now I see that input capacitance in that use is horrible with a guitar cord hooked up to the front end of a 50k input tranny hooked in reverse.  I'll just hook the 50k output lead as close to the grid input as I can mount it.  I just want to test and compare sound before going full tilt fab. 

I also have a mu-follower circuit.  Eric Barbour wrote "Brute Force Line Stage", in Glass Audio 1995.  I was going to build for a general purpose line amp based on 6sn7 tube bottom with a 6bx7 top tube.  It also drives 600 ohm without a Transformer out. He used it to drive his AKG 240 headphones direct no Transformer out. 

I plan on working on both circuits at the same time from an old donner tube PA intercom chassis. I'll use it for the power and faceplate.  It is just for fun.  I love the look of those old 6J7 tubes with ground cap.  It reminds me of your old Tube Preamps  EMRR.

 

[JayDubrek]

For my own amusement I sketched this up.....prefer a good 'ol birdsnest over a PCB for tubes meself......

Can't spot any mistakes but then you never know!

 

[emrr]

you guys are in the spirit; enjoy.

check the layout:

http://www.vintageaudio.net/members/view.php?item=45

 

[fazer]

JayDubrek  PTP pic

Looks like you need a jumper from pin 2 6sn7 to junction node of R13,R14, C7.3 

Also C1 .02 between input tx to G1 Control grid is not shown on drawing.  May not be needed with some tx's. 

Somebody earlier said the cap was the case ground on 6J7 but I see its G1 control grid now.  pin 1 is case ground on 6J7 by my  old RCA RC-14 book. 

The rest looks right but I'm at work now and need to comeback to this later. 

 

[emrr]

realized there's not a link to this:

http://www.groupdiy.com/index.php?topic=43092.msg536613#msg536613

 

[JayDubrek]

Thanks Fazer...corrected and updated!

 

[DaveP]

Has anyone else noticed the similarity between the WE 141-A front end and the V76?

I found a detailed explanation of the AC and DC separation in a late 50s early 60s textbook, it was described as elegant.

The voltage difference between the cathode and heater seems to be a critical factor, either elevating the heater or the cathode voltage.

Which came first, WE 141-A or V76???

best
DaveP

 

[emrr]

141-A dates to the mid 1940's roughly. 

 

[fazer]

I thought elevated heater, Cathode was to protect the tube from breakdown.
the recommended difference in cathode to heater on some tubes is quite low making them bad candidates for CF's.  I guess the same is true for mu followers.

Does the book (DaveP) suggest an enhancement in Audio quality as well?

 

[emrr]

DaveP's post has nothing to do with heater/cathode relationship.  With the 141, it's your basic DC biased AC heater, as seen in a million places.  It improves noise.  In this case it is not a safety issue. 

That which DaveP points out is a bias trick, which I am too brain dead and lazy to think much about this AM. 

 

[DaveP]

EMMR is right, this was described in the book as an elegant way to bias a cathode follower or concertina stage.

The academics who write these books don't generally have any commercial experience (unless retired from industry) so there was no mention of audio quality.

So the WE 141A was a wartime design (also licenced to Pultec for the MB-1) and used later in the late 50's by IRT for the V76 input tubes of both stages of the amp.

The fact that the cathode of the V76 is at 11V is probably incidental come to think of it.  In general I thought the cathode needed to be negative with respect to the heater to avoid diode effects within the tube, but I've forgotten the exact reasoning due to senility rather than lack of caffiene.

best
DaveP

 

[lassoharp]

Is there a coffee shortage today?

 

[emrr]

yes, or the coffee is bad, or the tolerance is high.  I'm about to make more and I'm still not awake. 

 

[fazer]

Thanks for the explanations on the DC bias guys.

You know when coffee doesn't work anymore, I find there is nothing like working on high voltage circuits to get you to wake up. 

I need to pickup my chassis to start work on this bad boy. 

I'm still thinking about emrr comment on the splat sound of a CF.  That's not what I'm looking for but gotta try a WE 141 before talking myself out of it. 

I like the simple PTP layout from JayDubrek.  I will use a tag board for some of it, and terminal Strip to get the show rolling.  There is a pwr supply in the chassis I want to power from.

 

[CJ]

i have orig xfmr data for 141

weird lams  input that is

 

[fazer]

CJ,  141 weird lams how?  That's the problem with building these things originals have sound due to many thing. 

Also any info on ADC 118 F transformers .  I know they are repeaters. they sound good to me.  Sorry to get of topic.

Also any UTC 0 16 grid input tx.  I have many UTC catalog pages but somewhere there is suppose to be a recommend Zobel network for R and C parts for this.  I can Figure something on the bench but I've heard there is info on this.  Never come across it on the internet.

Thanks

 

[dustbro]

i have orig xfmr data for 141

Please share! I'd like to see what's going on in there

 

[CJ]

the in and out iron goes for big bucks on evil bay

i think i have all he winding data,

as soon as i find my fugi usb cable i will post some stuff