All things LA2a related

[Biasrocks]

Have I done something wrong here?  or should I look elsewhere?

First, measure your DC voltages and compare to the schematic.

If the voltages look good, trace the signal from input to output to see where it disappears. If you don't
have a scope, you can build one of these.

http://www.groupdiy.com/index.php?topic=3278.2140

Regards,
Mark

 

[analogexplosions]

I'm not sure if i'm correctly grasping what this DIY scope consists of, I read that section of the forums, but still couldn't get exactly what it is.  Is it a signal generator outputing through an xlr cable with clips?

I am getting good DC readings from the Power section, so i started probing with my DMM to get readings on the tubes.  When I touched + to pin 3 of V1 (12ax7), Signal started coming through...  Does this mean i have a cold joint there?  Not sure why touching this pin lets signal pass.

Also, I'd like to get some readings and post them to see where any problems might be, but i'm not sure which points would be relevant...

Please forgive my "green-ness"  This is my first non-pcb build, and I feel there are FAR more chances to get something wrong.  Every other build i've ever done has just worked perfectly on first fire-up! 

 

[Biasrocks]

I'm not sure if i'm correctly grasping what this DIY scope consists of, I read that section of the forums, but still couldn't get exactly what it is.  Is it a signal generator outputing through an xlr cable with clips?

No, it's a signal probe that let's you hear the low level audio in a circuit. It's basically a capacitor to block DC and a guitar cable into an amp.

http://www.groupdiy.com/index.php?topic=44009.msg549882#msg549882

I am getting good DC readings from the Power section, so i started probing with my DMM to get readings on the tubes.  When I touched + to pin 3 of V1 (12ax7), Signal started coming through...  Does this mean i have a cold joint there?  Not sure why touching this pin lets signal pass.

Could be a cold solder joint or the tube socket not making contact with the tube.

Also, I'd like to get some readings and post them to see where any problems might be, but i'm not sure which points would be relevant...

http://www.waltzingbear.com/Schematics/Urei/LA_2.htm

There are voltages marked at different points in the schematic, verify that's what you're getting.

Regards
Mark

 

[analogexplosions]

Thanks for this schematic, it is FAR easier to read than the others i've looked at!

I'll report voltages as soon as possible.

 

[juanjovincenzo]

Hi all guys!!!
I am working in my first turret vintage project; I am using the classic Cayo-Costa turrets layouts - Teletronix 1968 La2a schematic http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_layout.jpg. I will buy sowter trafos....... but my doubt is what pair buy... the 68´s original remplaces  1009 - 1010 or  the La2 Drip version 4383 - 8940. 

I want the versatil soft compression and leveling but I dont want to lose the old Vintage character.
I have a good experience working with turrets and guitar amps but in PROaudio world the turrets projects will be a noisy chaos.
So please advise me.
Best Regards

Juanjo

 

[analogexplosions]

So, i took a few steps backwards in my build because voltages seemed kind of off.  I'm re-soldering everything on the tube pins now.  Before I go much further, I want to confirm my usage of the Sowter 1009 Input Transformer.

Primary
Red- to TB1/terminal one
White- to TB1/terminal 3
Blue- to TB1/terminal 5

Secondary
Grey- to ground
Green- to R6 (68k resistor) continuing to R7 (2.7k resistor) AND R5 (68k resistor) which goes to ground.

The primaries are self explanatory and I think those are correct as long as I've wired to the correct corresponding leads to the original UTC layout.  The secondaries are where I'm really confused and need to figure out what to do with.  I'm attaching a pic.

 

[Biasrocks]

The primaries are self explanatory and I think those are correct as long as I've wired to the correct corresponding leads to the original UTC layout.  The secondaries are where I'm really confused and need to figure out what to do with.  I'm attaching a pic.

I'm not sure what TB1 is, but with a balanced input on XLR assuming Pin 2 hot.

XLR Pin 2 = Red
XLR Pin 3 = Blue

And yes, Green goes to R6 and Grey goes to ground.

This assumes that RED on the primary and GREEN on the secondary are in phase.

I thought you were getting signal through to the 12AX7 already?

Regards,
Mark

 

[analogexplosions]

The primaries are self explanatory and I think those are correct as long as I've wired to the correct corresponding leads to the original UTC layout.  The secondaries are where I'm really confused and need to figure out what to do with.  I'm attaching a pic.

I'm not sure what TB1 is, but with a balanced input on XLR assuming Pin 2 hot.

XLR Pin 2 = Red
XLR Pin 3 = Blue

And yes, Green goes to R6 and Grey goes to ground.

This assumes that RED on the primary and GREEN on the secondary are in phase.

I thought you were getting signal through to the 12AX7 already?

Regards,
Mark

I was kind of getting signal to v1.  It was crackly and intermittent.  I rewired so i could double check all of my work and have a tidier work area inside this box.

My green is going to R6 and on to R7 currently.  Should green ALSO be connecting to R5 and going to ground?  if not, where does R5 fit into the picture with the Sowter 1009?

Also:

White, corresponding with pins 3 and 4 on the UTC, isn't really connected to anything... Is this correct?  I have it terminated to a pin which connects to nothing else as per the cayocosta layout.

 

[Biasrocks]

My green is going to R6 and on to R7 currently.  Should green ALSO be connecting to R5 and going to ground?  if not, where does R5 fit into the picture with the Sowter 1009?

R5 connects between GREEN and GREY. Pin 7 & 10 on the UTC.

White, corresponding with pins 3 and 4 on the UTC, isn't really connected to anything... Is this correct?  I have it terminated to a pin which connects to nothing else as per the cayocosta layout.

As I said earlier, it's the center tap (CT) and is not normally connected to anything, it should be isolated from the chassis, IE. not grounded.

Regards,
Mark

 

[analogexplosions]

Alrighty,

The New York Armory Art Fairs are over and I can get back to this thing!

So, my voltages are coming in pretty unruly.  I did some test with the t4b removed and a 1k test tone feeding into it.  (does not pass signal)

Here are my readings on the tube pins:

V1 (12ax7)

Pin 1- 192
Pin 3- 0.05
Pin 6- 129.5
Pin 8- 1.19

V2 (12bh7a)

Pin 1- 316
Pin 3- 284
Pin 6- 311
Pin 7- 36
Pin 8- 285

V3 (12ax7)

Pin 1- 233
Pin 3- 4.2
Pin 6- 232
Pin 8- 4.2

V4 (6aq5a)

Pin 2- 7.15
Pin 5- 140
Pin 6- 122




This makes me worried.  How the hell could I be so off?  I'm using the Hammond 369JX transformer, getting 120v from the wall, I have Black and Blue tied together and going to the switch and back to the live lug and White and Brown going to the neutral. all twisted.  Red/Yellow and Green/Yellow are going to ground.  Green pair is twisted and going to Fils and VU lamp. Red Pair is twisted and going to CR1 and CR2.  I'm pretty sure this is all correct, right? 

Thanks for all your help, I'm trying as hard as I can not to feel defeated right now.

 

[analogexplosions]

Doing my own troubleshooting (random probing with chopstick, half a pack of cigarrettes, and plenty of cursing), I got signal passing through.

Problem is, it is VERY low.  Logic's meter reads -45db on the return with gain maxed.  I am sending a 1k tone through to the input.  When I turn the gain knob on the unit, the signal goes from barely audible in headphones to -45 and gainy, which leads me to believe maybe the makeup gain section is working.  The distortion i get when maxed does sound like typical tube distortion, like when you max out a nice tube pre.

It seems college101 had a similar issue, and I was hoping to hear what it was, but then he just said it was a dumb mistake and didn't want to talk about it...  talk about a cliffhanger there!

Any ideas?

 

[analogexplosions]

Signal is fully passing!

...and much like college101's issue, it was a silly mistake.

There are still problems i'm experiencing though:

1- Grounding is either bad, or I need to ground the front panel.  If I hold a SM57 that is going to the input and the front panel at the same time, it gets quiet.  Have I messed up somewhere?  or will grounding my front panel do the trick.  any advice on the best way to do this?

2-  When gain goes above 2 o'clock on the Gain knob, I lose all signal and start picking up interference.  VU meter slams full-right.

3- Gain reduction still does not work.

My stupid wiring mistake was not grounding the yellow lead from the Sowter 1010 like pin 10 of the UTC A24.  Currently, I have that lead running to an isolated turret connected to R20 and R10, and have a wire going from that turret to ground.  Should I connect the Sowter leads together before it hits this turret? or is doing it the way I have sufficient?

 

[analogexplosions]

Bump for ideas?

 

[Biasrocks]

Signal is fully passing!

...and much like college101's issue, it was a silly mistake.

There are still problems i'm experiencing though:

1- Grounding is either bad, or I need to ground the front panel.  If I hold a SM57 that is going to the input and the front panel at the same time, it gets quiet.  Have I messed up somewhere?  or will grounding my front panel do the trick.  any advice on the best way to do this?

2-  When gain goes above 2 o'clock on the Gain knob, I lose all signal and start picking up interference.  VU meter slams full-right.

3- Gain reduction still does not work.

My stupid wiring mistake was not grounding the yellow lead from the Sowter 1010 like pin 10 of the UTC A24.  Currently, I have that lead running to an isolated turret connected to R20 and R10, and have a wire going from that turret to ground.  Should I connect the Sowter leads together before it hits this turret? or is doing it the way I have sufficient?

Grounding is important, wire routing is important. I suspect you have a combination of both problems.

Yes the the front panel should have a clear connection to the chassis, verify continuity.

The yellow wire on the Sowter output should be grounded.

The oscillation as you turn up the gain knob is most likely wire routing or grounding related. You should be using shielded cable for your connections to the front panel.

Here's my build

Regards,
Mark

 

[analogexplosions]

Thanks for your responses, biasrocks!

When this thing is done, I'll owe you many a beer.

I am using some Mogami Quad mic cable for my front connections.  The fact that it is quad cable shouldn't make a difference, right?  The two hot's are twisted together and the two neutrals are too.  On the Gain reduction pot, I am using two different cables with the shields tied together.  One is carrying signal from middle lug to pin 6 of V3, and the other is going from right lug to R6/R7.

As far as the front panel grounding goes, should I grind some paint off the screw connections too?  might as well, that makes sense.

And for the yellow Sowter 1010 lead, should I strip some sheilding and make a trip to gound BEFORE it hits the connection with R10, R20, and C4?

Thanks again.  Your build looks beautiful!  it somehow looks a million times more organized than a turret board build.

 

[Biasrocks]

Quad cable is fine, slightly more capacitance but the runs are short.

You want to minimize ground points if possible, attaching to R10/R20 gives you a proper ground.

Looking at my build you can see a black wire running from the back of the GAIN pot to the turret which is connected to chassis ground, you should do that if you haven't already.

Regards,
Mark

 

[analogexplosions]

Am i correct in grounding both the red/yellow and green/yellow (striped) from the power transformer?

 

[Biasrocks]

Am i correct in grounding both the red/yellow and green/yellow (striped) from the power transformer?

I don't know what kind of PT you have.

Regards,
Mark

 

[analogexplosions]

Sorry,

I think I mentioned earlier in the thread.  Hammond 369jx

 

[Biasrocks]

Yes, the center taps should be grounded.

Regards,
Mark