LA2A PCB ver 2.0 build thread

[wmb]

Does the transformer have the correct colored wires? What don't you know or is there something that doesn't make sense?

The primary wires are black. This is where the 120 vac comes into the transformer. The secondary wires are red. This is the high voltage ac output. The heater wires are green and output 6.3 volts for the tube heaters.

Every wire coming from the power transformer except the grounds which I think are red/yellow, green/yellow, gray and possibly a bare shield will connect on the pcb in a position marked with the color if wire for that point. The striped wires I listed above all connect to the chassis.

You should also grind the bottoms of the transformer "feet" down to bare metal. Similarly grind the finish of the chassis at the contact point of the feet to maximize contact between the chassis and transformer.

Maybe this helps or not.

cheers, wm.

 

[moogfrotee]

Hi wm,

look at page 10 of this topic, there i posted the picture of the sowter power.

The Sowter power is Gregs proposal for Europe.
But the color code is different from Greg´s manual.

I´m to much an amateur to make any experiments.

Cheers,
Max

 

[wmb]

Do you have a volt meter? This can be pretty simple if you are as careful as one should be when dealing with high voltages found in tube circuits.

What is your mains voltage? 240?

from the documentation...

"* FOR PRIMARY WINDING WITH SOLID CORE WIRE AND SLEEVING
You will note that the Primary is built up in sections and the two wires in the Blue,
White and Red sleeves must always be individually joined to make the primary circuit
complete. Spare connections not required can be cut short, each colour joined
separately and isolated. The solid wire inside the sleeving is coated with
polyurethane and needs to be stripped away and tinned if the leads are shortened. For
secondary windings with solid core leads please follow the same process.
FOR FLEXIBLE LEADS PRIMARY
Just cut short and isolate any spare connections "

Looks like you have the flexible leads primary from the photos.

EDITED
The BLUE and BROWN are the mains for the primary for a 240 volt mains line. This will connect to the PCB at the X-BLACK solder pads. You might trace the lines on the bottom of the board to assure the BLUE wire will connect eventually to the solder pad marked AC WHITE and the BROWN wire to the AC BLACK position.

The ORANGE wires on the secondary side connect on the points marked RED. The YELLOW wires are for the heaters marked GREEN on the PCB.

The YELLOW/GREEN, GREEN and GREY wires should be connected to the chassis.

I am fairly certain about all of the connections above and they satisfy all the PCB connections and earthing specified in the drip documentation.

From the sowter document it looks like you can cut all the unused wires on the primary side. I would do something to make sure they are completely unable to short circuit with anything as this could cause some serious problems.

Furthermore, I would confirm all the voltages with a volt meter before inserting tubes or t4b.

Post back here if you have a different mains voltage than 240.

Hope this helps.

 

[idphys]

I'm having a little trouble figuring out which Sifam meter to get (25SAQ?). Any suggestions?

Thanks,
Andy

 

[Stagefright13]

I used a Sifam AL29WF from meterdistributor.com about 50 some dollars. With the bezel kit.

I also bought the 6 volt backlight kit. Think $12.95

John

 

[wmb]

[quote author="wmb"]Do you have a volt meter? This can be pretty simple if you are as careful as one should be when dealing with high voltages found in tube circuits.

What is your mains voltage? 240?

from the documentation...

"* FOR PRIMARY WINDING WITH SOLID CORE WIRE AND SLEEVING
You will note that the Primary is built up in sections and the two wires in the Blue,
White and Red sleeves must always be individually joined to make the primary circuit
complete. Spare connections not required can be cut short, each colour joined
separately and isolated. The solid wire inside the sleeving is coated with
polyurethane and needs to be stripped away and tinned if the leads are shortened. For
secondary windings with solid core leads please follow the same process.
FOR FLEXIBLE LEADS PRIMARY
Just cut short and isolate any spare connections "

Looks like you have the flexible leads primary from the photos.

EDITED
The BLUE and BROWN are the mains for the primary for a 240 volt mains line. This will connect to the PCB at the X-BLACK solder pads. You might trace the lines on the bottom of the board to assure the BLUE wire will connect eventually to the solder pad marked AC WHITE and the BROWN wire to the AC BLACK position.

The ORANGE wires on the secondary side connect on the points marked RED. The YELLOW wires are for the heaters marked GREEN on the PCB.

The YELLOW/GREEN, GREEN and GREY wires should be connected to the chassis.

I am fairly certain about all of the connections above and they satisfy all the PCB connections and earthing specified in the drip documentation.

From the sowter document it looks like you can cut all the unused wires on the primary side. I would do something to make sure they are completely unable to short circuit with anything as this could cause some serious problems.

Furthermore, I would confirm all the voltages with a volt meter before inserting tubes or t4b.

Post back here if you have a different mains voltage than 240.

Hope this helps.[/quote]

I edited this post because I had indicated an incorrect position for connecting the primary of the power transformer. I was going from memory. As I was thinking of taking a nap I was mentally reviewing my original post. I remembered how I suggested connecting the primary and realized it was not correct. Hopefully this did not cause any problems.

 

[moogfrotee]

Do you have a volt meter? This can be pretty simple if you are as careful as one should be when dealing with high voltages found in tube circuits.

yes, i do.

What is your mains voltage? 240?

230 V

I edited this post because I had indicated an incorrect position for connecting the primary of the power transformer. I was going from memory. As I was thinking of taking a nap I was mentally reviewing my original post. I remembered how I suggested connecting the primary and realized it was not correct. Hopefully this did not cause any problems.

no accident so far :grin:

i have to analize what you wrote on the weekend, i´m to busy right now.
but thanks a lot!

 

[wmb]

For 230V you will use BLACK and RED from the primary side. BLACK replaces BLUE and RED replaces BROWN in my instructions.

Good luck!

 

[Purusha]

[quote author="Stagefright13"]
Yeah v1 and v2 are in the audio path. The BH7A and the 12AX7 #1. I can't tell you the difference in sound between the stock tubes and the ones I used. I built mine with NOS from the beginning. I put a 12AT7 in the 12AX7 spot because I already had some. Vintage NOS 12Ax7's can be pricey... But if ya go to tubeworld.com they have a pretty good explanation of the difference in sound between the various tube manufacturers. And in all my other tube equipment I have replaced the tubes and gotten better sound. Not usually night and day but with a good set of monitors you will notice the difference right away. Especially in the bottom end and highs.
[/quote]

Can I use 12AT7 for #1 and 12AX7 for #2 instead of both 12AX7 or 12AT7 in LA-2A or should I use both the same type?

 

[wmb]

[quote author="Purusha"][quote author="Stagefright13"]
Yeah v1 and v2 are in the audio path. The BH7A and the 12AX7 #1. I can't tell you the difference in sound between the stock tubes and the ones I used. I built mine with NOS from the beginning. I put a 12AT7 in the 12AX7 spot because I already had some. Vintage NOS 12Ax7's can be pricey... But if ya go to tubeworld.com they have a pretty good explanation of the difference in sound between the various tube manufacturers. And in all my other tube equipment I have replaced the tubes and gotten better sound. Not usually night and day but with a good set of monitors you will notice the difference right away. Especially in the bottom end and highs.
[/quote]

Can I use 12AT7 for #1 and 12AX7 for #2 instead of both 12AX7 or 12AT7 in LA-2A or should I use both the same type?[/quote]

If you are talking about just substituting an AT for and AX then yeah, no problem. There should be no penalty.

 

[Purusha]

OK, thanx.

 

[Stagefright13]

Yeah I have a 12at7 telefunken in #1 and a jjtesla ECC83s (12AX7) in #2. Sounds awesome. 2 different circuits. #1 is for audio. I would prolly stick with a 12ax7 in the #2 position cause of the slight gain difference. But it doesn't have to be an expensive one I would think. Cause the audio doesn't run thru it.

John

 

[moogfrotee]

Hi Purusha,

received the cases today!

looks even nicer than on the pictures!

many thanks,

cheers
Max

 

[chingon]

My Sifam AL29WF meter seems to have crapped out on me. Its been working fine until i noticed the other day. The needle just stays stuck to the left. The meter is has probably had only about 1 month use, which is when i put the compressor together. Has anybody ever experienced this?

Also I noticed when i first installed the meter that i would zero adjust and after a while the needle wouldnt go back to zero and i would have to adjust again. Did I do something wrong or is it possible i had a bummed meter to begin with?

Other than that the compressor still passes signal and functions.

 

[Stagefright13]

You should be able to test the meter by temporarily connecting it to the output xlr. I believe you will need a 3.6k resistor on it. If it works fine then you should check your wiring and solder joints. If not then the meter is bad.

 

[mata_haze]

Hi everybody,
I am now troubleshooting my La2A and i cannot figure it out why it's not shooting out a nice clean signal.

the sinewave I am using to test it goes through the first preamp stage V1 12AX7A runs through C2 and R15 and then ..disappears.
now on the original diaghram there is a connection to PIN 7 of the 12BH7A (V2) between C2 and R15, while on Drip board there is only a GRID resistor in that place (which I did not fit in as yet).
can anyone confirm that there should be a connection on that point please?

all the best
Mattia.

 

[idylldon]

[quote author="mata_haze"]can anyone confirm that there should be a connection on that point please?
all the best
Mattia.[/quote]

Yes. Install a jumper if you're not using the resistor.

Cheers,
--
Don

 

[mata_haze]

[quote author="Don"]mata_haze wrote:
can anyone confirm that there should be a connection on that point please?
all the best
Mattia.


Yes. Install a jumper if you're not using the resistor.

Cheers,
--
Don[/quote]

Works! thank you very much!

 

[Purusha]

Just curious. Do you guys put any terminating resistor on the output XLR in your LA-2A clones?

 

[David Kulka]

On homebrew LA2A's it will depend on the output transformer, whether a trimmer cap has been included in the feedback circuit, and other factors. If in doubt, try it both ways while looking at a square wave.

Teletronix recommended a termination resistor on their original LA2A's and after looking at many of them both with and without, I can tell you that the termination resistor will flatten HF response and clean up square waves a bit. Our policy is to install a resistor, and then tweak the trimmer cap for best square wave response at 1k.