All things LA2a related

[sleepingtiger]

Thanks! You rock hard! I missed this one...

 

[alexc]

Hi

here's some pics of my a*d*l 1000. Similar to original
but with pcbs, pcb-mount t4b and not sure what transformers.
Same valves, all in a line.

I like it. Classy sound on guitars and bass.

Never heard an orig, but this things is cool.

Going to build me an la2a and have two!

Bye

 

[cannikin]

[quote author="JerryPbury"][quote author="cannikin"]I still have some really odd values/readings, some are right on, all others are way off. I checked my connections and component values at least 8 or 10 times over the coarse of 2 - 3 weeks. Everything is correct.

I have using reference ground from various places over the unit and the readings are the same.

B21: 361v
B20: 360v :evil:
A1: 57.1v
A20: 288v :evil:
A14: 241v :evil:
A17: 100v
A19: 188v :evil:

How can some values be happening and others wacked?
Could a cold Solder joint cause this? I didn't see any - but I could have missed it - too much to look at.[/quote]

Welcome to my world. My voltages are still high. I have checked every component, ground, wire, heater connection and touched every solder joint over again, yet my voltages are way high. Cayacosta said it was probably my bias. I checked every related component, ground and heater connection and it didn't change. I even changed all related caps. The voltages still remain high. I'm just dyslexic. I hope I don't do the same thing to the second one.[/quote]

Well Scenaria & JerryP

I'm Making some headway....

I replaced the crappie ceramic Disks - 470pf (Parallel to the Neon) and 510pf (V4 to R3) with Mica, and my numbers are getting much better.
I also had a metal film 33K at R25, I replaced that with a carbon.

Check this out:

B21 - 356v
B20 - 282v
A1 - 57.1
A20 - 104v
A14 - 229v
A17 - 94.6
A19 - 132v

Though the numbers are much much better ... A20 is a little low and A14/A19 still a little high.

Any thoughts?

 

[sleepingtiger]

So I think I've narrowed it down considerably. V2, pin 8 cathode voltage is 120VDC instead of 1.1V and V1, pin 6 plate voltage is about 240VDC instead of 120V. All of the other voltage are a little (consistently) high but close to normal. I'm double checking the wiring and values around the problem area but so far I haven't found the problem...

 

[Guest]

if you could speak in actual la-2 (schematic) resistor numbers and not Bloo numbers im sure a lot of other people could help you.

i have the ciricuit memorized backwards and forwards

and i have no idea what the heck b21 and a14 are.


perhaps there should be a bloo la-2 thread intstead of a general

la-2 thread.

just a thought cause i sure would love to help.

 

[cannikin]

Hey driller45;

Silly me - Sorry, didn't think of that...

Anyway I'll try to figure out where they would be on the schematic...

But they are the Turret numbers on the Main Turret. Going Left to Right - 1 through 25, Back row A/Front row B.

Like this

http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_layout2.jpg

 

[Scenaria]

dude!

those numbers look just fine...

is it not working?

dont get caught up in the numbers game.. they are all approximations... remember all of us use diff caps and resistors.. the numbers are just ball park figures...

 

[Guest]

cool , now i can try to help :wink:

 

[cannikin]

[quote author="Scenaria"]dude!

those numbers look just fine...

is it not working?

dont get caught up in the numbers game.. they are all approximations... remember all of us use diff caps and resistors.. the numbers are just ball park figures...[/quote]

Its working great, I like chasing my tail (not :roll: really) I tend to get compulsive about numbers sometimes. Honestly all my problems went away when I replaced the 470 and 510pf caps. :razz:

 

[Scenaria]

you know what they say

if it aint broke...

CLOSE THE COVER and USE it

sheesh :roll:

congrats!

now show some pictures :green:

 

[cannikin]

Scenaria and everyone thanks for your help and patience with this.
thanks.

 

[Bluzzi]

David - Great to hear you are finished. It looks great! Like scenaria said "just use it"!

I am still at the parts gathering stage and just got the A-10s (building 2 LA2A). Next are the T4Bs and then I start on the power xfr, caps and resistors. It will take me months more but as I read all this progress it keeps me going.

Enjoy

Jim

 

[cannikin]

Thanks Bluzzi :grin:

Scenaria, Cayacosta, and everyone were so patient & helpful.

Building the LA2a was great fun, from buying the parts to trouble-shooting, and now using it on recordings. Whenever I finish a project and taste the fruits of my labor, the first thing I want to do is build a second one

have fun :wink: take your time, and when you don't understand something the answer is here at this knowledgeable and generous forum.

 

[Bluzzi]

Yes, the generosity and sincerity here is huge! I can't wait to finish mine and hopefully contribute some minute amount to this community. Thanks for the encouragement (typical of this place).

Jim

 

[sleepingtiger]

I've gone back through the output section and I still have the same high voltages at V1 pin6 and V2 pin 8. I've checked and re-checked all of the solder joints and component values and everything appears to be in order. Obviously, something's wrong and I'm not finding it. For now, I've officially run out of ideas... Do any of you have some insight for me?

Congratulations, Cannikin! Looks great!

 

[Scenaria]

forgive me if ive already asked you this before..

whats the values you put in for the following...

R29
R34
R36


and double check to see that you have a jumper going from A11 to A14

 

[cayocosta]

On the 12bh7 pin 1 you should have around 100V.

Check your ground from pin8 of the 12ax7, you should read 2.7k to chassis ground.

Check the 470k to ground connection from pin 7 too.

 

[sleepingtiger]

R29 - 4.7K/2w
R34 - 22K/2W
R36 - 1K

A11 is tied to A14

110VDC at pin 1 of 12bh7a
pin 7 of 12ax7 to ground is 2.7k
pin 8 of 12ax7 to ground is 468k

note that the last 2 values are the opposite of what you just gave me

 

[cannikin]

Thanks sleepingtiger,

I did a few things that changed my values, some may have made a difference/some not. Are you using any ceramic disks? if you are - replace those! 470pf/510pf.

Check R34 - I changed it to a 10K/2W instead of a 22K/2W. At that point A20 Value was in line.

The Next thing I did - might be black-magic - but I was using some PCb mount 1/2W resistors (the ones with thin leads) I replaced those and my values adjusted again about 2 to 5v.

After I did all the above, my voltages were happy, or rather I was Happier with my voltages.

If your VU meter isn't working correctly check how you wired the mini-turret to NEG Pin of VU. I ended up running a wire from A5 of the mini to the Neg Terminal and the meter behaved properly.

Also Really analyze the Wiring of this layout.

http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_layout2.jpg

Compare the your Wiring to the document, I changed some of my Ground Wire locations from R1/R2/R4 to exactly like cayocosta layout, noises went away and helped out.

I'm positive it was a combination of the above that created my extremely high voltages problem.

 

[cayocosta]

The 12bh7 is okay.

The 12ax7 is wired backwards - pin 7 and 8.

Pin 8 to 2.7k to ground.

Pin 7 to 470k to ground.