All things LA2a related

[Kingston]

Tapping on V1 gives the strongest response...

Anyone has any ideas?

V1 (a/b) are the ones making the high gain of the line amp stages so yes indeed most sensitive for microphonics. V3 is just there to turn on a "light bulb". very non critical.

Get some better ones. My recommendation is Tungsol reissue 12AX7. Completely reliable and very accurate.

http://uraltone.com/kauppa/product_info.php?info=p677_Tungsol-12AX7.html&XTCsid=fdtvu8pi13i16jpquqcdk77c46

(above link is just an example, they are sold in all good tube stores)

[edit]

that noise floor looks normal. I checked measurements from a non-p2p build of mine and it has a similar low frequency slope. G9 has a tendency for this as well. I think it's an artifact of certain types of negative feedback.

 

[radiance]

Thanks a bunch Kingston!!

 

[buildafriend]

I have:
                      2.75VAC
                                          at 60hz
                                                              on my output

- Output is connected to the next stage for proper loading (next stage is known good)

                      It sounds like my signal path is hitting ground somewhere right?

This is a Drip LA-2A PCB V3.

thanks regardless.​

 

[Jimi_kay]

Hello people. 

First off wow tons of info on here.  It's great. 

I have done the cayocosta layout with jenson transformers. 
I wanted to see if these voltages are correct.  I know v1 pin one is low, like really low.  But are all the others within the range?  Is the T4 pin 5 to much?

Test Point ME

C7B + 244.8

12AX7A #1 V1
PIN 1  4
PIN 2 1.4
PIN 3         1.4
PIN 6         85.8
PIN 7         0
PIN 8         1

12BH7A V2
PIN 1       116.5
PIN 2       0
PIN 3       3.1
PIN 6       236
PIN 7       86
PIN 8       120.3

12AX7A #2 V3
PIN 1         73.9
PIN 2         0
PIN 3         0.7
PIN 6         73.9
PIN 7         0
PIN 8 0.7

6AQ5A V4
Pin 1         0
Pin 2         5.1
Pin 5 134
Pin 6         92.5
Pin 7         0

Lamp (glowing) 51

Heater A 3.37 ac
Heater B 3.37 ac

R 29  242

T4 Pin 5 13.57

after diodes 357.4


What could cause V1 pin one to be so low voltage.  I've checked all the grounds, all the cabling... The zero meter adj works, I'm stumped.

Also I know it passes audio without the t4 in but should it already be clean.  I have a lot of noise.




Thanks for your time. 

Jimi

 

[Biasrocks]

12AX7A #1 V1
PIN 1  4
PIN 2 1.4
PIN 3         1.4
PIN 6         85.8
PIN 7         0
PIN 8         1


What could cause V1 pin one to be so low voltage.  I've checked all the grounds, all the cabling... The zero meter adj works, I'm stumped.

Jimi

A common error is to insert a resistor that should be for example; 470R with 4.7K or 470K. Trace the path to V1/Pin 1 and make sure all your resistor values are as they should be.

Regards,
Mark

 

[Jimi_kay]

Thanks for the reply Mark. 

I have attached a pdf of my resistors and caps list just to make sure before I start going over everything that I have the right numbers to start with.  Hopefully I have not mixed up things so early in the process. 
I tripled checked each thing before moving on when building, but never thought that maybe during the ordering part I mixed something up...

If someone could confirm this list, I will go over my orders from mouser.  This is where I created labels for each part like r18, c2, ect... When doing the build I went off the labels, not really double checking if that was the correct part.  I know I double checked before ordering, but damn who knows at this point where it went wrong...

I had no idea what I was getting into with building a point to point LA2A!  Ohh the rollercoaster ride it has been.

This is with the Jensen mod highlighted in yellow are those changes. 

I was told on the caps I can go up in voltages just not down.  So I did on some to make the parts easier to find.  Is that going to mess anything up?

Also I see now that there are some voltage changes on the jensen mod chart.  I will have to note those down and check against what I have.

Also with wiring should I have the wire bent and "floating" so that the heater is the only thing running along the chassis?  I did not do this.  I basically took the shortest path of wire I could make and I could cleanly solder.  I have a feeling I may have to clean up some wiring for the noise...

If there is anything else I can provide that can help I will. 


Thanks for your time. 

Jimi

 

[Bowie]

I have attached a pdf of my resistors and caps list just to make sure before I start going over everything that I have the right numbers to start with. 

Because checking your caps and resistors is very basic but time consuming, I doubt anyone is going to do it for you (maybe some kind Samaritan will prove me wrong).  Definitely do what Mark said and check the values closely.  I did everything perfect but mistook a 47 for 4.7 or something to that extent and it plagued me for hours. 

 

[Jimi_kay]

Thanks Bowie for the reply.

I have triple checked against the schematic, and the parts list and my list all values seem to match.  As long as raising the voltage wont matter on the caps, It should be good.

Now time to dig in and find all the resistors and check everything in the box.

About the heaters.  I read that it was supposed to have 6-7 volts off the heaters.  I thought this was combined so I thought my 3.37 was not bad. 

When I test my heaters, what voltage should I be reading at each point?  Also if it was wrong wouldn't all the other voltages to the other tubes that came out close be way off or underpowered. 

I am using the allied electronics 6K88VG.  I see it is 6.3 volts@ 2 A leads.  Is that what I should I be measuring at each cable, or is a lead considered a pair?

How can I test if my power transformer is good.

Sorry for all the noob questions.  You guys kick ass

Jimi

 

[Jimi_kay]

Update:

Checked EVERYTHING.  All correct parts in the correct place.

So I tried switching the tubes (12ax7a).  All of a sudden, I'm getting proper voltage readings in V1!

Ordered another replacement 12ax7a.

Ill let you guys know how it works when it's slapped in.

Thanks or the help!

Jimi

 

[Jimi_kay]

So guess what kind of tube my ART MPA GOLD uses....

Had one sitting in the garage, opened it up and slapped that tube in....
Boom.  I think everything looks good. 
Found one short with the shield of a cable and I just shrink wrapped it.  Hum gone. Sweet.  I came through loud and clear. 
Should I put the t4b in and see if I can compress?  Just want to check in.  Really don't want to break one of the more expensive parts if I can help it...

 
Test Point ME

C7B + 245.3

12AX7A #1 V1
PIN 1  109.5
PIN 2  0
PIN 3         1.02
PIN 6         107
PIN 7         0
PIN 8         1.03

12BH7A V2
PIN 1       117.1
PIN 2       0
PIN 3       3.18
PIN 6       236.8
PIN 7       86.1
PIN 8       120.5

12AX7A #2 V3 (Bottom Right)
PIN 1 84.5
PIN 2         0
PIN 3         0.7
PIN 6         84.5
PIN 7         0
PIN 8         0.7

6AQ5A V4
Pin 1         0
Pin 2         5
Pin 5         134.1
Pin 6         91.9
Pin 7         0

Lamp (glowing) 51

Heater A 3.38
Heater B 3.38

R 29  243

T4 Pin 5 8.24

after diodes 355

Thanks for all your help

Jimi

 

[Biasrocks]

Check for AC voltage on PIN 3 of the T4B socket while running a signal through and with the 'Peak Reduction' control half way up, this is your sidechain voltage, it should be in the 80-120 VAC range when compressing. It could be higher depending how much GR is being applied.

Check if pin 1,2,4,5 and 8 have conductivity to ground / check pin 7 to CCW lug of the gain pot.

There's a thread dedicated to this here

http://www.groupdiy.com/index.php?topic=30911.0

It's for an LA3A, but the same principals apply.

Regards,
Mark

 

[Jimi_kay]

Check for AC voltage on PIN 3 of the T4B socket while running a signal through and with the 'Peak Reduction' control half way up, this is your sidechain voltage, it should be in the 80-120 VAC range when compressing. It could be higher depending how much GR is being applied.

Check if pin 1,2,4,5 and 8 have conductivity to ground / check pin 7 to CCW lug of the gain pot.

All looked good but I was wondering about the grounds...

Do you mean 1, 2, 4, 6, 8 for the grounds on the t4b?  That is where the jumps are made.  The numbers I listed are all grounded for me. not pin 5. 

Thanks for your quick responses.  I cannot wait to build so much more gear!!!!

Jimi

 

[Biasrocks]

My bad, you have it right.

Regards,
Mark

Do you mean 1, 2, 4, 6, 8 for the grounds on the t4b?  That is where the jumps are made.  The numbers I listed are all grounded for me. not pin 5. 

Jimi

 

[Jimi_kay]

Plugged in everything and it works great.  Sounds awesome.  Clean signal.  Compression works fine on tones and program material.

Everything looks perfect but...

The meter just sits as 0.  No matter what I do.  I can adjust it with the zero trim when in gain reduction but when in +4 or +10 it just hangs out at 0.

First I want to make sure I have the right piece...

http://www.allelectronics.com/make-a-store/item/RSW-50/ROTARY-SWITCH-2-POLE-3-POSITION/1.html

2 pole 3 position switch.  I assume the poles are the two inside points.  And then 123 around the edge on the corresponding side.  On one side pins 1 and 3 are jumped so the cable going to TB2 pin 0 could go from either.  I have tried to switch the cable that goes to TB2 pin 8 to the other side of the resistor.  Nada.  All the other wiring and components are correct.  Do I have this switch wired wrong or maybe a bad vu?

Would a broken meter still be able to zero adjust?  Shouldn't it also be resting all the way left when the unit is off. Again mine just hangs out at 0 all day long...

How could I test the vu meter?

Thanks

Jimi

 

[Jimi_kay]

Just an update.  I had a cheap meter and replaced it with a hairball audio meter and all is working great!
Sounds awesome and works perfect.
I also put in 24 step Elma switches.
Thank you all for your help.
I think number two is going to go much smoother!

Jimi

 

[sr1200]

Im looking to do a point to point la2a, but am having issues finding transformers (input/output) I know sowter makes trannys for em, but since you can only get them from across the pond, i looked into more "local" source for transformers.  I spoke to dave from Cinemag, and he gave me a few suggestions (that guy should be in the legenday knowledge of audio hall of fame). 
his suggestions are this:
for output (inplace of the A24)
A CM2810 which is a 4:1 ratio (a little hotter than the 5:1 original) which he said is used in the 610.  Other options included the CM9589H (which is high nickel content which he says will effect the tonal coloration) and a CM9589L which is more balanced formula
http://cinemag.biz/output/PDF/CM-9589.pdf

for input (inplace of the HA100x or H10)
A CMQEE3440 which he says will give a bit more bandwidth.
CMQEE-3440A 37.5,150,600 : 50K -2 dBu Quad Eight vintage

Anyone have any thoughts on this?  Since Im not using all original parts anyway, im not so concerned with the sound being as accurate as most people that are trying to clone.  Just looking for a second opinion before i drop the money into it.

 

[Che_Guitarra]

Hi guys.  This thread is pretty epic so if I missed any advice on my flick through the pages I hope you can point me in the right direction.


I need some troubleshooting help with my Drip Opto 4 based LA-2A. I seem to have a problem within the peak reduction circuit - the front control pot is non-responsive and I can't swindle any compression out of the unit. The pot tests OK on a multimeter.

On the positive side, the gain control seemingly works fine, the unit is passing audio (at a reduced output), tubes are all glowing, and the unit has been on for a good fifteen minutes without frying a resistor, blowing a cap or emitting any bad smells.

With a double check I can confirm all cable connections to and from the PCB are correct, and the unit has bare metal continuity at every screw point.  T4B is correctly selected.



Any suggestions where to start looking?

 

[Che_Guitarra]

These are my voltage readings in bold - some are obviously askew.  Unfortunately i'm an electronics greenwood so drawing lines between dots isn't my strong suit.  Any help to resolve would be much appreciated.

Directly after the 1n4007 diodes: 370VDC… 342V
Directly after the 4.7K/2W: 265VDC… 285V

6AQ5 (V4)
pin2: 7.5V… 9.5V
pin5: 115V… 95V
pin6: 137V… 133V

12AX7 (V3)
pin1: 107V… 90V
pin3: .75V… 0.9V
pin6: 107V… 90V
pin7: 0V… -3mV
pin8: .75V… 0.9V

12BH7 (V2)
pin1: 93V… 120V
pin2: 0V… 0V
pin6: 216V… 122V
pin8: 98V… 123V

12AX7 (V1)
pin1: 90V… 106V
pin3: 1.53V… 4.3V
pin6: 100V… 235V

Obviously some areas for attention.... what should I try next?

 

[sr1200]

Could use some help here. Ive been over the schematic and diagrams at least a hundred times. My 12BH7 is getting the following voltages.
Pin
1) 276.5
2)
3) 274.5
4)
5)
6) 278
7) 209
8) 278

Wth!? Cant find what is wrong. Any help is greatly appreciated.

 

[Deepdark]

Hi guys. I have a really ode issue with my voltages. I did my la2a p2p, accordant to cayocosta layout. I have a utc a10 and a24, hammond 369jx. I took my voltage after de diodes, I got +\- 360v, which is nice. Then, I took the voltage after the 4,7k, and I got 317v, instead of 275v. And then, all my voltages aren't right. The only place that looks ok is the néon bulb at 52v. The t4b isn't installed. What can explain this voltages that high? Here are the tubes voltages:

V1
Pin1 = 136v
Pin3 = 1,84v
Right after the R9 I got 136v and 60v before the r22, r21

V2
Pin1 = 61v
Pin8 = 50v

V3
Pin6= 260v

V4
Pin6 = 260v
Pin8 = 42v

As you see, there is a real problem with those voltages. Can someone help me please? I double Check for band wiring, i change my 4,7k, too. When i took voltages on Any Heather connections, i got 0v...

Thanks a lot.