All things LA2a related

[sonicrevolution]

Hey Sr1200, I am not positive about that, and that is one thing that I am worried about.  I used a continuity checker and probed around the legs of the transformers, but  I admit, I barely knew what I was doing.  I am not sure of the proper way to check a transformer. 

 

[sr1200]

One way is to check  the resistance with your multimeter across the windings.

 

[sonicrevolution]

I am getting readings by putting the negative lead on primary and checking all other legs on the A24, varying from about 1.7 -2.5 with the ohm meter set to 20k.  I even hear a little static come out of the speakers when I touch the legs due to the xlr's still being plugged in and the unit patched (The unit is unplugged and turned off during all of this.) 

Matt

 

[sr1200]

Trace your signal path next.  Follow the schematic from input to output... see where the signal gets lost. 

 

[sonicrevolution]

Thank you for the reply, one quick question, when the unit is powered on, as long as I am grounded (to chassis I'm assuming) Is it fine to poke around all the components without sparks or being electrocuted.  That's been my biggest hang-up, is I am just getting used to working with high voltage equip.  thanks again.

Matt

 

[sr1200]

depends on what you're poking around with... I wouldn't stick my finger in there.  There is potentially lethal, high voltage in this unit.  Don't bend things till they touch another component or the chassis.  Always exercise caution, and if you're not comfortable with it... don't risk it.

 

[sonicrevolution]

good advice,
I was referring specifically to when I am using my DMM, and alligator clipped to chassis.  at that point am I free to probe values at all points?  Thanks for the help.

Matt

 

[sr1200]

make sure your meter can handle the voltage in dc for everything after the power section (B+ forward)

 

[DanKoVO]

Has anyone here done any diy upgrades to their LA2A clones like the Wa2a? Looking at the revive mod, it doesn't look to complicated to drop in some higher quality audio capacitors and some drop caps.

 

[RO4350]

Hey guys, I'm going to build a p2p LA2a and started sourcing parts. I'm curios why everyone around seems to use 2Watt pots.
After a short view on the schematic and a rough calculation I guess that 1/2W ones will do the job, even 100mW should be fine. I would have to take a close look to verify. So is there something I overlook?

 

[WILSON CALTRAN LOPES]

Hello friends, I'm from Brazil, and I want to build an LA2A for my home-studio, but I have a question, I can not know what element  "electroluminescence element" this is. I know it's a light for ldr, but it has no specification. Can someone help me? thank you!
http://www.tangible-technology.com/dynamics/la2_inputb.JPG

 

[kosi]

http://www.kenetek.com/T4AT4BUnits/KenetekT4BModules/tabid/300/Default.aspx

 

[dbonin]

  Anyone have thoughts on the 6.3v having continuity to ground?

Normal.

 

[dbonin]

How I calibrate my LA2A and my VU Meter for Output +4

NOTE:  this is done with 12AX7 on the input (not 12AY7).

Since most pro gear is rated for +4 I like to set my VU meter to accurately display 0 VU at about 1.23 VAC @ about a 600 ohm load.  (+4 basically translates into 1.23 VAC RMS when measured on a volt meter.)
Don’t go nuts about getting everything perfect regarding the voltages – get it as close as you can and you’re going to be fine!    Don’t go nuts about exactly 1 k for the input signal - close is good enough.  8)

Set up steps:
• Set your signal generator to generate  about a 1k sine wave at about 1.23 VAC – clip this on to pins 2 and 3 of your input XLR
• Turn PEAK Reduction all the way off (full CCW)
• Turn GAIN all the way off (full CCW)
• Make sure you have your T4B in place.
• Turn on the unit and let it warm up ~ about 10 minutes
• Set the METER switch to the +4 setting

I like to do this with two DMM’s,  but you can use just one -  just switch it over to the output after you have verified you have roughly 1.23 VAC on the input coming in on pins 2 and 3
Clip your DMM leads to the output xlr pins 2 and 3 – with GAIN and PR (peak reduction) knobs OFF you should see basically nothing on the output.
1. Slowly just start turning up the GAIN until the VU meter sits at zero.  Observe the output voltage on your DMM – it should be at or around 1.24  1.3 VAC  if it’s not, there is something wrong and you’ll need to check your work.  Make sure your meter resistors are correct.
2. Switch the meter over to GR mode (keep it there for the remainder of the steps)
3. Turn up  PR until your DMM reads about 0.4 VAC on the output XLR (this is the 10db drop you are looking for - if you can't do this something is very wrong!)
4. Look at your VU meter – if it’s not at -10 VU adjust the pot on R25 until it sets near -10  (I put a small 100k pot in the place of the R25 33 k resistor, right next to the meter to make this super easy.)
5. Disconnect the input signal and the VU meter will begin to rise toward zero – let it rise until it stops
6. Twiddle with the ZERO ADJUST pot to bring the VU meter back to ZERO
7. Hook up the 1k input signal again and watch the drop back to -10 VU on the meter

Repeat steps 4 thru 7 until the VU meter is about right under both conditions:
  - with the input signal applied  :  at -10 VU
  - without the input signal applied:  at 0 VU 

Cheers!
Don

Here is a quick and dirty video of the process... pls ignore the mess!  lol
https://www.youtube.com/edit?o=U&video_id=IeDPXvJMc3g

 

[dbonin]

My favorite excerpt from Jahnsen's la2a book!

"Watch for flames and sniff for smoke."

 

[dbonin]

With my latest build with (Jensens again, my fav for this unit)  I set C4 to a fixed 250 pF and the freq response was flattest there for me.  500 pF gave a slight lift above 15k and 100 pF gave a subtle roll off above 15k.  All by only a couple / few dB so nothing to get hyper excited about.  Just thought I'd share.

Cheers!

 

[laemmle]

I'm sure this is a basic question with a capital B but here's my situation...

I'm revisiting my 2005 build and it was pretty messy but more importantly there was some noise in the signal.
I pulled all the turret connections and started from scratch with a proper heater wiring job.

2 days later it's a lot quieter but here's the thing that is backwards...

The gain pot has signal at zero and as I turn it up it doesn't change until the end of it's rotation where it fades to zero.
Does that suggest anything?
I keep looking over the connections and I can't see the problem...

I just looked again.
That pot is wired correctly...

 

[laemmle]

I don't believe that the signal is making it to the peak reduction knob either.
So audio signal when the gain is set to zero stays the same until max where it tapers off to nothing.
and the peak reduction does not effect the signal.

Very encouraged by absence of noise though!

 

[laemmle]

I took some Measurements and definitely have some numbers that are out of whack.

I worked with the schematic voltage test points.

point                  schematic value                  my value
R9-R13                    205                                        201
r13-C2                      120                                        195
r17-c3                          220                                        450(!)
r15-r18                        100                                        200


Measuring the tube pins

V1
1 - 206
2 - 0
3 - 1
4/5
6 - 198
7 - 0
8 -1

V2
1 -200
2 - 0
3 - 7
4/5
6 - 445 (!)
7 - 26
8 - 213

V3
1 - 166
2 - 0
3 - 0
4/5
6 - 166
7 -0

V4
1 - 0
2 - 13
3 - 2
4 - 2
5 - 266
6 - 213
7 - 0

 

[laemmle]

So the answer was that I had read the diagram wrong and wired my gain and peak reduction knobs backwards (upside down).  I realized it when I looked at an interior photo I had taken before the rewire job. Embarrassing but better for it.
That is the exciting conclusion to this broadcast.