All things LA2a related

[thomasdf]

Hello LA2A Lovers (and for some, haters I guess  ;D )

I just started a thread with a few technical questions concerning a LA2A like that I am working on...

http://groupdiy.com/index.php?topic=54873.msg700411#msg700411

Any help is more than welcome, and I guess my questions are easy for you!

Cheers

Thomas

 

[kosty]

Does anyone know where I can get the mini-turret board next to the meter ?
No BOM seems to have them listed.

Any help is much appreciated !

 

[Unit7]

Does anyone know where I can get the mini-turret board next to the meter ?
No BOM seems to have them listed.

Any help is much appreciated !

Good question! I'm collecting parts for a P2P build too and searched for that board for hours without any success. Finally decided to order 5 of those 5-pin boards and glue two of them together with epoxy.

 

[kosty]

Does anyone know where I can get the mini-turret board next to the meter ?
No BOM seems to have them listed.

Any help is much appreciated !

Good question! I'm collecting parts for a P2P build too and searched for that board for hours without any success. Finally decided to order 5 of those 5-pin boards and glue two of them together with epoxy.

That's exactly what I ordered too, but now that the build is half complete, I feel the urge to use the proper board to be satisfied

edit: I think 15441K-ND or 15541K-ND on Digikey | 534-15441 or 534-15541 on Mouser might work

 

[kosty]

Another question to more experienced guys :

If you had to choose between a NOS A-10 from the 70's and a NEW Magnetika UTC HA-100X that Universal Audio uses in their reissue, which one would you use ?
I don't want to try them both out since they are in mint condition and using them will affect the resale value.

 

[sr1200]

I used cinemag transformers in mine and sounds amazing.  I can't see there being that drastic of a tonal difference.  Mine in comparison to a teletronix made in the 70's (well, bought in the 70's) is so negligible its not even worth discussing.

 

[sr1200]

http://i.imgur.com/NkR9cU3.jpg

Would there be any issue putting the tubes/trafos/in/out this close together?

 

[kosty]

This looks quite cramped. I guess soldering the wires won't be fun 

Regarding my transformer question : I decided to build 2 and to compare them, so I can decide which one to keep.

There is one part that I'm missing. Where did you get your insulated solder terminals and do you have any Part# ?

I ordered some that are too small at mouser #534-11501.
I found that "WEARNES CAMBION 572-4814-01-05-16" which are offered at Farnell might be the right ones http://uk.farnell.com/jsp/search/productdetail.jsp?sku=1310318 but they too much for shipping and don't wont to pay 7€ / piece in total (shipping + tax + handling fees).

Any suggestions ?

 

[sr1200]

I think i used these, theres no pic, so i cant be 100% sure, but i ordered them with the parts for my original build.  Theyre big, and have a screw hole on one end that i attached to the chassis w. a normal screw and a threaded end on theother which i connected the wires using the little Y looking connectors.

Mouser:  693-9641.6200

 

[beatnik]

I'm starting to build my first ptp clone.

I have scored most of the components but I can't find the trim capacitors 75-380pF. Can I just use a fixed value?

The BOM I have states for 2W potentiometers. Is that needed or overkill?

What is the NE-2 component in parallel to 470pF capacitor (in original schematic) ?

Thank you

 

[sr1200]

i got my trimmer caps from surplus sales of nebraska (its mentioned a few times in this thread... read through the entire thread, i know its a lot but Id be willing to bet any questions you might have will be answered.  As far as the NE2, that part is no longer being made, its a tiny little Neon bulb, NE3 and 1s are still available though according to Mouser.

 

[beatnik]

can i sub the ne-2 with ne-1 or ne-3 ?

 

[sr1200]

I hope so, i ordered the NE-3 (smaller package, same specs) but the bag they sent says NE02 so...

 

[sr1200]

Id say throw it on the scope, and see where the problem starts.  Its not a horribly complex circuit.  Follow the signal from the input to the TX and then thru the rest of the circuit until you see the noise come up.

 

[orangechili]

I have a ptp LA-2a i've been working on using Cayacosta's layout as a guide. Allied PT, Edcor in/out trannies. I can not get it to pass audio(no T4B installed). It just has a loud hum and start to oscillate as soon as I turn the gain knob up and the meter freaks out and pegs continuously at +20db. Help please! Any thoughts on what it could be? Here's a breakdown of my voltages.
After 1n4007 diodes: 370vdc
B+ voltage after 4.7K 2 watt: 280vdc
6aq5
Pin 2:6.1v
Pin 5: 155v
pin 6:108v

12ax7(V3)
pin 1: 80v
Pin 3: .93v
Pin 6: 80v
Pin 7: 0
Pin 8: .93v

12bh7a(V2)
Pin 1: 99v (shoots down to 44v after oscillation engaged with gain knob)
Pin 2:0v
Pin 6: 223v
Pin 8: 96v

12ax7(V1)
Pin 1:105v
Pin 3: 1.1v
Pin 6:114v

I poked around a bit with a chopstick around V1 and V2 and it didn't seem to do anything… I've double checked resistor values/transformer wiring numerous times, it's driving me nuts!

 

[orangechili]

Ok,
We can lighten up on the panic button a bit… I instead grid stoppers on the 12bh7a tube and the oscillation went away. I also changed the 10k 2 watt resistor to a 20k and that got the pin 2 voltage of the 6aq5 looking right. The unit now passes audio and voltages look better. I plugged in the T4B module and although I see the meter moving quite a bit in GR mode and output +4 mode I'm not really hearing any compression. Is there something i'm missing here? Any help appreciated.
Thanks in advance!

 

[sr1200]

Make sure the pots in the back of the unit are set correctly, i think they're supposed to be fully counter clockwise to start (i had a similar issue with the second one i build)

 

[orangechili]

Thanks for the tip! I do have them fully clockwise. I do believe the unit is compressing slightly now but have some issues… I really have to crank the gain almost all the way to get a healthy signal going into my daw. Also I have to crank my peak reduction knob almost all the way to get it to start compressing. I have checked my input transformer wiring and believe its correct. I don't think its normal behavior for the la2a? I am using a SM58 through a green pre before the LA2a. I wonder if it has to do with some slightly lower voltages on my 6aq5. The gain knob can overload my daw though when cranked all the way up. I'm using 100k log taper Alpha pots for each control btw. Any insight from fellow LA2a peeps would be greatly appreciated!

 

[orangechili]

I just saw your last post sr1200 and realized you said "fully counter-clockwise" on the back pots. Everywhere I read said fully clockwise, I tried them fully counter clockwise and got virtually no compression at all. With them fully clockwise I get compression from around 70-100 on the PR knob. In order to get compression I have to push my preamp really hard and the comp requires quite a bit of makeup gain. I have a feeling there's something going on with getting signal into the unit. If it would get into it more like it should it would compress sooner i believe… I'm using the Edcor WSM600/15k input transformer/Wsm15k600 output transformer combo. I believe the meter switch is wired correctly, if that was incorrect would that attenuate gain in a negative way?

 

[orangechili]

Ok, in the efforts(for my sanity) to track down lack of gain, especially on input of the unit I would greatly appreciate it if someone could verify the wiring on my Edcor WSM transformers?
Input transformer WSM600:15k
Pin 1: Direct to XLR socket pin 2
Pin 2 CT: No connection
Pin 4: Direct to XLR socket Pin 3
Pin 5: Goes to junction of R5 and R6(68k resistors on Cayacosta layout)
Pin 6 CT: No connection
Pin 8: Ground

Output Transformer WSM15k:600
Pin 1: to negative side of 10uf/450v cap
Pin 2 CT: No connection
Pin 4: Ground
Pin 5: To junction on terminal strip(xlr2/Meter switch)
Pin 6 CT: No connection
Pin 8: To Junction on terminal strip(Xlr3/meter switch)

I've attached a pic of the guts of the unit, not the prettiest wiring but the noise level overall on the unit is great, no hum, no hiss sounds very quiet, just overall too quiet Once again all voltages in the ballpark range. I checked and rechecked connections with continuity meter, I'm starting to wonder about the integrity of the input transformer at this point but don't have another handy to test it out…