All things LA2a related

made a few pictures of the areas around V1 and V3

i am pretty sure nothing is wrong in terms of components values and connections but i would be really glad if you can double check and help me out. thank you very much indeed

allright, next step, we will test your ground sections. Place a probe on the IEC bolt, where you ground your power transformer. with the other probe, test all of the sections which go to ground, there are a couple. They all should have continuity.

also, how do you wired your heaters? Pin 4-5 and 9?

ground points are all ok. i also checked that the ground is not going anywhere else

i wired the heaters to pin 4 and 5. pin 9 is left unconnected. is that right ?

omfg !!! i just realized i did the heater wiring wrong !

they need to be pin 9 and pin 4+5 as you pointed out.

i feel so ashamed !

i am going to fix that and report

thanks for your input

no trouble at all. 

man this is a mistake that i will remember forever. i lost so much time and i made such a silly error !

thanks for helping now the compressor works fine and all voltages are spot on

awesome enjoy the beast

Would I be asking for trouble if I used  5Y3 and a 6V6 instead of the 6AQ5? 
I have a transformer that puts out 70ma at the same voltages that would be expected in an LA2A.  It has a 2 amp 5 volt winding and a six volt winding with ample current.

I'm still collecting bits and pieces and this is a salveged transformer I've had on my shelf for years.  I have all the other major parts sourced with the exception of the VU meter and the 100KA pots

I'm a little, in fact quite late to the LA-2A game.  My research keeps turning up a reference to a book by David Jahnson.  Does anyone know of a live link or can you make it available for us new builders?  I understand there are errors, some significant but I think it would still,be a valuable reference.

Jonathan Horne
Quayhog53 at gmail

For a point to point version

http://www.recproaudio.com/diy_pro_audio/teletronix_la2a.htm

That is the schematic I am going to build.  I was looking for a link to the digital book by David Jahnson. Even though much of it is obsolete it should still be read by LA-2A builders to round out their knowledge.  It couldn't hurt.

the joys of down studio time means i can get back to projects.

my p2p is still not working. i've gone through and tested continuity following the RecPro re-produced schematic and layout and everything checks out (i found some problems and i fixed them). STILL the tubes are not all firing. Now, all my points i can read voltages on but for whatever reason i'm getting NO voltage reading on my heater circuit. that being said, the first tube that hits the heater circuit is heating up, the 6aq5 is glowing nicely, as is the VU meter.

what am i doing wrong? how can i isolate the heater circuit best? should i insert one tube at a time? so many questions. i'm ready to stay up way to late forever to finish this thing.

Heater circuit?

-The tube heaters are all in parallel. -How hard can it be?

yea, that's why i'm confused. i cleaned stuff up and i'm getting a solid 3.5v at each point so that should feed the tubes fine. however, only the 6aq5 is firing. what are things to look for as to why the tubes wouldn't be heating/glowing?

soundsactive said:

yea, that's why i'm confused. i cleaned stuff up and i'm getting a solid 3.5v at each point so that should feed the tubes fine. however, only the 6aq5 is firing. what are things to look for as to why the tubes wouldn't be heating/glowing?

Click to expand...

Are you getting that voltage at the tube sockets?  Is this a PCB or P2P?  Either way, I recommend looking at the Cayocosta layout so you can visualize the flow in your head.  If it's P2P, I'm guessing the wiring went bad at the sidechain 12AX7.  Something is fundamentally amiss. Posting a pic might clear it up very quickly. 

ill get some pictures up ASAP. this is all P2P a la Cayacosta's layout already.

Since my power transformer has a center tap on the winding for the heaters, which is grounded, i should only be measuring 3.5v on the heaters, correct? due to my reference to ground/center tap?

soundsactive said:

ill get some pictures up ASAP. this is all P2P a la Cayacosta's layout already.

Since my power transformer has a center tap on the winding for the heaters, which is grounded, i should only be measuring 3.5v on the heaters, correct? due to my reference to ground/center tap?

Click to expand...

I'm not an expert, but I built a LA2 w Caycosta's layout last year, and the way I remember it, for the heaters I used one filament (two cables) from the Hammond xformer that gave 6.3V AC. Not the center tap.

thanks for the help!

i'm getting 7.1v now at the heaters, which is a touch high. the 6aq5 is getting 7.1 fine, however the 2nd 12ax7 (v3) isn't. Yea I finally found a location of my problem. Continuity tests fine across all the heater stuff. i'm going to get pictures tomorrow with some light and get them up. would this be a problem if i blew out this tube on initial power up? i caught some grounding problems and fixed them after first power up.

as for the center tap, on the hammond 369jx transformer i'm using there is a center tap on the heater wiring that i'm sending to ground. that's correct, right?