Stereo/Dual LA2A Thread

Hello everyone,

I almost created this thread awhile ago. Since most LA2As are built P2P in a single case and there are many Bloo kits out there, there's not much interest in a dual LA2A (in one chassis). However, with the Drip Electronics PCB out, the dual LA2A is now more feasable.

I'm building a dual LA2A using the drip electronics PCB and it is not without its challenges.

I'm going to document some of these issues and my findings here so that I can solicit feedback and future builders can refer for help and learn from my mistakes.

LA2A PCB:
http://www.groupdiy.com/index.php?topic=10774

The power supply issue is the biggest hurdle to conquer. This thread ALMOST points you in the right direction, but stops just short of giving you an EXACT model number. PRR also does an amperage calculation which is very nice.

http://www.groupdiy.com/index.php?topic=5809

The problem with powering a dual LA2A with 1 transformer is that the HT voltage is relatively low compared to the demand for the relatively high filament current.

After searching and searching, I'm now settling on 2x of the allied 6K88VG. This is the spec'ed tranny, they will fit on the back of a 3U case they're cheap and I'm tired of searching.

Other almost/close/good options are:

*Sowter M024, which is actually quite nice but is very expensive.

*Hammond 370DAX its a little short on filament current at 3.5A

*A combination of a beefy HT transformer and a separate filament transformer like the Hammond 167Q6

Here's my current front panel layout...

The VUs are central and I'm kinda doing a mix between a mirrored layout and non-mirrored layout. non-mirrored being the best for intuition, mirrored for good looks!



I've moved the R37 controls to the front for usability and consolidated both of the meter switches into one 4PDT switch (i would have gotten rid of it, but you need +4 (or +10) for stereo calibration) I've gotten rid of the +10 control. I'm only going to be using the GR mode generally.

The stereo calibration switches will be rear or side mounted as I've been told that once you set 2 la2as for stereo, you dont need to adjust them again. (Hope this is true)

The meter zero switches will also be side mounted. Set 'em once and forget it.

Each channel has its own limit and compress switch.

the Stereo/Dual switch is a DPDT switch (can bd DPST) that connects terminals 6 and ground. This is like connecting an RCA cable between the two units.


Critiques are welcome.

Here is where things get a little difficult.

I want to avoid magnetically coupling the 6(!!!) transformers I have mounted to the back. Thank god I don't have tubes back there too.

Two issues here:

Power supply to signal coupling and output to input coupling.

Here is my current idea:



The power transformers (Allieds) are mounted left, and I could have mounted them vertically, but I think the mounting screws will have to also go thru the top and bottom lips of the case (thinking way far ahead here to par-metal's configuration). I could put notches in the lip and mount the trannies vertically.. hmmmm. need to think about that.

I've staggered the input (Harmon kardons on the right) and output trannies (sowters in the middle) to give maximum distance between them to minimize cross talk.

The smaller circles are the Neutrik XLR chassis mount connectors. Each is wired to its closest horizonatal transformer.

Critiques are needed here, please! My Electromagnetic fields class was a long time ago.

You can use one power transformer for both L and R.
If you series the tubes to run on 12.6 instead of 6.3, your problems
are over, if you can find the right B+ volts and current and a
12.6 volt winding.
You don't have to cut traces becuase the heaters are hard wired.


cj

CJ, thanks for the reply!

Just a double check to make sure I understand you correctly.

I need a 250-0-250 @ 2x 40mA = 80mA

with a 12.6 filament still at 2A??

This I think can be found, will search.

Any comments on the input/output transformer lay out on the back?

Another issue is packing 8 tubes inside a par-metal case.

I'm looking at installing an AC fan (so i'm not sapping secondary power) in the case to blow hot air out of the side of the case.

mouser part number: 433-3E-115S looks good

all 4 tubes run cool enough to lay cables across

CJ youre a prince!

thanks,