D-LA2A Support Thread

[diyfanatic]

some nice french dudes made a group buy a month ago
you can have all the info about the tx
http://www.groupdiy.com/index.php?topic=32314.0

I have not bought it for now, still working on a 1176 and a D-aoc.

there's an email on the first page, try to shoot in english and ask for the ref of the tx (in the pict)

 

[stitch-o]

the saga continues:::
so the relays are getting power, thats not the reason why the left channel isn't working....
but I think the 6AQ5 on the left channel isn't getting juice.
what are the voltages I should be reading there?
all the tubes are now getting power. Still no compression in the right channel.
The funny thing is I know the signal is in the channel because the gain knob does its job and the bypass works as well.
now Im searching for bad solderpoints...

EDIT pt2: looks like I cooked a trace on the gridstopper 1k resistor on the 12bh7a.
so my question is: where do either end of the gridstoppers for the 12bh7 go?

the 5814a and 12AT7s are reigning the gain in a lot but I'll be ordering 25k pots and 75k resistors on Monday.

cheers!

 

[stitch-o]

so my question is: where do either end of the gridstoppers for the 12bh7 go?

So I traced the paths on the other channel's gridstopper 1k resistor, then jumped the shorted connection on the problem channel and.......
still no compression on the left channel.

When the Peak Reduction pot is turned ALL the way up, there is slight distortion so I know the pot is in the circuit. All tubes are lit up.

Maybe I missed a connection somewhere in the sidechain/control voltage circuit?
Where in the circuit is that located?

Any ideas of where to troubleshoot from here would be greatly appreciated   :-\


Cheers!

 

[deuce42]

Diy Fanatic and Red Noise - thanks for your posts regarding the 2u spaces. I understand that there is a need to lower the tube socket mounts if using a 2ru case, but my TB4's from drip seem even taller than the to height of the 6005 valve. Anybody else notice this? My guess is it may still be possible but very tight indeed.  3ru seems like just such a waste of space!!!! Am I overdoing this?

 

[[silent:arts]]

2U D-LA2A with UREI T4Bs:
http://www.silentarts.de/DIY/D-LA2A/IMG_3980.jpg

2U D-LA2A with Drip T4Bs:
http://www.nrgrecording.de/nrg/viewtopic.php?t=405&postdays=0&postorder=asc&start=92

it is doable, depends on the inner hight of your case

 

[[silent:arts]]

When the Peak Reduction pot is turned ALL the way up, there is slight distortion so I know the pot is in the circuit. All tubes are lit up.

Maybe I missed a connection somewhere in the sidechain/control voltage circuit?
Where in the circuit is that located?

you already swapped your T4Bs between the left and the right channel ?

 

[stitch-o]

When the Peak Reduction pot is turned ALL the way up, there is slight distortion so I know the pot is in the circuit. All tubes are lit up.

Maybe I missed a connection somewhere in the sidechain/control voltage circuit?
Where in the circuit is that located?

you already swapped your T4Bs between the left and the right channel ?

Late last night I did swaps of tubes and T4Bs with same no compression problem.
So I know the problem isn't the tubes, transformers, relays, frontpanel switches or pots or T4Bs.
Its really weird cause the channel bypasses normally, and when "IN" will gain properly....just no compression.

I'll double check all the tubes and T4Bs later today but if you, or anyone, has a suggestion as to what else I can focus on while the hood is up, I'd be muchly appreciative  ;D

 

[[silent:arts]]

schematic
if you have wired the gain & threshold pot with the suggested molex connectors swap them for a test.
compare voltage on pin3 of the T4B on both channels with the same signal applied.

 

[stitch-o]

schematic
if you have wired the gain & threshold pot with the suggested molex connectors swap them for a test.
compare voltage on pin3 of the T4B on both channels with the same signal applied.

Perfect I will do all of that! (tho I use no molex)
+ thank you for your schematic. I have many versions of the La2a floating around here....

Cheers!

 

[stitch-o]

I'm starting with some continuity testing (thanks again for the schematic, it makes it much easier):
I'm not quite finished but I did find a descrepancy between the working channel and the non-compressing channel.
Reading the schemo:
Resistor R132/R232 says it is to go to, on the V103/V203of the 12ax7, one leg to pin 2 and one leg to pin 7 on the tube socket.
On the non-working channel I have connectivity between R132 and pins 2 and 7 of V103 only.
On the working channel, I am getting one leg of r232 is connected to pin 2 of V203 but the other leg shows connectivity on 4,5, 7 AND 9. Can this be right?

Edit: The only thing I can think of is that there is a short to the heater run on channel 2 but it works fine and why pin 9 getting signal?

Back to beeping...


EDIT 2:: After some focused critical evaluation and re-soldering a few points (this seems to be the issue specifically C181 and C182), the left channel works!!

Now, my issue seems to be that I'm not really seeing any gain reduction (on both channels) til its up near 7.5 (out of ten) on the PR pot. I still have so much gain as to incur distortion in the signal so I'll be ordering the pots tomorrow for the gain mod and will go from there.

BTW: this thing is dead quiet...

Edit 3::: ooops, the 12AU7 and 12AT7 were switched between 101/203 and 103/203....back to sounding almost excellent.

EDIT 4:::: I had 12AU7s in the V101/V201 position.
I now know it REQUIRES a 12AX7 or equivalent to drive the compressor stage. heh heh....uh....
I also now know I had a bum tube.
Found a spare seet of cheapo 12AX7s and Everything is as it should be til I do the pot mod.

pheew and Cheers!

 

[[silent:arts]]

sounds like good news ;D

 

[deuce42]

Stitcho - sounds like your almost there

I have a question which I hope isn't too stupid. Since these things put out huge amounts of gain the alternatives seem to be subbing a 12au7 instead of a 12ax7 at v1 and the Volker 22k/7k resistor pot mod. But why not just keep the gain pot very low - like on 1 or 2? Does it matter keeping the low settings on the gain knob instead? Am I missing something obvious?

 

[crisotop]

I think Volkers gain mod uses a 75k resistor in series with a ~25k Pot (but I guess your 7k was just a typo). The only big advantage you get is better gain control - if you have a full ~300° range for ~26dB or about 10° makes a huge difference, the latter is nearly impossible to recall and you need a calm hand to set makeup gain

cheers
christoph

 

[Kingston]

What about another option for the make up gain? People seem content with the 25k pot + 75k resistor trick.


How about a simple balanced pad on the output *after* the transformer? Common sense would dictate it would "sound better" since the output transformer and last gain stage are driven a lot harder. The 25k pot + 75k resistor trick makes all the standard (and very expensive) LA-2A output options pretty pointless, for example those sowter replicas.

 

[deuce42]

Yes my apologies - meant 75k not my 7k typo. I am thinking too that the pad switch makes more sense in terms of driving the transformers.

Anyone got any views on doing this?

 

[stitch-o]

Stitcho - sounds like your almost there

I have a question which I hope isn't too stupid. Since these things put out huge amounts of gain the alternatives seem to be subbing a 12au7 instead of a 12ax7 at v1 and the Volker 22k/7k resistor pot mod. But why not just keep the gain pot very low - like on 1 or 2? Does it matter keeping the low settings on the gain knob instead? Am I missing something obvious?

Thanks bro! Yeah, ALMOST is the operative word. Its that last 5% that will mean the world to me. ;D

The biggest part of my problem was that I had a 5814a in the V1 pos (a 12AU7 variant).
This did not generate enough signal to drive the compression. As soon as I stuck a higher MU tube in the V1, all of my non-compression issues were solved: so from what I gather, V1 NEEDS a 12AX7/ECC83/7025/etc to drive the compression circuit (I believe this was the crux of my problems)
In my comp, V3 now has the 5814a and I am ordering2 x 12AX7 RCA long greyplates. Also, 2 x 25k log pots and 75k resistors today as I still have MASSIVE amounts of gain.

The solution of just leaving the gain on 1 or 2 doesn't quite work for me. There has to be some flexibility for me in the process of dialing in proper gain staging between this piece, the one before it, and the one after it. I, seriously, can't get the gain past .75-1.25 before it redlights out.
For you, maybe this isn't important...dunno

 

[diyfanatic]

noob question to myself about the gain...

if we use a 600/600 input tx, the gain should be decreased?

and in the case of the 600/10k input tx, why not taking a 100K pot or upper value instead of 25k pot?

 

[stitch-o]

noob question to myself about the gain...

if we use a 600/600 input tx, the gain should be decreased?

and in the case of the 600/10k input tx, why not taking a 100K pot or upper value instead of 25k pot?

You need to impedance match the input to the circuit that's why the 600-10k value transformer.

My question is tho: if the original La2a had input trunnies that were 50k secondaries (UTC A10 or HA100x are 10:1 600-50k-->maybe my info on the UTCs is not accurate?) and this design is exactly an original La2a, why the 10k secondary sowter 4383 trunnies? I need to look over some schematics...

Edit: ...which I just did. This project is identical to the original design from what I can see, tho I might have missed something as I was focusing on the input network.

 

[stitch-o]

Common sense would dictate it would "sound better" since the output transformer and last gain stage are driven a lot harder.

But in this case WAY to hard, IMVHO.
Mine was getting not-so-nice distortion and loss of all bass because of too much drive.
The tube mu downgrade ended 95% of distortion and gave me back low end.
and my gain setting still isn't past 1.5!!!!
I'm going to mouser now for the pots and resistors...

Try the different things out and see what works.

Cheers!

 

[deuce42]

Actually I had wondered similarly about the input tranformers secondary 10k when the schematic seems to want 50k. Guess it is just a matter of trying it and finding what works best for your ears.

I note that there was a post somewhere else on this board where SSL Tech or someone else seemed to be of the view that the best tranny for LA2A was a 600/15k set up becuase it was easier to dig into signals that were less "hot sounding". It appeared as though there was consensus on this board that numerous permutations had been tried and in the end the 600/15k was the best.

Wish I could find it here.