la2a - oh gawd!

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studmuffin

Member
Joined
Aug 24, 2009
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24
So I bought this La2a clone  from the US on ebay.  I'm in the UK. On paper it looked like a good deal; built with NOS parts - very credible.  Needed a voltage converter so got one from Maplin in readiness.  The unit arrived this morning.  Plugged it in and it passes audio fine, but there is no gain reduction going on and the make-up gain is soooo senstive that I can't wind it up much more than a hair or so off the zero position before it runs out of headroom.  Jeez.

I feel like a bloody fool.  :-[ Of course I've contacted the seller, but there's a time difference and I'm upset and want my new toy to work.

Any clues anyone?  Anything a non-techie like me can check visually?  I'm hoping this is something simple I could rectify here because if I have to return it chances are I'll have to bear the cost which will be around £100 to the USA and I've waited so long for it to arrive.

Help please anyone?
 
First off, count to five and calm down.Ready? Tell us what you are feeding the compressor (test signal, CD, radio, DAW track .. what dBu/RMS level if you know..)

Do the tubes look like they have survived the journey? Any cracks or ..?

Have you used an LA2A before, e.g. do you know how much to crank the peak reduction to actually get reduction etc?

Did the seller indicate that it was calibrated or just "working"?

I don't think you came here to receive a barrage of questions, but I don't think you'll get great answers without some of the above answered first.
 
baadc0de said:
First off, count to five and calm down.Ready? Tell us what you are feeding the compressor (test signal, CD, radio, DAW track .. what dBu/RMS level if you know..)

Do the tubes look like they have survived the journey? Any cracks or ..?

Have you used an LA2A before, e.g. do you know how much to crank the peak reduction to actually get reduction etc?

Did the seller indicate that it was calibrated or just "working"?

I don't think you came here to receive a barrage of questions, but I don't think you'll get great answers without some of the above answered first.

Thanks for the reply.  Calming down  :-\

Ok, I hould have thought to check the tubes.  The packaging wasn't that great to be honest.  Anyway, I can't see any obvious cracks but a very gentle shake reveals that one of the 12AX7s (the one nearest the large - I'm guessing - mains tranformer) and the 6AQ5 both exhibit a slight rattle.  I don't know anything much about tubes, but I'm guessing they shouldn't do that right?  The 12AX7 glows, but I can't see any light coming off the 6AQ5.  They both get warm.

To answer your other question, I was feeding it a test signal ie line level output from a mic amp.

What do you think?
 
A dead 6aq5 would mean no compression if I recall correctly. Do you know anyone who could lend you one? Studio / guitar tech / electronics store ... ?

This is how mine looked like (these are two la2a's on one board)


More when I get home..
 
baadc0de right, with blown 6aq5 it won't compress. I had to replace one in mine the other day.
You could try wiggling the 6aq5, maybe it came lose in transit.
If your local i can lend u a tube to test.
 
mrclunk said:
baadc0de right, with blown 6aq5 it won't compress. I had to replace one in mine the other day.
You could try wiggling the 6aq5, maybe it came lose in transit.
If your local i can lend u a tube to test.

That's really kind, but I'm in Bromsgrove so no good.  They're not the most expensive valve going so I'll take a chance and order one - see if I can get it overnight.
 
packing with the tubes left in is not advised unless proper bubble wrap and packing peanuts is used. I would bet that the tubes got damaged as a rattle in a tube is no bueno. Replace the tubes and try it again.  if that does not fix it, it is an LA2A, a fairly simple circuit that has been written about over and over. Not much that can really go wrong in those units and plenty of info on how to fix them.
 
pucho812 said:
packing with the tubes left in is not advised unless proper bubble wrap and packing peanuts is used. I would bet that the tubes got damaged as a rattle in a tube is no bueno. Replace the tubes and try it again.  if that does not fix it, it is an LA2A, a fairly simple circuit that has been written about over and over. Not much that can really go wrong in those units and plenty of info on how to fix them.

Thanks, I'm sure you're right.  I managed to order a couple of cheap tubes to test out.  Hopefully they should arrive tomorrow.
 
A little update (feling more calm today).

Last night I ordered a 6AQ5 and a cheap Chinese 12AX7.  My thinking is it's worth a punt on a cheap tube to diagnose the fault.  If it works then I can hit the orginal seller to replace the money tube (it arrived with a nice looking British NOS Mullard). 

I got the front plate off and had a mooch inside.  You're right pucho812, even to my non-techie eye, I can see it's a simple circuit.  Not much there really. I'm posting some pics and I'd be glad if anyone would comment on the general build quality/wiring.  It's got UTC A10 and A24 input out transformers that look in lovely cosmetic condition.

Thanks for your help and comments everyone.  I really appreciate it.
 
Pah!  Disappointment  :'(

Tried the 2 new tubes; the 6AQ5 and the 12AX7 nearest the power supply with replacements.  Still no compression.

As I mentioned, I can't move the make-up gain much above zero before it's out of headroom.

Hmmmm......
 
I know with the La2a clone I have and other La2a I have used is that it runs pretty hot and people mention that they use it as a preamp. So maybe it could be your gain staging for the sensitive issue you mention??

 
canidoit said:
I know with the La2a clone I have and other La2a I have used is that it runs pretty hot and people mention that they use it as a preamp. So maybe it could be your gain staging for the sensitive issue you mention??

Fair point and perhaps worth a try, but the signal I was sending wasn't that hot really.

My local techie is going to take a look.  I found a schematic for him and although he doesn't know the La2a, he has done a lot of valve repairs on guitar amps.

By the way, how do you control the side filter on these?  I recall you could make them more or less sensitive to high frequencies?  I can see a couple of screw controls on the rear; VR1 and VR2.  I'm guessing it's one of those.  If that's correct what does the other one do?  There is a meter calibration screw on the front, so it can't be that...

Oh - as for the T4 optical cell - forgive this dopey question - should it light up or anything?  I can't see that it does, but I'm probably being dim.  Not really hanging in the right place with you tech guys - hope you don't mind!
 
In order to get an la2a or a clone to compress it needs a pretty hot signal.  Crank the threshold knob while hitting it hard and see if you have any compression. 

What is the meter switch set to? GR?

What are you going into afterward? Are your converters running at +4 or -10?
My gain control rarely makes it past 9 o'clock at +4.
 
Your photobucket link doesn't seem to work so I can't see what your build looks like.

If the T4B is housed in a see-through can (it should be shrouded in complete black/darkness but at least for the time being it will help you test) feed it a hot signal and crank the peak reduction all the way up.  The EL panel in the T4B should light.

If the T4B is correctly housed in a dark can with no light getting in, feed it a hot signal and crank the peak reduction all the way up.  Then try the same test with the T4B removed.  An LA2a without a T4B is basically a line amp and will pass signal.  Checking level with and without the T4B will tell you whether you're getting any compression at all.

If you could fix your photobucket link, that might help.
 
studmuffin said:
By the way, how do you control the side filter on these?  I recall you could make them more or less sensitive to high frequencies?  I can see a couple of screw controls on the rear; VR1 and VR2. 

Give these  a quick turn before you send it to the shop. My units wont compress if these adjstments are not set correctly.
 
Jim50hertz said:
In order to get an la2a or a clone to compress it needs a pretty hot signal.  Crank the threshold knob while hitting it hard and see if you have any compression. 

Done that - no good

What is the meter switch set to? GR?Yes - it doesn't move

What are you going into afterward? Are your converters running at +4 or -10?
My gain control rarely makes it past 9 o'clock at +4.Lavvry AD10 running at +4.  But I can't raise the output more than a hair off zero before distorting.  No way I could get it even close to 1/4 to
 
test t4's in a dark room and see if they light at all when you feed it a +4 signal build looks real solid.  My only negative comment would be the t4 itself... A clear see through cover looks nice and all but will cause the t4 to not function porperly or worse in this case not at all. Find a way to replace that with a non see through black cover or something. Paint it anything to keep the light out. also check the 12bh7 tube. Had to replace those with in a year on some new LA2A reissues we got at work
 
take the lid off the T4b and see if the EL panel is flashing when you run music thru the compressor and have the compress knob at full tilt boogie.

 

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