All things LA2a related

[yodermr]

Half the fun is making it look like a very expensive unit and knowing you can do it fairly cheap. Then I can sell the cost of doing it as an investment to my wife.

Here is another example I have. (excuse the braggin for a minute) Just some stainless steal, oak and new knobs, oh yea and my next door neighbor's EDM machine. Looks expensive but it wasn't.
http://www.demofactory.biz/Wedge.JPG

 

[Siegfried Meier]

Couple of questions. Testing the units out and they both sound very good. However, 1 is much louder on the output, even though all the front panel pots are equal (including output gain and gain reduction and meter cal). Does the C4 adjustment have anything to do with this? What exactly does C4 do? Are there any other adjustments besides the 2 calibrations mentioned in the manual?

Thanks.
Sig

 

[Paul B]

[quote author="CJ"]What happened to Paul B? We either fixed him, or he jumped off a bridge somewhere.
:?:[/quote]

Yeah, you fixed me, for the most part :grin: :grin: :grin:

Now comes the hard to admit part. The crux of it is right here:

[quote author="CJ"]Paul, do you have a mic and a mic pre laying around?
If so, plug that in and see what happens.
It could be that your LA2 has been working as it should this whole time. I have seen that happen before. Maybe you are just over driving it,
cj[/quote]



Ive been feeding the wrong kind of signal into the box. I have been taking the CD output signal from the monitor/speaker outputs. I had in fact run tests with a preamp but it was with the same signal. I used a mic into the preamp and then into the box. The signal has to be really hot for the box to compress to the levels I've seen posted by others. The signal coming out is still very hot but it is usable. The gain is working much smoother. The unit is VERY quiet.

So I would bet my box has been working for the last 6 weeks or so.

Now I could use a cooler input level, and just crank up the PR (with lower input levels I need to turn PR to 70-80 to get about 10dB reduction) because i probably wont be needing 20-30 dB PR, to tell you the truth! I've not messed with it much so Im heading out to go twiddle knobs and flip switches here in a minute.

CJ, do you think it would help to put the 100k's back in for R9 and R13? Im still running the AY7.

 

[CJ]

If you clip a bit, the 100K's will make the clipping more mellow. You will not notice much benifit form the 100K's until clipping, but they do reduce gain a bit, and make the distortion more symetrical.

Don't stress the overdriving. Others will benifit, including you and me, from the troubleshooting extravaganza we did.

 

[SSLtech]

C4... that's the trimmer cap -right?

If so, sweep a sine wave until you find the rolloff at the top end (about 18kHz... look at the output with the peak reduction turned all the way off, (...and the T4 removed if you want to make double sure!)

Just before the response takes a dive at about 20kHz, there's probably a peak, that's where the trimmer lets you set it for flattest response, or add a touch of brightness just before the dive, if you like.

Keith

 

[AnalogPackrat]

Great news, Paul! Don't sweat the running around in circles debugging stuff. Every time I look at the circuit I learn something, even if it's small. And all the discussions are good mental exercise. I think I'm going to dig out a 12AY7 from my stash and give it a go, too.

Anywho, belated congrats on getting it working :grin: Now post some pics of your wiring for everybody to see!

A P

 

[Guest]

hi there ,

built an la-2 all the voltages check out
except 12bh7a pin 6
mine is showing 230v

reissue is 222/schematic 220/ la-2 195 /la-2a 200

you think the 10v difference is ok ?

all the othervoltages check out , but im hung up on this one.

:?:

thanks for any input.

 

[AnalogPackrat]

Driller,

If your other voltages are ok you seem good to go. 10V on 230V is like 5%. No biggie. Is this a Bloo kit or from scratch? Are the tubes in yet? T4B installed?

A P

 

[Guest]

ah thanks , i was worried and reviewed my wiring a million times ,
seems like every thing is correct.

its a home build point to point.
with all tubes and t4b plugged in.

i was looking at a reissue , noticing they have added 1k resistors
here and there.
on the 12bh7a pin 2 they added a 1k .

my voltages are more or less matching that of the reissue
except that pin 1 being 10v higher

ill post all the reissue voltages latter today just for refrence for others.

 

[Siegfried Meier]

Had the 2 of my Bloo LA2A's running for 6 hours last night and they worked great. Fired them up today and one of them blew a fuse immediately. Then, after 5 minutes the fuse blew in the second one. Any idea what the deal is?

The fuses I'm using are F-GMA-00.500
GMA-500 MA

Should I be using something else? Am I under the correct assumption that if they worked for this long and haven't been touched or moved that nothing else could be wrong with them?

Thanks,
Sig

 

[dissonantstring]

i just wanted to give a big thank you to steve for pointing me in the right direction. :thumb: :guinness: :guinness:
i redid my rotary switch wiring and also relocated my C5 cap (meter turret squishing it) and also redid my wiring from R2 to A15. i now have beautiful compression which is reflected in my VU meter when i adjust my PR. awesome sounding! still have no meter movement at +4 or +10? i'm convinced, as steve has stated, that it's in my rotary switch so i'll have to re-re-re-check against the schematics, instructions and cayocosta's layout. still more to do.
one really puzzling question for anybody who has done the bloo kit, did you link turrets A11 to A14? i saw it in the cayocosta layout and verified it in the schematic in the bloo manual, but when i checked pg. 14 in the manual it doesn't list it as one of the main turret board links? can anyone verify this link for me? my unit sounds pretty awesome without it so i'm wondering if it was removed from the original schematics for some reason.
thanks again to steve for his generous help and his awesome bloo kit!
thanks to Keith and CJ for their sharing their wealth of knowledge and calming my fears.
cheers,
grant

 

[SSLtech]

Yeah A11 andA 14 were linked in the LA-2a. The unit works without the link or with. The output drive clipping level is changed, IIRC. For fully-authentic LA2a behaviour, link A11 and A14. For 'warm-n-dirty' without having to blow the front end out of the following device, try it without.

Keith

 

[dissonantstring]

thanks for the info keith!
i was a bit surprised when i found that i hadn't linked A11 to A14.
i think i'll leave it out for now as it sounds great without it, but maybe
do the link in the future if i find i want a little more grit.
cheers!
-grant

 

[Bluzzi]

[quote author="SSLtech"]Yeah A11 andA 14 were linked in the LA-2a. The unit works without the link or with. The output drive clipping level is changed, IIRC. For fully-authentic LA2a behaviour, link A11 and A14. For 'warm-n-dirty' without having to blow the front end out of the following device, try it without.

Keith[/quote]

Thats great info Keith! Thanks.

Do you think it would be OK then to install a switch to be able to have both choices? Or would that cause a huge spike in the output when switching?

Jim

 

[Siegfried Meier]

I can't ever seem to get the Search Topic function to work. Looking for info on the Grid Stoppers for the 12BH7A - I'm using Sowters. What pins exactly do I place the 1k in series with? Thanks so much.

Siegfried

 

[AnalogPackrat]

Siegfried,

Pins 2 and 7 are the grids of the 12BH7 triodes. You know how to do this or do you need some pointers?

A P

 

[Siegfried Meier]

Thanks. I believe I need a 1k resistor wired in series to each pin. So, lift the wires going to pin 2 and 7, solder the 1k to the pin, and solder the wire to the other side of the pin. Correct?

 

[SSLtech]

[quote author="Bluzzi"]Do you think it would be OK then to install a switch to be able to have both choices? Or would that cause a huge spike in the output when switching?[/quote]
Actually I'm not sure... -there's only one way to find out though, and it can't hurt to try it out!

-First person to wire a switch and test it wind a bag of jelly-babies!

Keith

 

[AnalogPackrat]

[quote author="Siegfried Meier"]Thanks. I believe I need a 1k resistor wired in series to each pin. So, lift the wires going to pin 2 and 7, solder the 1k to the pin, and solder the wire to the other side of the pin. Correct?[/quote]

That's correct. Go for it.

 

[Siegfried Meier]

Ok. Grid stoppers are in, but unfortunately not stopping anything. 1 unit is fairly quiet, with a pretty high buzz (almost sounds like a dimmer switch with flourescent lights through a guitar pickup kind of noise) but the other unit has a really loud hum - reminds me of when a power supply fails and has that weird kind of hum. It kind of fades when I wiggle the output XLR around. Could there be something not making proper contact there?

Man, it's really tough sorting out these noise problems. I think I may need to hand these over to a qualified tech. It's just too difficult to sort out on my own - and not enough time.

Thanks for any help.
Siegfried