All things LA2a related

[emx]

Hi, I've got an output transformer question. I'm building an la2a and thought I would try cinemag trans for both the in and output. I got the Cinemag CMLI-15/15b input and the CM-9589 output(Cinemag recommended it). Basically what I've found is that the output(the 9589) has no center tap. Is there a way around having this? It's supposed to have one going to the terminal strip so is there a way to rig a substitute?

 

[CJ]

Just snip the darn thing. It's a tap left over from the fifties.

Go end to end for your output signal.

 

[AnalogPackrat]

I think your output is ok--just leave the CT terminal unconnected. You only need it for unbalanced connections anyway (right?). The other connections go to + and - signal. I'm not convinced your input iron is going to work well, though. Should be at least a 1:4 ratio if not higher (the original was 1:10). Hopefully one of the real experts will chime in here...

A P

 

[CJ]

What experts?
Please tell me your not calling me an expert? :shock:
Dude, I'm like high school edikated.

Man, your house almost came a few days a go. Two people hear at work got woken up by the rain from 2 to 5 am , right when everybody is sleeping.
They said it looked like a waterfall outside.

Actually, I have never seen your property so i don't know what the f i am talkin about, eh?

what is the transformer issue here?

 

[emx]

OK, I'm actually thinking about just re ordering trans(Probably sowters) for the job. Someone recommended that input tran of one of the forums for this unit for some reason. Now that I'm looking at the papers on it I'm not so sure(it's a 1:1). I think the output tran would do the trick but not the input. The output is a 4:1 I believe or maybe a 1:4 depending upon wich direction your looking through it. I would still like to have that center tap just for any possible use it might have so I'll probably get the sowters. I'm attempting to build a stereo pair from the Cayocosta layout and want something I might be able to run a full stereo mix through when I get both running so if anyone here has suggestions other than the sowters for this use please let me know.

 

[CJ]

What's wrong with the cinemags?
Can you link some data sheets for those guys?

 

[AnalogPackrat]

[quote author="CJ"]What experts?
Please tell me your not calling me an expert? :shock:
Dude, I'm like high school edikated.[/quote]

And Ye Olde School of Life (experience)--it counts for something, dood.

Man, your house almost came a few days a go. Two people hear at work got woken up by the rain from 2 to 5 am , right when everybody is sleeping.
They said it looked like a waterfall outside.

See, that's why you hire a geologist before buying property up here. It cost me a few bills to get: 1) a nice two hour on-site geology lesson, 2) site walk-through, 3) check of all known geo hazards, and 4) a 10 page official report of findings. Worth every damn penny. Of course there are no guarantees, but it's better than the alternative...

We had a couple or three of inches of rain. No biggie. Last December we had 11.25 inches in two days. December 2002 we had 15+ inches in four days (three big storms in a row). The first one packed 80mph gusts which knocked the tops out of a bunch of big trees and uprooted a 130' Doug Fir 60 feet from the house. It fell away from the house and parallel to the driveway and took out a bunch of oaks and laurels on the way down. Our power was out for five days. That was a scary storm.

what is the transformer issue here?

emx here has some Cinemags for his LA-2a. The outie looks ok, but the input he's got is a 15k:15k thing which doesn't seem right. Here's a link to the Cinemag line input datasheets. He's got a CMLI-15/15B.

A P

 

[CJ]

That 15/15B is absolutely perfect.

You want a 1:1 ratio so you can reduce the UTC A-10 1:10 ratio to the point where the gain knob does something useful.

But when you say 1:1, most joe's, probably with lace on their panties, think 600:600.

Well, the 600 on the input is perfect, but 600 on a grid, well, that might be a problem.

But this Cine mag is 15K both sides. Our source wants 600 or above, so the 15K will be luxerious as far as work done on the pri side. And the 15K sec side will be better for the high impedance grid circuit.

That 20db rating should be plenty if I know these Reichenbach characters.
Over-stackin fools, nobody told them the price of Ni these days!

 

[analag]

[quote author="CJ"]That 15/15B is absolutely perfect.[/quote]

CJ, now that's something we can both agree on.

analag

 

[emx]

So what your saying CJ is that they both might work? I should probably point out I'm new at this. Also I'm wondering what the center tap was used for originally?

 

[CJ]

The input will work great.

I just looked at the output specs and the output will work great.

0.1 % THD at 20 hz and 29 db is incredible.

 

[HappyTom]

A couple of days ago I finished my LA2a but waiting on my VU and frontplate. I use a sifam AL39 and a really ugly homemade faceplate at the moment.

I power up and get:
Without tubes:
A24-A25 (Heaters) = 6.84
A1 (neon) = 54.9
B21 (Between CR1 and R29) = 372
B20 (Between R29 and R34) = 366

That seems to be good, now with tubes in:

A24-A25 (Heaters) = 6.something.. Bad note.
B21 (Between CR1 and R29) = 349
B20 (Between R29 and R34) = 270
A1 (Neon) = 54
A20 (V4 pin 5) = 73
A14 (Between R17 and C3) =225
A17 (V3 pin 6) = 96
A19 (V4 pin 6) = 115

V4 pin 5 is really low, the Bloo-manual says 131V...
And pin 5 is the only thing connected so there might be the tube?

I didn't find the issue with the low voltage on V4 pin 5 so i run some music thru the unit. No low freq and alot of hi freq..
Here is a freqsweep on my unit. Left channel is my soundcard and right channel is my LA2a:

http://www.rollingthunder.se/la.jpg

I got Sowter in/out.
R29 is 4K7 2W
R34 is 22K 2W

Any ideas?

HT

 

[AnalogPackrat]

You haven't installed your T4b, right? If not, then the HF rolloff is somewhere in the main audio path (V1 and V2). What did you use for C5? Double check C1, 2, and 3. What voltages are you getting around V1 and V2?

As for the V4 pin 5 thing...did you use 1k for R36? You didn't install R38 or C13, right? Did you adjust R3 and R37 as it said in the manual?

A P

 

[HappyTom]

No T4b.
C5 is 20uF 450V sprague atom.
R36 is 1K
R38 and C13 is installed as written in the manual, 22K between V4 pin 6 and center terminal of V4 then a link from center pin to C7d (+) and 510p between V4 pin 6 and r3 pin1.
R3 and R37 is fully clockwise. I read it in this thread, it is not mentioned in the manual.
As I don't have an internet connection in my "lab" I'll take some measurements today and post tonight.
I'll check all things around V1, V2 and V4.

Thanks!

HT

 

[HappyTom]

OK, 1 down 1 to go.
As you (AP) said I should look at C1, C2, C3 and I had one fault there.
I had no connection between C3 and R17. I look in the manual again but I didn't find it mentioned. Now it's connected with two gatorclips and a wire.
Put some audio thru it again and no difference at all.

As suggested before in the thread posted by CJ I traced the circuit and marked all connections with a pen in the shematic.
I found one more sucker at the inputtransformer. No connection from the black wire to ground.
Put audio thru it again and there it is, now i got the lowfreq.
Here is a new sweep:
http://www.rollingthunder.se/saker/LA2a.htm
Left channel is soundcard, right channel is LA2a.

No change at V4 pin 5.
Is my 6AQ5 drawing to much current and it might be a failure in the tube?
Can i test it someway?

HT

 

[HappyTom]

I've poked around in the unit and i don't get it, mostly because I don't know how a tube works.
I know that pin 5 on the 6aq5 is the plate, but i really don't know what the plate does or how it connect with the other pins at the tube, I'll think I have to do my homework.... Til' then I might need some help with this. I did these readings with no t4b, no signal and all tubes in.

Pin    V3        V4
1      97        NC     
2      0         8,7       
3      0,8       Heat              
4      Heat      Heat                   
5      Heat      73,2                     
6      97        115                 
7      0         0             
8      0,8                                   
9      Heat

Do you guys see something funny?

Thanks
HT

 

[CJ]

Looks reasonable.

 

[The Kid]

R34 is 22K 2W

I don't know what schem you have, but on mine, it's supposed to be 10K; which would explain the low voltage. Think about it; if you put a higher value resistor in, you'll have an increased voltage drop across the resistor, which would in turn lower the voltage at pin 5. (Kirchoff's law.) Algebra in action! :twisted:

 

[CJ]

10 K and 22 K were both used depending on the year, which is listed in the back of Jahnsen's book.

. Probably becuase they ran out of resistors.

The cathode resistor will bias the tube correctly, no matter what size the plate resistor. Well, within reason. A 1 meg plate resitor would not be good.

 

[AnalogPackrat]

[quote author="HappyTom"]No T4b.
C5 is 20uF 450V sprague atom.
R36 is 1K
R38 and C13 is installed as written in the manual, 22K between V4 pin 6 and center terminal of V4 then a link from center pin to C7d (+) and 510p between V4 pin 6 and r3 pin1.
[/quote]

If you are using 22k for R34 and 1k for R36 on V4, you shouldn't have R38 and C13 installed. Those two were added with a later rev that changed R34 to 10k and R36 to 470R. These details are in this thread, so search it. This may well be what is making your V4 voltages a bit wonky. Someone posted a table of revisions to the LA2a somewhere--probably also in this thread. Use Ohm's law to calc current through R34 and R36 and you'll a) learn something and b) determine whether your V4 is behaving within the realm of happiness.

As for your last table of voltages--check that you did not wire R3 and/or R37 (or any of the pots, for that matter) backwards. Otherwise, it looks close. I think you could safely install the T4b and give it a try.

A P