All things LA2a related

[AnalogPackrat]

Mark,

Can you bring us up to speed on what you're doing? Do you have all the tubes and the T4B installed? Have you checked DC voltages? Does the thing pass audio? Does it seem to compress? When you say "noisy" do you mean hum/buzz or hiss, pop, spit, crackle, scratch? Remember, no one here can see your unit or hear it, so you have to give gory details and measurements :wink:

A P

 

[Siegfried Meier]

Ok. I have the schematic and the Sowter Input Tranny document from Cayacosta and I'm good with wiring the input of the LA2A. However, I'm not entirely sure where the yellow and green wires come from for the output, or where I'm supposed to put this 10uF cap across. Any ideas? Is there a similar pic like the input?

Much appreciated.
Siegfried

 

[SSLtech]

Look at the schematic.

The 10µF cap goes IN SERIES with the primary of the output transformer.

There's really nothing to wirring the Sowters, a transformer's just two coils of wire when you think about it. The Schematic reflects this aspect so beautifully well that you really can work it out quite easily...

Just do it! -The Cap is there to block DC from the primary. That's it!

Keith

 

[Siegfried Meier]

Haha. Thanks. I'm not so hot with reading schematics, although you're right. There's really nothing there and it's quite easy.

Thanks again!
Sig

 

[yodermr]

Sorry, I was posting in the wrong place before

Tubes and the TB4 installed. DC voltages...

C7B+ 257
12AX7 #1
1 - 110
3- 1.1
6 - 108
8- 1.11

12BH7
1- 117
3- 3.3
6- 250
7- 81
8 - 121

12AX7 #2
1- 82
3- .79
6- 82
8- .79

6AQ5a
2- 5.8
5- 143
6- 106

It passes audio (sounds very nice) but noisey. Seems to be unity gain with the output at 10 o'clock (no compression). With Output and G/R at 1 o'clock its the least noisey. From there going counter clock wise with the Output control its noise like an open line (like a guitar cable not connected to an amp) going clockwise its more brighter crackly noise, hard to describe brighter would be my best decription. Similiar change in noise character change with the G/R control going clock wise or counter clock wise from the 1 o'clock position.

I have some work to do on the meter, apparently I have a DC movement type and I need to build a rectifier (waiting on geranimun diodes) to get it to work with AC signal (I'm learning here) Anyway it does compress as I can see the range calm down on my AD converter down stream.

 

[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

 

[cannikin]

Hey Scenaria, CJ or anyone

Where have you guys purchased terminal posts? the ones that you can attach directly to chassis? Are they still manufactured?

thanks :thumb:

 

[Paul B]

David and everyone else,

theres a lot of cool stuff including tag boards at

www.turretboards.com

Paul

 

[Paul B]

[quote author="AnalogPackrat"]Paul,

I assume C5 is an electrolytic with at least a 250V rating, right? Is it wired with the positive end toward the tube and the neg toward the otuput transformer? Everything else seems fine. Did you replace the BH7? Did you recheck the wiring on the BH7 socket?

A P

p.s. Is there any way you can post some pics with closeups so we can have a gander at your build?[/quote]

C5 is a 450V electrolytic w/ the pos end toward the tube. Replaced BH7 and re-checked the wiring there (and everywhere else)

Ill try to get some pics up in a few days

Paul

 

[Paul B]

[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]

Hi CJ,

I will give it a try. I have been feeding a signal straight in from a CD player per your suggestion, and the ol 440 and 1k sine wave. The hum is gone now, but if I feed a weaker signal into the box I wont get the t4b to fire up and compress, and the VU meter wont register a signal.

gonna finish that chart now...

 

[CJ]

Cool. I think I slipped a decimal point on the upper cathode of the bh7, IF YOU GET SOMETHING DIFFERENT, IT MIGHT BE OK.
CAPS OFF.
:razz:

 

[Paul B]

OK, here's the chart.

R9 and 13 are 220k again.

I also am running the 4.7k 10k and 470 for R's 29, 34 and 36.

I put back C13 and R38

V1 is a 12AY7

http://photobucket.com/albums/a5/PaulB_/?sc=3

 

[CJ]

Is your gain pot an audio taper?
Put it at half way on and measure the resistance from wiper to both ends.

 

[SSLtech]

[quote author="CJ"]Is your gain pot an audio taper?
Put it at half way on and measure the resistance from wiper to both ends.[/quote]
...but ONLY after disconnecting it. -The parallel pots and the 1k resistor and the input TX secondary etc all make readings useless otherwise.

Keith

 

[dissonantstring]

hello.
a couple of days ago i did my first check of voltages after completing
my wiring on my bloo and some seemed a little high, but keith said it wasn't too high, so i put in the tubes. my voltages still look high and now i'm concerned with the some of the voltages. can someone tell me if these voltages are o.k. something looks wrong...

A24 and A25= 6.83VAC
B21 = 368 VDC
B20 = 286.3VDC
A1 = 55.5VDC
A20 = 83.2VDC
A14 = 232VDC
A17 = 106VDC
A19 = 123.8VDC
pin 5 on T4 socket = 33VDC (too high?)

please help! :sad:
thanks,
-grant

 

[CJ]

tubes circuits can have a 20% tolerance on voltages and not be affected.
tweak that 4.7k pwr resistor between the two filter caps if you have a thorn in you Levis about it.
otherwise, wait for Emron to get back in business. :grin:

 

[dissonantstring]

coolio!!
thanks CJ! your knowledge is greatly appreciated and much respected (so is your sarcasm ) :razz:
this is my first tube project so all the high voltages scare the be-jezus out of me. i know i'm being paranoid and you'll give me a lashing for this, but i read earlier on in this post that pin 5 of the T4 socket should be around 8.5VDC, but mine is 33.2VDC! :shock:
i haven't put in my T4 module yet, though i also read it shouldn't fry the T4 since the ELs can handle more voltage than that.
Should I put in the T4? :?
(i keep hearing a chant in the back of my head saying, "do it! do it!")
-grant

 

[Scenaria]

heck...

you can shove 120V AC into the EL panels... and they will light up just fine :green:

 

[CJ]

Do any surfing in Beumont? :razz:

 

[dissonantstring]

thanks a bunch CJ and Steve!
i will be doing some tests with the T4 module in.
i can't wait!! :green:
-grant