All things LA2a related

[CJ]

Here is the voltage chart for no Gain Reduction and no T4 plugged in.
I used 500 hz instead of 1000 because 1 khz was too iritating to listen to.

I also did a resistance chart. Unplug the unit and T4 for the resistance chart comparrison with your LA2. Infinite ohms on the V1 plates and top V2 plate.

cj

http://vacuumbrain.com/The_Lab/TA/LA2/la2_resistance_1.jpg

 

[dissonantstring]

hello.
i am building the bloo LA-2A and i'm on my final leg!!
powered up and no smoke and neon lights!
did the basic testing and wanted to check if these voltages looked o.k.
(without tubes of T4B):

AC Volts between A24 and A25 = 7.27VAC
B21 = 396VDC
B20 = 392VDC
A1 = 54.9VDC

both of the voltages on the B+ looked a little high so i thought i'd better
check first. do these voltages look within safe levels to put in the tubes
and T4?
Thank you for your help and patience (in advance, in case something
blows up later and i need mucho help!)
-grant

 

[SSLtech]

All of them look a tiny bit (though not disastrously) high. One two-part question: Which power transformer do you have, and did you connect it for 110V or 120V?

My gut feeling is that you either have high AC mains on your area, or maybe you've got it rigged for 110V and you 're feeding it something closer to 120? (which would elevate your voltages all round a little... That 6.3V is usually about 6.8-7.0 unloaded... 7.2 suggests that it too is a little high.)

Keith

 

[dissonantstring]

hi keith,
thanks for such a quick response and any help.
i have the power transformer which came with the kit (allied 6K88VG).
i didn't know this power tx could be wired to anything other than the 120V on the primary.
:shock:
have i wired this incorrectly? :sad:
-grant

 

[dissonantstring]

oops! forgot...
if the allied tx is wired correctly for 120V and i am getting 120V from my
AC lines (according to my furman AC rack power led read-out, however accurate that is :roll: ) is there something else i should be checking?
-grant

 

[Siegfried Meier]

Does anyone have a simple diagram of how to connect the Sowter input and output transformers? I've seen the schematics on Sowter's site, but they don't really help me much as I have a hard time reading these. Does Cayacosta have a simple diagram for this?

Thanks.
Sig

 

[SSLtech]

grant,

Like I mentioned, the coltages do not look all that worryingly high. -If you could select a slightly higher primary voltage, that would knock about 8% off them and put you right in the usual range, but you're still plenty within the acceptable range... And if the voltage sags a bit, you'll stay there longer than most people! :grin:

Sid,

As regards the Sowters, I wired one up with Scenaria once: He's out of town, so I can't sneek a quick peep, or I'd post a redux for you.

Also, I think that Ron (Cayocosta) has left the building, taking down his posters as he left, so there's no diagrams any more. -He also only did the UTC layout, no Sowters

Keith

 

[AnalogPackrat]

Sig,

You can check my Bloo build using Sowters by following the link I posted in this message (about half way down the page):

http://www.groupdiy.com/index.php?topic=95160

I used a 3 position terminal strips mounted near each transformer to connect components which were wired across the transformer terminals in the original.

A P

p.s. SSLTech--didn't realize Ron had also bugged out. Bummer.

 

[Siegfried Meier]

Okay. Got the pics. But I still don't understand where exactly the leads of the transformers are supposed to wire to. The Bloo manual only refers to UTC's. Any idea?

Thanks.
Sig

 

[Siegfried Meier]

Got the data sheets for the Sowter's.

Thanks.
Sig

 

[Siegfried Meier]

Data sheets aren't helping me much. I need specifics on where to connect all of these leads of the transformers. Can't see a thing on the pics that anyone has either.

Sorry to be a pain.

Thanks,
Sig

 

[Scenaria]

Sig,

sowter input

brown and green go from the input xlr

dont worry about the pink (just shrink wrap the end)

blue and grey goto the circuit

and black gets tied to chassis ground

for the output

yellow and green come from the circuit

grey and pink to the output XLR

black gets tied to chassis ground.

cant get much easier than that

 

[dissonantstring]

hi keith.
thank you very much for calming my fears. i'll do a couple of more
checks on voltages at the T4 module and 12AX7 sockets, as well
as the voltage drop across R33 per some of the other comments earlier
on in this thread. if all looks well i'll drop in the tubes and check voltages
per the bloo manual and then hopefully do the T4 module checks and calibration.
thanks for your assurance and wealth of knowledge keith! :thumb:
cheers!
-grant

 

[Siegfried Meier]

Making sense now. I just found the schematic as well. Thanks so much!

Sig

 

[yodermr]

What is the significance of the 470pf cap across NE2? I built the LA2a off of the Jahnsen book. He did not list it but I noticed it on the original schematic and on Caycosta's layout diagram.

Additionally, I was wondering if the wires coming off of SW3 (compress/limit) should be twisted or shielded. I mounted the switch on the front panel.

I'm trying to track down some noise and meter problems ( currently researching how to build the VU rectifier) and came across these differences.

 

[AnalogPackrat]

[quote author="yodermr"]What is the significance of the 470pf cap across NE2? I built the LA2a off of the Jahnsen book. He did not list it but I noticed it on the original schematic and on Caycosta's layout diagram.[/quote]

It's probably just providing a path for HF crud to ground. That neon tube is just providing a DC reference to offset the meter needle when it's switched to GR.

Additionally, I was wondering if the wires coming off of SW3 (compress/limit) should be twisted or shielded. I mounted the switch on the front panel.

Yes! That's your input signal path and any noise/hum that gets injected is going to get amplified.

Analog Packrat

 

[AnalogPackrat]

What happened to Paul B? Vacation? Hangover?

 

[yodermr]

Thanks for the help Packrat

That didn't seem to help reduce the noise. I tried moving wires around with a chop stick but I could find anything that helped


When the output and G/R pots are at 1 o'clock this thing is pretty quite, move them away from that position and it gets quite noisy

Any thoughts on why that would be or where to look?

Guidance much appreciated :grin:

 

[Paul B]

The saga continues....

Before I throw up my voltage and resistance next to CJ's (with the T4B removed, PR at 0) let me see if there's anything to say about the following:

C7a-d are axial, and I have all the [-] terminals on one side across the tag board nearest the power socket

C5 is a 22uf

In place of C8-1 and 2 I have a single .033uf

I've re-done the turret board and replaced most all of the components. The basic problem still remains: the box is blasting loud and distorted if I turn the gain past 10, and barely registering on the VU, although better than before. It seems to be compressing just fine, just have to nail this amp problem...

 

[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?