All things LA2a related

[Scenaria]

jim, make sure both pots on the back are turned fully clockwise... then re-check that A20 reading

 

[Jim50hertz]

will do steve, thanks

unfortunately i can't get to it again till tomorrow evening, will post results and it should hopefully be good news!

jim

 

[QNote]

Would anyone care to share the approximate time they have invested in their LA2A kit? Was curious as to how my timeline is moving along versus the average...

 

[Scenaria]

depending on skills... 12 to 18 hour build time..

 

[dukasound]

Hi
Does anyone have drawing of original front and back of LA-2A.
I made first with comparasion from pics.
Thanks
Duka

 

[Guest]

 

[AnalogPackrat]

QNote,
I'm in for about 8-9 hours so far--taking my time and checking my work as I go. I'd guess I'm a couple of evenings from first test. If you go with non-UTC trafos expect to spend a little extra time laying out and drilling your blank plates. If you use Sowters you might think about mounting a small tag strip on the inside of the plates for easier connections (especially the input one which has some resistors tied directly between the trafo and other points). This is a fun project--can't wait to use it!

Analog Packrat

 

[Tom]

A couple quick questions for y'all.

I am using Sowter transformers, and found on the Sowter web site a copy of the schematic showing color coded wiring specific to their transformers. Cool.

In studying this schematic, which appears to be a later revision than the one in the bloo manual, I note an unlabeled variable capacitor that is strapped in parallel across R30 (47k). This branch of the circuit appears to relate to the stereo parallel option. What value are you all using here? Or are you omitting this value.

Also, in an earlier version of the schematic, at pin 2 of the 6AQ5, there is a 1k in parallel with a 50uf cap. (r36||C10). In the later schematic I downloaded from Sowter, I note the values changed to 470 in parallel with 100uf. Is there a consensus on a preferred version to use here?

Best regards

Tom

 

[cannikin]

This doesn't answer your question but its good to have, I got this from Cayocosta, :grin:

 

[Jim50hertz]

hey steve

tried the rotation of the 1 meg pots and....

same result

A20 - 73V
A19 - 82V

my other readings are as follows...

B21 - 348V
B20 - 270V
A1 - 57
A14 - 220
A17 - 86

are the rest tolerable if I can find the problem r.e. low voltages on A19 and A20? Don't really want to drop the T4B in and go to audio/meter testing till i solve this one...

Cheers

Jim

 

[Tom]

Cannikin:

Thanks for the graphic. I sort of fudged this connection by using an isolated stud and a ground ring.

I attached the ground ring and isolated stud to the top screw on the input trafo plate. I connected R7 between the isolated stud and the T4B. Then I soldered R5 and R6 in an "L" shape, soldered the free end of R6 to the stud and the free end of R5 to the ground ring. I soldered the black and grey wires from the sowter transformer to the ground ring, and connected the ground ring to the chassis ground around the T4B. Finally, I connected the blue wire to the R5/R6 connection.

I think my solution should work out there, but yours sure does look cleaner. Thanks for sharing the graphic.

btw, what did you do with the pink wire from the Sowter trafo? Trim it back? Cap it off?

Best regards

Tom

 

[cannikin]

For the Pink Wire I cut it back to about an Inch then put Heat-shrink on the End so it couldn't make any contact.

 

[Tom]

Greetings:

My BLOO kit is coming along, but I ran into a small snag that I hope someone can help out on.

I have completed the kit assembly instructions.

When I turn the gain up from its lowest setting just a tad, e.g., to a setting of 10, the meter pegs and sort of bounces.

With tubes but no T4, I get:

B21-331V
B20-244V
A20-130V
A19-114V
A14-197V
A1-60V


These voltages are a tad low, but they appear in the ballpark to me and the neon lights fine. Any thoughts?

The only thing that I knowingly did that I did not see in the instructions was to provide a jumper between A11 and A14 to link R17 to C3.

I am going to start tracing through the circuit tonight, but hoped someone had seen this before and could provide me a good starting place to begin debugging.

Thanks in advance

Tom

 

[AnalogPackrat]

Tom,

Did you put in the 1k grid stoppers on the 12BH7? Check back through this thread and you'll find a few posts about it. Apparently the Sowter output trafo wants to cause the white cathode follower to oscillate.

Analog Packrat.

 

[AnalogPackrat]

LA2A builders,

I'm on the home stretch on my Bloo kit and have a question. I'm making all of the shielded connections from the front panel into the chassis and I'm really hating the cable I have :evil:. I have some belden 2 conductor mike cable which is really thick and the insulation for the inner conductors is really stiff which makes it hard to work with. I also have some RG179 which is nice and small, but also has the same inner insulation issue. Does anyone have a source for decent small and flexible insulated cable?

Analog Packrat

 

[sleepingtiger]

I used 2-conductor shielded audio cable that's readily available from Radio Shack. Whenever only one conductor was needed I just twisted the two together (the shield is a third conductor). It was easy to use, works well, and with a little heatshrink it looks nice. Luckily, Doug was visiting town and he saw the Belden cable I was about to use and he suggested I try something else. I'd used the RS cable before and since there are no other shops in my area I grabbed a spool and went to work...

You guys are going to love the Bloo... I can't stop compressing things :grin: ...

 

[AnalogPackrat]

Thanks, sleepingtiger. I still buy a few odds and ends from RatShack. I'll stop by on the way home to see if they've got this stuff.

If they do, I'll be firing this thing up tonight.

:thumb:

Analog Packrat

 

[Tom]

AnalogPackrat:

Thanks for the tip. Tonight, I will review this entire thread and give the resistors a try. I gotta believe that I am close.

Btw: I used mic cable for the shielded wires and hated it too. The cable is pretty thick and bulky, but it was all I had and stores around me were closed for the holidays.

Thanks and good luck with your build.

Best regards

Tom

 

[AnalogPackrat]

Tom,

I see that you never got an answer on the 470R vs 1k question on the 6AQ5. The choice here depends on what you did with the grid stuff (the later version has the series R and parallel cap on the grid) and what plate resistor you used. Look back on this thread and you'll find Scenaria's answer to this one, too. I built the later rev which has the extras on the grid, a smaller plate R (bigger power rating, though) and the 470R on the cathode.

As for the stereo link, I put it in mine. Scenaria made the sane suggestion that I mount the jack on one of the trafo plates. With the Sowters I was able to fit the jack on the output trafo plate. I used a fixed 100pf cap instead of the variable one. Looks like it's just there to shunt HF grunge on the link to ground. If I ever build another Bloo I might have to parallel another cap on there if I have trouble with the link. No biggie.

I hope we'll both have a working unit tonight.

:guinness:
Analog Packrat

 

[Tom]

AnalogPackrat:

You were right on. Those 1k grid stoppers were all that was needed to have what appears to be a fully functioning BLOO.

I plan on doing some recording over the weekend, thus I will know more then, but in my initial tests, the unit works exceptionally well.
Thanks to everyone for their help.

Best regards

Tom