All things LA2a related

[AnalogPackrat]

Whoops. That's what I was afraid of. When you measure the heater voltage you should measure across the leads--one test lead on A24 and one test lead on A25. That will give you 7.2 volts AC which is what you want. Your heater wiring is fine. Hook it back up. Sorry 'bout that, but it just dawned on me as I finished typing that message.

 

[AnalogPackrat]

So on to the other voltages...

B21 is a bit high and B20 is too high. Look at the circuit. You've got 378V onone side of R29 and 316V on the other. That's 62 volts across 4700 ohms which is:

I = V/R = 62/4700 = 13mA

You should be dropping 85-100V across R29 for 16-21mA.

A1 is fine (my neon was 53.4V). A20 is a bit too low. A14 is way too high. A17 is OK. A19 is too high.

I'm fading here. Need to get to bed. If you're still up for it tonight, make a little diagram of just the parts of the circuit that feed from c7a and c7b out to the plates of the tubes. Work out the current through each resistor in this part of the circuit using your voltage measurements and the resistor value. Do it again using the voltages on the schematic and compare currents. Somewhere you'll find a difference and that will lead you to the problem area.

Have fun.

Analog Packrat

p.s. If you used Sowter transformers, be sure you hooked up the grid stoppers correctly. You may have some HF oscillations at this point that are causing your weird readings. Someone else had this problem a few months back and it's on this thread.

 

[wave]

6.8 VAC on A25/25 (meter leads attached to both turrets)

 

[wave]

OK new voltage readings
B21=363
B20=304
A1=53.7
A20=106
A14=304
A17=94.4
A19=136.8
Readings with all tubes in / no T4B
I am using sowters w/o grid stoppers in place
I have turned R3 & R37 full each way and differences in reading are very minimal
zero adjust is working fine and when meter swich is in GR mode the needle swings up to zero when I power on
I think I could get my voltages a little better though....
will continue tomorrow
AP thanks for the help!!! :sam:

 

[SSLtech]

[quote author="AnalogPackrat"] ... with a nut on the backside ... [/quote]

Wh-oooah, Nellie!

-THAT's an image I'm going to have to work hard to get rid of!

Keith

 

[AnalogPackrat]

SSLTech,

Man, you'd fit right in where I work. Bunch of non-PC guys who are always "reinterpreting" each other's statements. Or maybe it's just the MJ trial hangover :shock:

Analog Packrat

 

[AnalogPackrat]

wave,

I think you are close to getting this thing working. Right now it looks like your 12BH7 is not drawing any current (you have the same voltage on both sides of R17). Either something isn't right with the wiring or it's oscillating. If it were oscillating you should see some drop over R17, so I'm guessing something's just not wired right. What DC voltage do you have at A7? What about pin 3 of the 12BH7? Make sure R19 and R20 are the correct values and correctly wired.

Analog Packrat

 

[CJ]

Do you have a chart with all these B-21 etc points mapped out?
I am too used to

1plate
2grid
3cathode
h
h
6plate
7grid
8cathode
h

to be of any use here.

 

[audioforge]

hello.
I am sourcing components for my la2a clone.except 12bh7a t4b and in/out transfos missing i have nearly all others components coming from old tube milatary radios surplus in a junk yard.
I have some questions about few caps.i will refer to Ron schematic:
first: i miss C13.i ve read its a mica cap.i have some 300 330 470pf mica's
does the 510pf value is strict or a 470pf or 560pf will be ok?
or can i put a 510pf polystyrene cap there?
Second: C2 and C3: i have old 0.1mfd 400v cornell dubilier film caps .is it OK instead polypropylene orange drop?
third: can the 470pf ceramic (parallel to the neon) be 470pf mica? .
THanks a lot for your help and for the great infos found on this forum.
audioforge.

 

[AnalogPackrat]

It's the secret Bloo turret board numbering code. :wink: I'm officially not supposed to reproduce the manual :roll: ... I hope Scenaria doesn't mind.

Here's the decoder ring. Look at Cayocosta's diagram on your site (see, you had it all along). The top row of the turret board is the "A" row and is numbered 1-25 from left to right. The bottom row is the "B" row and is numbered similarly. So:

B21 is C7a
B20 is C7b
A1 is the Neon
A14 is 12BH7 plate
etc.

Analog Packrat

 

[AnalogPackrat]

Audioforge,

I used fixed value mica caps for all the pf range stuff except for C4 (it's a trim cap). I think I used 380pf for C13 and the one parallel to the neon. Polystyrene would be overkill since these are in the sidechain and the meter circuit. For C2 and C3 I used some surplus 0.12uf 400V film caps--I'm not even sure whether they are polypro or polyester. They work fine.

I also used Sowter transformers--you can buy them from Prodigy Pro. Just click on the icon in the upper left of the forum page. One note about the Sowter output trafo is that it may cause oscillation in the output driver (12BH7). To kill this, use 1k grid stoppers on the 12BH7 triodes. You should be able to find some 12BH7s at your local surplus places. They seem to be fairly common.

Good luck,
Analog Packrat

 

[audioforge]

thanks analog for your answer.
for C4 i have some big 20-120pf and an arco464 (45 to280pf).as CJ (i think) says a fixed 100pf could do the trick i am sure the 464 will be nice enough.
C13 C12 you said:"Polystyrene would be overkill since these are in the sidechain and the meter circuit".
for sure but i have some with thoses values but not in Mica.
to ask in others words :if i change some ceramic to filmcaps or mkt i have, in the audio pass will i deviate too much to the "original la2a sound" or as some said: all is in the iron?
thanks . audioforge.

 

[AnalogPackrat]

I don't have any experience with a real LA2A, so I can't really answer your question about comparing the sound of various types of caps. Just looking at the circuit, I think you could use just about anything you wanted for C13, C12, and the neon bypass cap.

I think if you're going to hear anything from caps it will be C1, C2, C3, and C5. In the original and reissues C5 is an electrolytic. I used a film cap that I got surplus. I believe it is metallized polypropylene. I didn't compare it to an electrolytic. To my ears, my Bloo kit sounds very nice.

Some of the other guys here have far more experience with LA2As than I do. Hopefully some of them will chime in. I'm curious about some of the same questions you are asking.

Analog Packrat

 

[CJ]

The side chain caps drive the EL lite, so there is no real benifit in using designer caps there, in fact, you may do more harm than good. Response time might change. I do not know the various response times of say, ceramic vs poly, but I bet it is different.

 

[wave]

Well the battle has been won! :grin:
Thanks to all your help (CJ,Analog Packrat, Scenaria)
I was looking in the 12BH7 area as per AP's comments and I found that I had accidentally wired
V2 pin 8 to turret B8 and
V2 pin 7 to turret B7 (dumb mistakes)
Here's my new voltages
B21=354
B20=283
A1=53.5
A20=129
A14=87
A19=107
:guinness: :sam: BEERS ALL AROUND :sam: :guinness:
Now I think it's time to plug in the old T4 and get some sound in this box!!

 

[AnalogPackrat]

Hold on....everything looks good up to A14. Do you have a typo in the A14 and you menat A17 (which should be about 100V+/-)?

 

[AnalogPackrat]

wave,

Gotta head home. I'll check in when I get there. I think you're pretty much done. Listen out for distorted audio or other oddness which may mean oscillation to be killed with grid stoppers.

Later

 

[wave]

Ok
Here's something wierd,
I plugged the unit into my patchbay, took a vocal output from my interface into the Bloo, out of the Bloo back to the bay on to channel 10 of my mixer.
When I plug the cable into the Bloo, the output meter (in GR mode ) drops to zero and looks as if its showing the peak reduction in reverse (rising while compressing). When I pull the output (either from the bay or the back of the unit) the meter works correctly in gain reduction mode. The meter does not work at all in +10/+4 operation.
Other than that, the unit sounds AWESOME! No noise, lots of gain and some serious squash.
Another observation, it seems like I get more compression in compress mode as opposed to limit (i.e. when in compress, I can hear it working more when I've got the peak reduction knob up).
Any thoughts?

 

[wave]

[quote author="AnalogPackrat"]Hold on....everything looks good up to A14. Do you have a typo in the A14 and you menat A17 (which should be about 100V+/-)?[/quote]
Yes...sorry
A14=231.5
A17=87
A19=107

I'm gonna disconnect it and get back inside this baby

 

[Scenaria]

check your limit/compress switch.. is it upside down?

as for the meter.. this is where a lot of builders make mistakes... triple check your wiring on the rotary switch... its not uncommon to have just one lead attached to the wrong pole.

it should work harder as you turn the peak reduction knob clockwise..

when you switch to GR on your metering.. it should sit at zero and deflect to the left... or downward showing more compression... turn your PR knob all the way down the meter should be at zero.. as you turn the knob up it should begin deflecting to the left (or downwards) are you sure the rising isnt the compressor returning during its release?

I think all of your metering issues are going to revolve around that rotary switch..

you have no oscillations? did you use grid stoppers? im really impressed if you didnt need the grid stoppers and theres no oscillations (very cool!)

and yea... the la2a's have TONS of gain!

congrats!

:guinness: :guinness: :guinness: