All things LA2a related

[quote author="Cainester"]Does closed = compress and open = limit?[/quote]
Correct.

Keith

Okay... couple of observations here:

The GR display doesn't change in compress or limit modes... just ggives an approximation. (check it out on the schematic: the limit switch changes only the effectivienss of the GR, not the amount of light in the EL panel, therefore the seperate metering LDR is unaffected by the switch... just the signal)

The test for the value of the limit resistance is to feed in a tone via a level control BEFORE the compressor, switch to compress and note that the signal rises slightly as the input is increased.

Repeat the test switched to limit, now note that the output signal should more or less stop a little above the threshold... if the signal actually starts to turn down above the threshold, reduce the value of the resistor. If it still climbs slightly, increase the resistor.

Use a pot to gauge the setting, then replace the pot with the nearest value fixed resistance.

Keith

Actually Caine, I thought you meant you had no compression or limiting - sorry about that. Otherwise, you did great - congratulations! R7 is bypassed in compress mode - with the switch closed.

[quote author="cayocosta"]Actually Caine, I thought you meant you had no compression or limiting - sorry about that. Otherwise, you did great - congratulations! R7 is bypassed in compress mode - with the switch closed.[/quote]

And to answer Cainester's earlier question, yes it is because of this bypassing that the resistance falls to 0 ohms at the pot.. (think about it! :wink: )

You don;t say how you're observing the gain reduction... if you're looking in "meter-gain-reduction" mode, you won;t see anything. -If you're looking in "Meter-output+4" mode, you'll see it.

Keith

Yeah, I'm looking at +4. In limit, I've not found the happy medium yet. The output voltage gradually increases as more gain is applied to a point and then the out voltage starts to decrease. It seems it will always do this to some extent, so I must find where it does it the least and stick that value in there..

Keith, thanks for the heads-up on the limit mode test - I've never heard of that before. In fact, that might be the first new bit of la2a info since CJ's T4 hack! How about some sweeps?

http://www.vacuumbrain.com/The_Lab/TA/Rons_LA2a/la2a_sweeps.jpg

Ok,
The unit sounds awesome, but..
When I run a tone and scope the output, the single wave has turned into 4.
What could cause that?
Caine

Hi folks
I´m planning to built a Bloo LA but could not yet find out which transformers to use for in/out. Can the UTCs be found anywhere? What - if not - are good replacements? How do they fit in the Bloo kit/enclosure?
thanks
Jens

ebay dude...

theres been a ton of A10 and A24's on ebay the past few days..

do a search for UTC...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67815&item=5742525285&rd=1

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67815&item=5742340383&rd=1


http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67815&item=5741006007&rd=1

Hello,

I am wiring up my input and output XLR's on my L.a.2.a.

(Warning: Lack of sleep and extreme solder inhalation!!!)

If I read on page 2 of the "L.a.2.a. re-issue" manual it lists the barrier strip pinout. (1) is input + and (8) is output + . (5) is input - and (10) is output -

Now, look at the 1968 schematic. barrier strip (1) input goes to pin 6 on UTC input. (8) goes to pin 6 on UTC output. (5) goes to pin 1 UTC input. (10) goes to pin 1 UTC output.

I have 3 sources of info on this wiring that are all conflicting. I'm wondering if all 3 ways are correct?

sources:

1) bloo info
2) cayocosta diagram
3) info above I found: (old schematic and reissue manual).


Thanks,

Z
:?

It appears that the 1968 schematic illustrates the A-10 (input tranny) primary labeling inverted - with tranny pin 6 hot. Older schematics as well as photos of reissue units illustrate tranny pin 1 hot.

If I'm correct, then this is the way to wire the input for XLR pin 2 hot:

XLR input pin 2 --> barrier strip terminal 1 (optional) --> HA100X/A10 tranny pin 1 (primary) HOT input to tranny
XLR input pin 3 --> barrier strip terminal 5 (optional) --> HA100X/A10 tranny pin 6 (primary)

(My layout reflects this, and this supports the reissue manual's instructions to use barrier strip terminal 1 for hot)

Now, the output side is always illustrated the same way in all the schematics (without the XLR) - barrier strip terminal 10 to tranny output pin 1 hot. However, photos of the reissues (with XLR's) indicate that XLR pin 2 is wired to barrier strip 8 (pin 6 of the output transformer), which would reverse the polarity from the output tranny.

This is how (in photos) the reissue unit's output is wired for XLR pin 2 hot:

XLR output pin 2 --> barrier strip terminal 8 (optional) --> A24 tranny pin 6 (secondary)
XLR output pin 3 --> barrier strip terminal 0 (optional) --> A24 tranny pin 1 (secondary) HOT output from tranny

(My layout reflects this too, and this also supports the reissue manual's instructions to use barrier strip terminal 8 for hot)

But, the trouble here is that pin 6 of the output tranny secondary is not the hot side.

Has the signal's polarity been flipped by the time it gets to the output transformer primary, so that reversing the XLR connections puts the signal back in phase on the way out?

If so, then it makes sense.

Reissue:

I don't know much, but I believe a triode inverts phase at the anode, and a cathode-follower output is also inverted. So, the signal is ultimately out of phase (2 triodes and a CF) going into the output tranny primary - and that's the reason for the switch at the XLR output.

Thank you for your input cayocosta, I appreciate it! I shall wire mine the way you have discussed unless proven otherwise.

Z

Looks like the Seattle contingent is all working on LA2A ( I'm back to workin on mine again) =)

ju

[quote author="cayocosta"]...a cathode-follower output is also inverted. [/quote]

Polarity is only inverted when the signal is taken from the plate, not the cathode. A cathode follower's output is in phase with the input.

But, we're not looking at a cathode follower in the LA2a, are we? (I'm not familiar with the reissue.)

Oh... maybe we are looking at a cathode follower -- but it's a "white" one, eh?

Trying to figure out what is causing me a low voltage reading at turret A17 which is supposed to be 90vdc but it is 5vdc with all my tubes in but T4B is not installed. I double checked all wiring, shorts and components etc. I was very careful wiring this thing up.

Across turret A17 and B17 is a 220K (R33) which is reading 260vdc on the B17 side which appears to be proper.

A17 goes to V3 (pin 6) which is jumped to V3 (pin 1) which then goes to a .02
capacitor on to pin 3 or bottom contact of R37 (potentiometer) etc....

:?

Hope to figure this damn thing out ...so I can drop the T4B in and fire her up. Time to break out the magnifiying glass.... :roll:

Z

The tube's cathodes (pins 3+8) should be tied together and wired to the 1k resistor and bypass capacitor(s) that go to ground. Check to see that your ground there is good.

Be careful with the voltages in there.