All things LA2a related

[alexc]

Hang in there ..it does happen ..  re  'roadbloacks'  along the way.

Keep going thru the schemo and your build  ..  make sure things are mostly OK and go from there.

The result is well worth the travel.

Good luck with it ...

 

[andow]

Sorry if this is obvious, but how do you calculate the current through the Zener diode replacing the neon bulb in the meter circuit?  :-[

 

[dbonin]

Sorry if this is obvious, but how do you calculate the current through the Zener diode replacing the neon bulb in the meter circuit?  :-[

https://groupdiy.com/index.php?topic=3278.2440


its all there... I asked the same question awhile back! 

 

[andow]

https://groupdiy.com/index.php?topic=3278.2440


its all there... I asked the same question awhile back! 

Thanks! I don't know how I missed that.... :-[

 

[andow]

I guess it's just semantics, but in the manual by Universal Audio it says about the side-chain pre-emphasis:

This potentiometer is factory set for a “flat” side-chain response (clockwise). Increasing
the resistance of this potentiometer by turning it counter clockwise will result in
compression which is increasingly more sensitive to the higher frequencies.

Looking at the schematics however, the response for high frequencies stays the same and compression gets less sensitive to lower frequencies.

Am I reading the schematics correct?

 

[dmp]

Yes,  the pot in parallel with the cap make a HPF.
It does make the compression more sensitive to high frequencies, as the low frequencies are filtered out.
To maintain the same amount of compression as the pot is turned up, the peak reduction would have to be turned up.

 

[andow]

Yes,  the pot in parallel with the cap make a HPF.
It does make the compression more sensitive to high frequencies, as the low frequencies are filtered out.
To maintain the same amount of compression as the pot is turned up, the peak reduction would have to be turned up.

Thanks for the confirmation! 

 

[FarisElek]

Having trouble finding this:

How do you wire a 2 way (On On On) switch for +4/GR?


Ryan

 

[FarisElek]

Not able to drop B+. I've got Neon lighting up, but for some reason there is no drop after 4.7k or 10k. Getting a solid 365ish. Using Edcor Xpwr136 for my PT. Tubes not installed and TB4 not installed. I looked around on here and found some posts with people in similar situations but I checked all those problems. Could using not great wire cause this? I'm using rather thin wiring that I remember being rated fairly high so I didn't think it would be a problem.

 

[scott2000]

....for some reason there is no drop after 4.7k or 10k.

What is the voltage before the 4.7k?

 

[FarisElek]

Around 370. Same voltage after 4.7k and 10k. I compared with the 369jx, it's wired right and closer to the original than the hamlond. I found a wrong value resistor on V4. 33k instead of 22k. I don't have a 22k on me though. Could that be it? I don't see why that would do that though.


Ryan

 

[scott2000]

Is there a reason the tubes aren't installed? Seems necessary to get any real readings apart from some basic checks to see if stuff is going where it should....

 

[FarisElek]

The tubes were installed but when I saw 320ish at the 6AQ5 I figured I needed to get my B+ right first. But I guess the tubes would need to pull current to get the B+ anyway?

 

[scott2000]

Not sure what area of the 6AQ5 you're reading. But yes, I'd maybe try trouble shooting what your exact problem is which would be that I guess? Unless there was something else...
.
Not sure about what the resistors are doing where you substituted...maybe someone else knows and could tell you if it'll have any effect.
Could always parallel another resistor to get you closer but idk....66k ish sounds like a weird one to have on hand..
unless you had multiple 33k laying around...could series and parallel to get your 22k

 

[FarisElek]

Not sure what area of the 6AQ5 you're reading. But yes, I'd maybe try trouble shooting what your exact problem is which would be that I guess? Unless there was something else...
.
Not sure about what the resistors are doing where you substituted...maybe someone else knows and could tell you if it'll have any effect.
Could always parallel another resistor to get you closer but idk....66k ish sounds like a weird one to have on hand..
unless you had multiple 33k laying around...could series and parallel to get your 22k

Sorry about that. Didn't have a schematic out when I responded. I have 33k for R38 which is supposed to be a 22k resistor. It is a grid resistor off of pin 6. Pin 5, which received B+ off of the 10k resistor (R34), was reading somewhere around 335v. Very off.

 

[scott2000]

Pin 5, which received B+ off of the 10k resistor (R34), was reading somewhere around 335v. Very off.

yeah... sounds  off....

You probably already did checks for resistor values, open resistors, bad connection, bad 6aq5 ..?

Maybe could post a schem with your voltages with everything in and idling? Someone may be able to help easier with that language...

 

[FarisElek]

yeah... sounds  off....

You probably already did checks for resistor values, open resistors, bad connection, bad 6aq5 ..?

Maybe could post a schem with your voltages with everything in and idling? Someone may be able to help easier with that language...

Yeah, I did all that. My wiring isn't too bad and I have everything zip tied so it's very easy to look around but everything looks fine. I'm sure it's probably one very easy to miss stupid thing that I've already looked past several times thinking it looked good  : . Tomorrow I get the 22k resistor and I did some basic solder touch up today. I'll post voltages tomorrow. A bad 6AQ5 would be annoying because I purposely got a NIB Sylvania one that was apparently tested, but you never know.

So, just to be clear, without the load from the tubes in it, is impossible to get the B+ to what the schematic says should be 175 after the 4.7k? I'd rather not destroy my tubes If I can get that B+ without them in.

Ryan

 

[scott2000]

came across this...not sure if you've seen it...

may have some relevant bits of direction or clues to help....??

https://groupdiy.com/index.php?topic=35180.0

 

[FarisElek]

came across this...not sure if you've seen it...

may have some relevant bits of direction or clues to help....??

https://groupdiy.com/index.php?topic=35180.0

I did see that. I just read into the comment about following the resistance off pin two found a bad connection just now. I just tested it and the voltages are so much better. I'm still getting 30v too high on pin 5 of the 6AQ5, but I'm just about there. While reinstalling the tubes I dropped my output tube and it shattered into a million pieces so i can't test audio yet  :'(

 

[scott2000]

arrgh... I had that happen once....Nice pair of smoked glass 6sn7s the seller taped together  and in my excitement I separated them with too much force  and one shot off to the tile floor...

Hopefully your close.... I've read the tolerances can be as much as 20% in this thing so maybe...