All things LA2a related

[college101]

Sowter-input t=1009- 1;9.1       600/50k   +15dbu
Sowter-Output t-1010  5:1       15k0/600  +50dbu


So..basically cut into an old Xlr cable, attach a headphone jack to the xlr, plug the headphone jack into the cd player....turn the cd player on, and volume up high

This is the way to go for signal?

nothing connected when I checked it...  What if I hooked up my microphone, and set it infront of my stereo systems, and cranked some volume..?

 

[Sredna]

What if I hooked up my microphone, and set it infront of my stereo systems, and cranked some volume..?

Naah, think about your neighbours!  ;D

The Cd player would work, dont turn up the volume too high as the input transformer is a step up one.
Connect the tip of the phono plug to pin 2 in the XLR and the sleeve (and ring) to the XLR pin 3.

Ideal would be a tone generator with a known output level.

How do you monitor what's coming at the output?

 

[Kingston]

quote of the month right there

 

[college101]

Ok..nice quote...Cranked some music playing from my stereo!

I will try the cd player method for sure...Just wondering if there is another way..I already have to re-wire everything!

 

[Sredna]

quote of the month right there

;D ;D ;D

Just wondering if there is another way..I already have to re-wire everything!

If it's too much work making connections for your gear, why build it in the first place?

The XLR input on the LA2A in not a microphone input (it could be used as such though) but for
a balanced connection from for example a mixing desk. For easy connections you could install a stereo guitar jack in parallell with the XLR-IN, then you could plug in a CD player or a bass or whatever...

 

[college101]

Funny guys...you know what I mean...Im at work doing three things at once....Trying to hide out from my boss.. still trying to get this 2A built!

Just seen you edited your response Sredna up above

I dont have a scope or a tone generator...Wish I had one though!!! Probably would make this a lot easier!

In terms of monitoring at the output...Im taking suggestions on what to do!

I also want to ADD here...I have been humbled regarding electronics! This stuff is ten times harder then what I thought...I gotta give it up to all you analog guys....

 

[Sredna]

Don't worry about it, we're all still learning... 

The output should be connected to a ~600 Ohm balanced line input but for testing you can
make an adapter XLR->guitar plug and plug it in a guitar amp or line in on a mixing desk.

 

[college101]

Oh man..That's so simple...I will do that as well tonight...I hope my 2A does pass audio!!!

So..Cd player...into Input...Gain on 5, peak reduction on 5, XLR output to guitar amp

Obviously my metering is not working correctly, but should I put it on +4 or Gain Reduction for this?

 

[lassoharp]

To me, these subtle difference may be what is causing it to not work, inless you guys think otherwise?

Transformer shield tie arrangements shouldn't be affecting whether you're passing audio or not.  It sounds like a miswire or cold solder joint somewhere.

I dont have a scope or a tone generator...Wish I had one though!!! Probably would make this a lot easier!

Easy fix(several).

1) Get someone to send you a 1K test tone as an mp3 - burn that to a CD = viola!  Simple tone generator.

2)  Got a synthesiszer/keyboard of some type?  Record a note around 1K onto CD.

3)  Play a music CD - measure the avg output with an AC voltmeter first - it will jumping around all over the place but you should be able to get a common avg reading, say .25V.  This is very crude but this is only for sig tracing purposes and should at least tell you where the signal is being lost


Before investing in a lot of (possibly $$) NOS tubes - recall that +/- 15% voltage readings relative to the schematic is considered normal.  In fact,  I'd be willing to bet that not many La2a's left the factory with the exact same voltages.  Tubes are just that way and they will often vary slightly from day to day but that doesn't necessarily mean a faulty tube.  Very normal behavior.

 

[college101]

Lassoharp...Good idea...Ill have to get that going as well with the 1k tone to cd idea

This is from a few pages back...its the voltages i checked at pin points on the tube sockets, with tubes installed, but no signal going threw...so basically its at idle
???Look at 12ax7#2-Pin 1 and pin 8 are way off--- there is no way that less than a volt can be right! I would have to go out on a limb and say thats outside of the 10 to 15 percent of spec= which is what makes me think I need to atleast replace that tube....I did buy it off ebay, and the guy had the tube tested- the test came back 60/58

First I re-solder'd my ground off of the main power tranny...then check'd the tube voltages!

c7B+=353 dc

12ax7a #1
pin 1=155 dc
pin 3=1.06 dc
pin 6=268.4 dc
pin 8=5.05 dc

12bh7a
pin 1=115.4 dc
pin 3=4.24 dc
pin 6=263.7 dc
pin 7=86.9 dc
pin 8=120.7 dc

12ax7a #2
pin 1=.116 dc
pin 3=.959 dc
pin 6=115.8 dc
pin 8=.956 dc

6aq5a
pin 2=85 dc
pin 5=111.5 dc
pin 6= could not get a valid reading-it just kept draining all the way down to zero

 

[Sredna]

Before investing in a lot of (possibly $$) NOS tubes -

get some cheap ones!  ;D It's always good to have some backups...

@ college.. With 12ax7#2 do you mean V3? (the one close to 6AQ5A) Then the voltages are not that off, pin 3 & 8 should have 0.75V according to the schematic. V1 should have ~1.2V on pin 3&8. (Pin8 too high)

What worries me is the 85V on pin 2 on V4 (6aq5a). Did you measured that again after changing R38?
But that's the sidechain amp, try to get the makeup amp to pass sound first.

 

[college101]

[@ college.. With 12ax7#2 do you mean V3? (the one close to 6AQ5A)/quote]
YES..V3 is the second 12ax7a I was talking about

I forgot to measure that last night

What worries me is the 85V on pin 2 on V4 (6aq5a). Did you measured that again after changing R38?

 

[lassoharp]

The plate (pin 1 & 6) voltages on 12ax7 #1 do seem to be higher than they should be.  That shouldn't stop it from passing audio though.

Did you use a 68K resistor for R16? 

 

[college101]

Did you use a 68K resistor for R16?  

YES I did..and I have tripple checked it...


One thing I did find...is the hammond 369JX PT which i am using...I am showint that it is rated at 60ma instead of 40ma(info from tube town site)..could this be causing the higher voltages?( maybe this is just a typo on their site)

This is from the Digikey site
Specifications for Hammond Manufacturing Chassis Mount Power Transformer 50 VA 369JX Vendor Hammond Manufacturing
Category Transformers
Mounting Type Chassis Mount
Type Standard
Primary Winding(s) Dual
Secondary Winding(s) -
Series Output Voltage @ Current -
Parallel Output Voltage @ Current -
Maximum Power 50 VA
Lead Free Status Lead Free
RoHS Status RoHS Compliant
Other Names 369JX
369JX

 

[lassoharp]

Maybe slightly higher overall voltages, but I wouldn't guess that much of a difference for that small a difference in current rating.  Of course, if the other side chain tubes are pulled, the overall supply voltages will rise - the 6AQ5 having the biggest effect.

The audio path tube voltages look like they should be passing signal.

You can quick check with an AC voltmeter.  Tie one lead to the OT pri ground side.  Inject sig of some type.  Use other lead and follow from input xfor pri > sec > V1 grid, V1 plate etc.  This will show whether the audio amp is amplifying and where it's losing signal.

 

[Biasrocks]

Build one of these, along with a small amp really helps to trace signal through a circuit.

http://www.diyfactory.com/data/mbsignaltracing.htm

Mark

 

[college101]

So I start at the input tranny and touch points to see if it passes audio back threw tester, and out the amp?  I can build that...

 

[Biasrocks]

So I start at the input tranny and touch points to see if it passes audio back threw tester, and out the amp?  I can build that...

You've got it. That way you can figure out where the audio is disappearing.

I omitted the volume pot and just used my amps volume control, works a treat.

Mark

 

[lassoharp]

So I start at the input tranny and touch points to see if it passes audio back threw tester, and out the amp?  I can build that...

. . if you have a DMM and a CD player/keyboard tone you don't have to go to that much trouble of building Mark's tester.  Just inject and read the signal voltages.  Very quick and effective.

Looking again at your V1 voltages, the pin 8 (cathode) reading looks more off than the elevated supply voltage would seem to cause.  So, maybe half of that tube is sick.  I'm rough guessing with 2X plate voltage  you would be seeing maybe 2.2-2.6V instead of 5+V.  Only other thing I can think of there is cathode resistor value(R14). 

 

[Biasrocks]

. . if you have a DMM and a CD player/keyboard tone you don't have to go to that much trouble of building Mark's tester.  Just inject and read the signal voltages.  Very quick and effective.

True enough, but a signal tracer has an additional benefit of actually hearing the audio which makes it easy to detect distortions, etc. Something a volt meter will never tell you. In the absence of a scope, it comes in handy.

Hopefully, this will get him on the trail to a working piece of kit.

Mark