All things LA2a related

[glazer56]

Hey all!
About a year ago I completed my LA2A, with a lot of help reading through this forum. Since then the compressor has worked beautifully, and sounded great... until about a week ago, my studio partner/engineer Don was powering on the unit, and, no love... The unit just failed to power up. I checked the fuse, and saw that it was blown, replaced it and... nothing. Another blown fuse. I decided to check to see if one of the tubes went bad, so i pulled them and powered up the unit. All good! lights came on, and it was happy. I started replacing tubes and checking power until I came to V3. Once V3 tube was in it was popping fuses again. I replaced it with a new 12AX7, and no luck. Now it appears to be blowing fuses regardless if V3 is in or not.

I haven't sat down to check the power supply yet, but I wanted to probe the forum for a little help in case anyone has experienced the same problem first. Any suggestions? Many thanks in advance!

 

[mnats]

My Bloo came with that Allied transformer everyone uses so I had to substitute a Hammond 369JX. I've had it running all night and the power transformer is still around room temperature. The measured line voltage is 238.5 Volts. Guess building it on a PCB is no guarantee of a trouble free LA2  :

Blowing fuses after working OK? I would look for an intermittent short somewhere - maybe around your V3... If you use the old series light bulb trick you can possibly keep it from blowing fuses long enough to locate your problem?

 

[Beauvais]

Hmm, no blowing fuses here. The thing works and sounds fine, the transformer just gets reaaally hot. I've not cut the unused wires on the secondary, they're tied together along with the two wires serialling the primary. Of course there's no contact there, but could that be a problem, the wires being very close to each other? It just seems strange, I'd expect the thing to not work properly if there was a short somewhere?

btw, the other box I've just finished is a dual 1176 (working great) with your rev j boards, thanks for that too!

 

[amishsixstringer]

Hey guys, I've read ALMOST this entire thread in the last day.  WEALTH of knowledge here.  Anyhow, I built my La2a with the 1968 schematic and pictures of a reissue la2a and ron's layout.  UTC a-10 and a-24.  I fired it up today for the first time with no tubes or T4b.  I have 6.9V on the heaters which is perfect and about 52V on the neon and it lights up.  I'm reading about 360V and 370V on R34 and R29.  Everything seems cool so far. 

My only concern is that I can't get the VU meter to "0" out.  Must the tubes be installed?  I'm concerned that I may not have the VU meter wired correctly, but I've checked it a few times.  The switch I have for SW2 is a 6 position rotary switch that has a 'shim' in it to limit it to 3 positions.  It has 2 poles, which I have wired to the negative side of the VU and the other pole is wired to R24.  I checked for continuity for the poles and found the corresponding pins for each position of the switch.  Could the VU meter be bad?  It is brand new, but the needle does not move at all with any sort of adjustments.

I have all the tubes coming this week (NOS RCA's), and I have the T4b, but not installed.  Are there any other values I should measure before putting the tubes and T4b in and putting audio through it? 

I haven't attempted to pass audio, but I'm not sure that's a good idea with no tubes or opto in place. 

For the record:

R29 = 4.7K

R34 = 10K

Both of the Pots on the back of the unit are fully clockwise right now. 

If I back off the meter adjustment screw on the front panel the neon goes out. 

Where do I go from here?

Thanks!

Neil

 

[amishsixstringer]

Also, is there a way that I can test that the VU is getting voltage by checking the voltage drop across the + - terminals of the meter?  I'm not at the workshop now or I would just try it, but I'm thinking it could be possible.  If so, about what reading should I get to put the meter at 0dB and keep it there?  Thanks again guys.

Neil

 

[amishsixstringer]

Ok, at the lab I gathered this:

I was pondering.  The Meter should be in a parallel circuit with the neon and R1 when in GR mode.  (There are some components in parallel as well, but it is the main idea.  Anyhow, I don't know how the meter works exactly, but I'm able to adjust R1 until there is almost a 0V drop across the terminals of the VU meter.  When this happens, there is 55Volts on the neon and it is lit up.  The neon isn't as bright as it is when R1 is pegged, but it is lit.  I'm reading about 13.5Volts from ground to each terminal of the VU meter when I have R1 where there is almost no drop over the meter.  i'm ASSUMING that this means that the meter should read 0 or close to it.  However, there is no reading on the meter.  This thing is junk huh?  Sucks. i paid 80 bucks for it brand new. 

This is my find.  Can anyone back this up?


Neil

 

[amishsixstringer]

Ok.  More info.

The meter should read 1.28VAC across it to read 0dB.  I was measuring DC, because I wasn't thinking and my meter defaults to DC when I turn it off then on.  I'm not getting that. Instead my meter just goes crazy.  I'm thinking I may have my switch wired wrong. I'd like to try getting the meter to work by taking the switch out of the circut and hardwiring it to GR.  The meter is most likely ok.  If I 'jiggle' it, I can get the needle to move around.

Neil

 

[amishsixstringer]

Ok.  Smoke.  I thought I had a bad chassis ground on R1, so to test it I soldered a wire from the front panel to the chassis (I don't have a hinge on my case, so there's nothing connecting the chassis together).  R34 starts to smoke.  I only had power to it for about 3 seconds like this.  I think all my voltages are still good.  What should the voltage value of pin 6 on the 6aq5 be?  I have 350V with no tube installed. 

Still no meter function.  I'm getting frustrated and running out of ideas.  Someone help?

Neil

 

[AnalogPackrat]

With no tubes or T4b installed you should have no current through R34 (or R35 for that matter).  No current means you should read the same voltage on pin 6 (and pin 5) of the 6aq5 socket as you do at the R29-C7B node.

R1 shouldn't have any effect on anything without tubes installed.  Something is wired incorrectly--perhaps around the T4b socket or the connection from V4 to the T4b.  Also check R1--the center lug (wiper) should only connect to pin 2 of V1.

You should have ground connection between the front panel and the rest of the chassis and this should not affect operation.  Somehow you seem to have provided a path for B+ to ground that shouldn't be there.  You need to fix this before you worry too much about the meter.  Put on your debugging and logical analysis hat and keep one hand in your back pocket (you do have an alli clip for your meter's ground lead, don't you?).

With things set to "not smoking" mode measure DC voltages at both ends of R29 and at all of the plate pins of the tube sockets.  Also measure the three lugs of R1--maybe that will provide a clue.

A P

 

[amishsixstringer]

I found a cut in a piece shrink tube on the power transformer that was shorting to ground causing some smoke to happen.  It seems happy now.  I ran a ground strap between the front panel and the main chassis.  I have 370V at R34 and R29 on the B+ side to ground.  Across R29 I have 3.5Volts. 

Please excuse my ignorance, but the plate is pin 6 on all 4 tubes right? Should I be looking for AC or DC voltage there?

I do have the same voltage on Pin 5 and 6 of the 6aq5 as the R29/C7 node. 

I should have tubes here this weekend.  I have the t4b already, but I'm assuming it might not be a good idea to put this in before the tubes.


Thanks guys.  I think I'm on the right track.

neil

 

[AnalogPackrat]

Measure DC volts.  On the 12AX7 and 12BH7 there are two triodes in each bottle.  Pins 1 and 6 are the plates (of course V3 has the triode strapped in parallel for more current drive, so they'd better read the same ALL the time on that tube).

I would not put tubes in until you've really checked this thing over.  If you move R1 (slowly) does the voltage at R29/C7 change at all?  We're looking for a change indicating something wired wrong at R1.  Don't crank it until something smokes.  If that voltage moves when R1 is adjusted without tubes in something is not right.

Keep at it, it is a finite problem space and you can figure it out.

A P

 

[amishsixstringer]

Measure DC volts.  On the 12AX7 and 12BH7 there are two triodes in each bottle.  Pins 1 and 6 are the plates (of course V3 has the triode strapped in parallel for more current drive, so they'd better read the same ALL the time on that tube).

I'm not at the lab yet, but I will make sure of this.  I'll post values later tonight for each tube.

I would not put tubes in until you've really checked this thing over.  If you move R1 (slowly) does the voltage at R29/C7 change at all?  We're looking for a change indicating something wired wrong at R1.  Don't crank it until something smokes.  If that voltage moves when R1 is adjusted without tubes in something is not right.

I do know that the voltage at R29/C7 does NOT change with R1 adjustment.  I have throughly checked this already.  I will check again tonight just to be sure.  I don't have the tubes until probably monday, so that's no problem at all.

Keep at it, it is a finite problem space and you can figure it out.

Thanks very much. I'm pretty sure I'm in the ballpark right now.  Everything I've checked up to this point has been good.  I'll get the plate voltages of all 4 valves tonight.  I have CJ's voltage chart to check it against, but this is with tubes installed I believe.  What should I be looking for with no tubes or opto?

Thanks so much guys,

Neil

 

[amishsixstringer]

I have verified that turning R1 does not effect Votage at R29. 

Here are some voltages from the tubes (with no tubes or t4b installed)

V1
Pin 1: 362
Pin 6: 362

V2
Pin 1: 0V (Not sure if this is good)
Pin 6: 370V

V3
Pin 1: 362V
Pin 6: 362V

V4

Pin 5 370V
Pin 6 370V
No other pin on V4 shows DC voltage  I have good AC to each of the heater pins on all tube sockets.

R29 on the B+ side is 372V and on the 'A' side it is 375V.  Measuring voltage ACROSS R29 gives me 3 volts.

Does this all look ok?  All the plates seem to be getting equal voltage except for V2 pin 1, but this seems that it may be right by looking at the layout. 

Neil

 

[AnalogPackrat]

V2 is fine--it is wired as a White cathode follower and the second plate has a DC path to ground. 

Maybe I'm dumb, but I can't see how it is possible to get a few volt drop between B+ and the various plates with no tubes (an no other DC path to ground).  For instance, you've effectively got a 10V drop across R33 which implies it has about 450uA flowing through it somehow.  Did you make these measurements over a short period of time without changing the position of your meter's ground attachment?

Other than that it seems OK.  How's your meter behaving--can you zero it now using the trim pot?

A P

 

[amishsixstringer]

Could it have anything to do with C9 or C12 storing energy?  Also, there is a path to ground through Pin 1 of V3--C9--C12--Jumper to R3--C6--R30--Ground.  I don't know if this somehow does not ground due to components, but there is a ground there eventually.  Also, if I measure ACROSS R33, there is no measurable value.  Yes, I left the ground on the same ground screw with an alligator clip and tested pretty quickly.

I can't zero the meter yet, but from what I've gathered the tubes and T4b must be installed for the meter to zero.  I haven't actually gotten anyone to confirm this though. 

Thanks so much.  My box has come a long way this week!

Neil

 

[AnalogPackrat]

If you've had the power on for more than a few milliseconds C9 and C12 should be charged to a stable value.  Capacitors are open circuits with respect to DC so there is no DC path to ground that includes a (good) capacitor.

The meter "zeroing" is just a bias applied when the meter is switched to read gain reduction--the needle swings negative so you are just bumping it up to read 0dB with no gain reduction.  You don't need tubes or T4b to do this.  Just switch the thing read GR and adjust.  If it doesn't move, you may not have a real dB meter (should have diodes--do a search on this site--many people have run into similar problems).

I may well have no network connectivity the next couple of days.  Maybe someone else can pick up from here...

A P

 

[amishsixstringer]

Well, The meter is a sifam vu meter (AL39).  It is the same one that is used in the recproaudio build.  It says VU on the face.  I can't get the meter to move in GR mode.  I have checked the wiring on the rotary switch as well as bypassed the switch attempting to get it to zero with no success. 

I have the switch wired so that with GR mode it makes contact with a pole and pin 2 as well as the other pole and the other pin 2.  I have checked the switch with a continuity meter prior to wiring.  When the switch is GR I should be effectively connecting the + terminal of the VU to R25, as well as connecting R4 with R24 right?  My R25 value is 33K right now.  I know this resistor is often changed for calibration, but I don't think it should effect me in this regard. 

Neil

 

[AnalogPackrat]

Net connection still up...

OK, Sifam = no problem with the meter.  Did you measure your resistors before soldering them in place?  If not, it's a good habit.  For instance, just this week I was breadboarding a little FET based treble booster circuit for a stompbox build.  I could not get the output level beyond a couple of mV.  I disconnected part of the output circuit and, hey presto, everything's dandy!  After futzing around for a little longer I found that I had used a 50R instead of a 1M resistor for the switch pop-stopper.  :-[  Someone (OK, it had to be me) had put a couple of green-black-brown marked resistors in the 1M drawer.

So my advice would be to re-check the resistor values that could affect the meter.  R24, R25, R26, and the 1M adjustment pot.  If you turn off the power and drain the C7 caps, you can measure R24 in-circuit by switching off of GR metering (assuming nothing is connected to the output of the unit).  You can also measure R26 to "close enough" accuracy in-circuit (keeping the meter switch off of GR) since the parallel path is much higher impedance (~1.3M).  R25 and the 1M pot are parallel to each other so you can't really measure them independently, but if you don't get roughly 330k || (1M + 33k) for the measurement then something is off.  Of course you can also lift one end of each resistor and then measure without any potential in-circuit pollution of your measurement.

A P

 

[amishsixstringer]

Thanks a lot for taking your time with me on this, as I'm obviously a novice.

I removed one leg of each component for testing, instead of getting a 'close enough' measurement.

R1:  1Mohm

R24:  3.88Kohm

R25:  33Kohm

R26:  330.5Kohm

This all seems to check out fine.  I'm getting really confused.  Especially since I have neon light.  Thanks again.


Neil

 

[stitch-o]

So after readding all 96 pages of this thread, these are my questions:
-These are all (Bloo, Drip, [Silent:Arts], etc) for the most part, part for part remakes of the original La2a. The original La2a's used HA100x and A10 UTC input transformers.
I have read many posts of people complaining of too much gain by using these originals (600:50k 1:10 step up) and wind up swapping for a lower value input transformer (1:4 600:10k-15k step up)
The original La2a's don't have this gain issue at all and are fairly moderate with the amount of gain.

Why, if all these repros are exactly the same, would they have gain issues from using the original input transformer?

and, if you wanted to downsize the impedance of the A-10 to something more manageable (like 10k), what values would have to be used at R5 and R6 (I think these would be the spots to change R value)?
R5 and R6 are 68k in all the schematics I've looked at.

Cheers!