All things LA2a related

[dripelectronics]

ive never tried it , but started to notice some people
use non polarized poly pro film caps for C5 (10UF).

is it ok to use a non polarized in place of this cap wich is a polarized
electrolytic?

g.


.

 

[Svart]

as long as your V rating is sufficient you are golden.

 

[AnalogPackrat]

[quote author="mcfarlane_audio"]ok so after paroozing all 110 pages of this thread, i have a question, my la2a is working great, save for one problem, its quiet, that is to say, im loosing alot of signal somewhere, at max it pushes about -7 db output (with a 0db input), with no reduction, any bright ideas seeing as ive checked all my voltages to be correct..?? thanks eveyone[/quote]

Something is definitely wrong--LA-2a is a gain monster. Does it pass the full spectrum (20-20kHz)? Have you popped the cover off of the T4b, turned off your lights and watched the EL panel brightnes modulate with signal?

Can you tell us some more about your build? PTP or Drip's board? What input/output iron? Pictures? If you have a signal generator and a scope (or AC voltmeter) you can try trace the signal to see where it gets lost. Somewhere in this thread CJ posted a nice markup of the schematic with AC signal voltages (for a given input level with PR at minimum and nominal gain setting).

A P

 

[John Peacock]

It was about this time last year that I assembled my BLOO kit. Trouble shot, and with the help of you fine folks, got it working. So I installed the meter bulb, racked it up, and finally got a chance to use it a month or two later.

So I patch it in to the signal chain and, "That's funny... It's passing audio but no compression." Not wanting to go back down this frustrating road, I let it sit for about 9 months. Now I'm back. Did some tests:

1. Powers up
2. Voltages are where they were before when it was working
3. Passes audio
4. neon lights up
5. No smoke/blown fuses
6. When I take the cap off the TB4 and shine a light on it, I get gain reduction.
7. TB4 does NOT emitt light in a dark room when I apply gain reduction.

So there's my problem. The light emmitters in the TB4 are not getting the signal. Now how to fix it? I of course will go back through this thread (which is hours and hours of reading) starting tomorrow, but in the mean time, if anyone has any thoughts on where I should begin to look for a problem, it would be greatly appreciated.

Thanks.

-Jp

 

[CJ]

you dont have to fix the whole circuit, just part of it.
the part with the parallel plated 12ax7 and the part with the 6aq5.
put some signal into the unit, turn up the GR full boogie, get out any ac voltmeter set to ac volts, get on the plate of the parallel plated 12ax7a, which would be pins 1 and 6 tied together, see if the signal is getting there, if it isnt, back track, if it is, keep moving forward til you catch it, look at the plate of the 6aq5, if nothing, maybe the pot wiring for gr, if you are good on the plate, make sure the volts are getting to the T4 socket, and you are done.


Then let it sit for another nine months, then plug it in and tell me whats next.
But I'll probably be dead by then, so farm it out to another wannabe technical hero. Like that freak that lives up in Santa Cruz with busted rickies.

:thumb:

 

[John Peacock]

Thanks for the fast response, CJ. I liked every part of it. You are spelling things out at just the right speed for me.

Ran some audio with gain reduction at 90% and measured some AC:
1. Plate of 12AX7A was at around 1.2 V (good , yes?)
2. Pin six of 6AQ5 was around 1.35 V (again, seems good - am I following it right?)
3. Pin five (plate?) of 6AQ5 continuously cycled through a bunch of numbers and overload on my multi-meter. (bad?)
4. Pin two of 6AQ5 (I'm not sure what this one is, but measured it anyway) essentially showed 0V.
5. From the plate of 6AQ5 we follow a wire to C11. On the wire side of C11 we are of course are still cycling on the multi-meter.
6. On the other side of C11 (also known as pin 3 of the TB4), I'm essesntially metering nothing, which is I think what we expected based off of my previous post.

So here are some of my guesses, purely for entertainment purposes:
1. If the multimeter cycling isn't a bad thing (and maybe just more is coming off the plate of 6AQ5 than my meter can handle), then my C11 is bad. Swap it out; problem solved. This initially was my strongest guess, because C11 did not ship with my kit and was actually purchased from Radio Shack. But it does measure right at .01. If I had an extra, I'd already have swapped just to test this theory.
2. If the multimeter cycling is bad, then clearly we've got a problem over at 6AQ5. Bad tube? Perhaps something is wrong over at pin 2 (or whatever it is connected to)?

And the answer is? Will wait anxiously for your response.

[quote author="CJ"]
Then let it sit for another nine months, then plug it in and tell me whats next.
[/quote]

I teach high school nine months out of the year. I trouble shoot my LA-2A during my summers off. :wink:

-Jp

 

[CJ]

Good work!

Sounds like something on the otherside of C11 is shorted to ground.
Measure ohms, with pwr off and caps bled, on the down stream side of that cap.

the wise cracks are just boredom, sorry.
cj

 

[John Peacock]

[quote author="CJ"]
Sounds like something on the otherside of C11 is shorted to ground.
Measure ohms, with pwr off and caps bled, on the down stream side of that cap.[/quote]

Hmmm... I may have gotten lost just there. Isn't pin 3 of the TB4 immediately down stream of TB4? When in doubt, follow the salmon, right? But in all seriousness, you may need to actually tell me where to put the probes of the multi-meter on this one.

And all of the measurements around 6AQ5 are okay?

[quote author="CJ"]
the wise cracks are just boredom, sorry.
[/quote]

Please, keep 'em coming. When they stop, I'll know that I'm in serious trouble.

 

[John Peacock]

A bit of a bump to this thread.

Nothing on my end has changed. Any help is greatly appreciated.

In the meantime, I would like to change out my .01 C11 capacitor. Even if it is not bad (which it might be), I'd like to swap it out for something of a little better quality. I've got a lot of options on Mouser. Any suggestions for a capacitor type, make, model, url, etc. would be great.

-Jp

 

[AnalogPackrat]

C11 can be pretty much any film cap with a rating of, say, 300V+. It's effectively in the sidechain (feeding the EL panel) and won't affect the sound of anything in any noticeable way. Just get a reasonable polyester job and be done with it. Give me a minute to look back over what you've been working on--maybe I'll come up with something to try.

<addenda>
Well, when say you're meter is "cycling" it sounds to me like you might have a low freq oscillation thing happening in your sidechain amp. Do the CJ chopstick dance and poke around the wiring going to the 6AQ5. Look for loose wiring/bad solder joints on 6 or 8 components and wires connected to that tube. If you have a spare, swap it out. Did you adjust R3 and R37 per the LA2a manual (available as a pdf somewhere on the web--google for it).

BTW, is this a Bloo kit? Photos around the 6AQ5 area might help us evaluate the situation!

A P

 

[CJ]

electronics is one big sewer line.
someone with a procrastionary attitude sits down to a 3 lb London Broil.
A week later, that 3 lb brown steak wants out!
But it gets stuck somewhere around turn four, and the professor (professors are famous for procrastinating) starts feeling like he's been on the Atkins for too long.

Well, its the exact same thing with troubleshooting.
Only that 3 lb Brown Trout is a short or open circuit. You just go down the sewer pipe until you find the clog.
With Rooter Rooter Electronics, you'll be on your way in no time.

get it?
logical troubleshooting is all it is.
or logical roto rooter. take your pick.
and if your good, you can cross over into plumbing for a few extra bones on the weekends for DIY transformers, because transformers are all that really matter, now that Bill Walsh is gone.

 

[Purusha]

Maybe someone here can give the proper explanation...

When talking about the 110V/220V and the null pole, why does it matter which wire goes where from the IEC to the LA-2A board?
If this is really that important than we have to also measure which one is the positive and which is the null every time we plug the
unit into the wall as this is not the same in every wall socket.

In my country we don't have GND sticking out of the wall sockets... so you can turn the plug either way you want.

 

[Greg]

[quote author="Purusha"]In my country we don't have GND sticking out of the wall sockets... so you can turn the plug either way you want.[/quote]
Dangerous. I just bought a house in New Orleans that was built in the 1930s. The majority of the receptacles are not grounded. I'm slowly running new Romex and changing the receptacles to fix this potential hazard. I have a modern circuit breaker panel, so I just have to run new electrical.

Purusha, if s power transformer on any of your pieces of gear shorts out, then your chassis will probably become live. Ouch! Do you have a circuit breaker panel or a fuse panel on your house?

 

[Purusha]

I mean the house wiring has GND and also wall sockets they do have GND but on the
side of the plug, not in the middle where some countries have a metal sticking out which
goes into the plug for GND. So basically I can turn the plug 0° or 180° to get the +/- either
way into the unit.

I do have a circuit breaker panel in the house. This saved my gear many times from the lightning
strikes near by. :thumb:

 

[burdij]

The IEC connector has three poles labelled H, N, and G (H- hot, N- neutral, G- safety ground). The power system has a hot line, a neutral line and an earth ground (called Protection Earth or PE in England, in US called Earth or Safety Ground). Because of sloppy wiring which does occur occasionally, the hot and neutral in an outlet may be reversed. The appliance will still operate. Therefore, the hot or neutral should never be referenced to the frame or ground of the appliance.

The reason why the third wire is provided, as someone mentioned earlier, is to protect against internal component failure in the unit by bypassing line current to the safety ground and popping the fuse or circuit breaker. The only place that the neutral pole of the mains circuit should be referenced to the Safety Ground (and to an external ground rod) is at the mains fuse panel in the building. This is a requirement of most electrical codes.

In electrostatic terms, where it makes a difference to us in audio, the safety ground, since it is returned to an actual earth ground, can also be used to conduct the earth ground to your local Faraday shield (the metal box around your audio project). This type of shielding is used principally to conduct the local electrostatic field interference to earth and has no effect on local magnetic interference. It is also effective against local RF fields, collapsing the E vector (the electrostatic part) of the E-M wave. The difference between the two can be perceived in the audio output from the unit. Interference caused by electrostatic sources can usually be heard as a buzz at 60Hz. Interference from a local magnetic field source can be heard as a fairly pure hum at 60Hz.

The discussion of the "Pin 1" problem in Bill Whitlock's AES lecture is related to the safety ground/electrostatic field issue.

 

[Purusha]

Because of sloppy wiring which does occur occasionally, the hot and neutral in an outlet may be reversed. The appliance will still operate.

I am aware of this. So there is no bad effect on the sound if the hot and neutral are reversed
in tube audio units? If this is true than I wonder why Greg marked hot and neutral on the PCBs...

 

[John Peacock]

[quote author="AnalogPackrat"]
Well, when say you're meter is "cycling" it sounds to me like you might have a low freq oscillation thing happening in your sidechain amp. Do the CJ chopstick dance and poke around the wiring going to the 6AQ5. Look for loose wiring/bad solder joints on 6 or 8 components and wires connected to that tube. If you have a spare, swap it out. [/quote]

Unfortunately no spares. Do you think it would be worth tracking one down to give it a try?

[quote author="AnalogPackrat"]
Did you adjust R3 and R37 per the LA2a manual (available as a pdf somewhere on the web--google for it).[/quote]

I just turned them according to the advice earlier in this thread: fully counter clockwise. Do you think it would make any significant difference. As it stands, it's not working at all.

[quote author="AnalogPackrat"]
BTW, is this a Bloo kit? Photos around the 6AQ5 area might help us evaluate the situation![/quote]

Yes. Not sure if these will help:
http://www.flickr.com/photos/35001164@N00/969173804/
http://www.flickr.com/photos/35001164@N00/969172856/in/photostream/

 

[AnalogPackrat]

[quote author="John Peacock"]
Unfortunately no spares. Do you think it would be worth tracking one down to give it a try?[/quote]

Well, I'm a packrat and like to have spare lying around. Maybe you should be on the lookout while we examine other possibilities.


[quote author="AnalogPackrat"]
Did you adjust R3 and R37 per the LA2a manual (available as a pdf somewhere on the web--google for it).[/quote]

I just turned them according to the advice earlier in this thread: fully counter clockwise. Do you think it would make any significant difference. As it stands, it's not working at all.[/quote]

Have you changed them since it last worked? There's always the possibility that you wired one or both backwards (I do that fairly often). If you have the "stereo adjust" (R3) turned the wrong way (electrically), it grounds the grid of the 6aq5 and you won't get anything out. R37 adjusts the high frequency response of the sidechain and won't cause things to completely stop working. So check the wiring on R3 and turn it the other way and see what happens.

[quote author="AnalogPackrat"]
BTW, is this a Bloo kit? Photos around the 6AQ5 area might help us evaluate the situation!

Yes. Not sure if these will help:
http://www.flickr.com/photos/35001164@N00/969173804/
http://www.flickr.com/photos/35001164@N00/969172856/in/photostream/[/quote]

That helps. Is R38 soldered to the center lug of the 7 pin socket? Looks dry. I don't see anything else right off the bat.

A P

 

[objectx]

Hi guys, my name is Michael and I've been lurking here at these forums for a while and finding the info very helpful.... I've just finished putting together a diy la2a using various sources and layouts as a reference. I fired her up and took some voltages (heaters/b+) and it all looked good so I put in the tubes... nothing burned up and voltages looked ok from the figures I'd been seeing on this thread, so I chucked in the t4b (thanks greg!).

I'ts passing audio ok and seems to have reasonable gain. The gain knob seems to work as expected however I'm not getting any compression and the meter isn't working (I can get it to zero, but only with a 100k resistor as R25, its a cheap meter.

If i move the wire going to pin 3 of the t4b i can get a change in the behaviour of the gain knob; it seems to have a big jump in gain at about 1-2 o'clock and the peak reduction will then have some effect on the sound as well. I can't really tell if the el panel is lighting up. It has sowter trafos and I've installed the 1k grid stoppers on pin 2/7 of the 12bh7. That stopped some bad oscillation.

here are some voltages, with r3/r37 at full CW and all heaters active.


c7-A: 352V
c7-B: 266V
c7-C: 206 V
Neon: 57V (when light)

V1
p1: 112V
p2: 0.2mV
p3: 1V
p6: 103.5V
p7: 6mV
p8: 1.2V

V2
p1: 98V
p2: 0-20mV
p3: 3.7V
p6: 220V
p7: 66.5V
p8: 103.2V

V3
p1: 79V
p2: 0V
p3: 0.84V
p6: 79.2V
p7: 0.2mV
p8: 0.84V

V4
p1: 0-5mV
p2: 5.7V
p5: 151V
p6: 101V
p7: 0-5mV

I'm using 10k for r34. I've triple checked my wiring and grounding and now I'm at a loss... any help would be very much appreciated.
:grin:

 

[CJ]

all the voltages look ok, so it must be wiring.
try turning r3 backwards, if it s wired wrong, you will get no GR.

i wish they would take that silly thing out of there, all it does is waste time and money, neither of which i have a lot of.
.

then if no joy, chck the ac voltage on the downstream side of C11.
you need about 60-70 volts to start lighting up the el panel to visibility.
take the lid off the crooked T4 and see if the panel glows.
douse that light.