DIY LA2A Help.

[BigMuff]

Hello, I had posted this on the gearslutz forum (http://www.gearslutz.com/board/geekslutz-forum/421548-diy-la-2a-setup.html).

I want to make sure everything is operating correctly before adding the T4 module.

I took some measurements via mexicola's recommendation on the gearslutz forum.  My measurements are in bold.

directly after the 1n4007 diodes: 370VDC (379)
directly after the 4.7K/2W: 265VDC (this is your B+ voltage) (278)
6AQ5 (V4)
    pin2: 7.5V (7.25)
    pin5: 115V (137)
    pin6: 137V (120)
12AX7 (V3)
    pin1: 107V (80)
    pin3: .75V (0.87)
    pin6: 107V (80)
    pin7: 0V (0)
    pin8: .75V (0.87)
12BH7 (V2)
    pin1: 93V (134)
    pin2: 0V (0)
    pin6: 216V (227)
    pin8: 98V (138)
12AX7 (V1)
    pin1: 90V (99)
    pin3: 1.53V (1.16)
    pin6: 100V (102)

-------------------

It passed audio through but has a buzzing noise to it. I used a Hammond transformer for the Power Supply. It does not have a center tap on the filaments so I put 1/2W 100 ohm resistors to ground on each leg. Some sites said 2W resistors but my past tube amp projects have only used 1/2 W. Could that be a problem?

Also, the VU meter doesn't seem to be responding. The zero adjust knob works, but audio does not trigger the VU at all.

-------------------

My grounds are good with every ground point having continuity (Including the front panel). I've attached a photo.  If anyone would like higher quality ones to review, please email me and I can provide them.

Thank you all for your help!
-Noah

 

[kazper]

There is a great big LA2A thread... this should be in it already 20-30 times..

But there is a great guy here, known around these parts as CJ and in his signiture is a LA2A voltage chart.

Just in case you have a hard time finding one of his more recent transformer disections...
ouch... it's down.. lets see if there is a way to find it...

Kaz

 

[CJ]

dang Ipower web host, started out great, then they changed the ftp interface to some horendous bells and whistles thing, the old system was so much beter, i could upload ten pics instantly, but with the new system, ytou have to go in and out of directoris each time, took forever,

then they said, "recession>pay us 19 a year,

then i go to pay and thery say economy is back,

pay up 161.95 now

so they have my stuff hoastage, byt no biggy, eb=veryone has seen it 1000 times, hasnt changed much, ami rambling?

man, this brazilian coffee at 16 bucks a pound is worth every penny, so sweet if you add one grain of sugar, you cn't drink it,

Red Rock coffee folks, they blow away  petes and barfucks. ;D

oh, the voltages look great, check the VU switch, that is where everybody seems to getshung up,

 

[kazper]

man, this brazilian coffee at 16 bucks a pound is worth every penny, so sweet if you add one grain of sugar, you cn't drink it,

Try Kopi Luwak coffee

 

[BigMuff]

VU switch is wired as it should be. Still no response. If the zero adjust pot works then that means the VU isnt fried right?  It's just not getting the right signal there.

While I was poking around with a chopstick, I noticed the wire from the arm of the gain pot to Pin 2 of V1 made the buzzing louder when i touched it.  I used some Mic cable for this since it had a shield.  Could this be what is causing my buzz problem?

Thanks again...

 

[CJ]

humm is probably the pwr trans being close,

pin 2 is the first grid, the most sensitive pin of all, so the hum when messing anywhere near that pin is normal,

get a hi-liter pen and trace over the vu section, if your meter is good, this will find your problem.

 

[BigMuff]

If I touch the transformer when powered up I feel slight vibration and can hear a little bit of buzz.  Does this mean I've got a noisy transformer?  I'm using a Hammond 269JX.  I've seen others use these for their LA2A's and didn't see a complaint.

I also read someone replaced filter caps and that cured the buzz.  My filter caps are some big Sprague Atom caps. Any help there?

Haven't traced the meter 100% yet. So far I haven't found an issue.  The T4 being out wouldn't prevent the VU would it?  Would the rating of R25 have anything to do with this?

Also, some say their neon lamp lights up.  Mine doesn't, ever. The voltage there is correct but it doesn't illuminate at all.

One last thing.  The limit compress switch would buzz loudly when I touched it. Should I scrape off some paint to make sure this is grounded to the front panel?

Thanks again to everyone.

 

[raysolinski]

I would replace the neon as it has lit on all of the LA2's I have built....is your unit passing audio as a line amp with no t4 in it? It should and be fairly quiet....

Ray

 

[BigMuff]

Ray, I got the NE-2 from Mouser.com (PN 36NE002).  Does the NE-2 have to be rated at a certain value?  Even though it doesn't light the voltage measured across it is within range (see my initial post).  It passes audio with a loud buzz.

In my previous post I mentioned the transformer itself buzzes when power is applied and small vibration is felt, should I try a different transformer?  One online build (http://www.conditionedresponse.com/DIY/LA2/index.html) used a transformer from Allied Electronics (http://www.alliedelec.com/search/productdetail.aspx?SKU=2270113#tab=Overview) and had good luck.  It seems to be shielded.

Thanks.

 

[BigMuff]

My NE2 was rated at 90V and everyone elsewhere said 65V.  Got that ordered. This is most likely why the NE-2 wasn't lighting up.  This may also be why my meter wasn't working...but it didnt work on the either two meter modes either.

Other than that, the meter section is correct. No other errors. Now, I tested the meter out of circuit with DC using a variable power supply.  Needle moved. So I know the meter is operational. could it be the meter wont operate on AC?  Could I assemble some sort of rectifier for it?  Here's the link to the meter: http://www.mouser.com/ProductDetail/Jewell-Modutec/MSQ-AVU-000/?qs=dGTOHwujGHCbrl2lDWHBPw%3d%3d

 

[BigMuff]

My 12BH7 broke (literally). The above voltages being slightly off wouldn't have overloaded it would they?  I'm hoping this was due to my own clumsiness thats I cannot recall.

Swapped NE-2 with one rated at 65VAC 90VDC.  The previous one was 90VAC.  Any ideas?

 

[raysolinski]

Well...your power transformer should not buzz or mechanically vibrate....either it is bad or you have something down the line that is shorted..did you miss a ground?...does the transformer get hot as well?  Make sure your output cap (C5?  not sure if that is the number) is oriented correctly with the positive side facing the circuit..if it is backwards you get a bunch of dc in the output and it buzzes like crazy... Not sure about your meter...I just grab any old vu meter that looks cool for a project since I judge compression by the sound and not the meter  ;D....I hate to say it but you have to trace everything from the begining...I know personally I have done some wrong things on the pcb that seemed right....wrong value resistor...missed a jumper, missed 1k's around the tubes...I know it is frustrating but keep plugging away...it sounds beautiful when it is done!

 

[BigMuff]

I have triple checked connections and all grounds are good, connections are made, solder joints are good as well.  I have a different transformer, this one with a center tap on the filaments and shielding.  I was getting ready to swap this one when I noticed my 12BH7. Is it possible for something in the circuit to do this or did I more than likely break it by accident?  It shattered a little bit at the base, but enough to have one or two pieces of glass be separated.

Any certain meter you recommend? I'll get on and order an old one to put in.  I've read elsewhere that LA2A's are "finiky" with meters.

Yes C5 is in the right place, positive facing circuit, negative facing the output transformer.

I'll trace through again tonight.  Thanks.

 

[BigMuff]

I replaced the 12BH7, I'm thinking that was my own clumsiness.

I replaced the power transformer with one from Allied Electronics, seems quieter now, a LOT quieter. The voltages are also closer to the specs I was given.

I replaced the NE2 with one rated at 65VAC/90VDC versus the one I had originally, 90VAC. However, it still does not light up.

I purchased a Sifam AL29 (the drip electronics people recommended it). Still not response from the meter. Does this require a rectifier for the meter? Could certain resistances need to be adjusted for proper function.

The unit passes audio with little noise.

Any other help?