LA2a Help-No gain redux showing on meter

[Freddy G]

Hi Folks,
I just finished building my second LA-2A clone and everything seems to work and it sounds good. The problem is that when I switch the meter to gain reduction mode it just sits at zero where I adjusted it with no deflection even though I can hear compression going on.
When I open the box I noticed that the Ne-2 was not glowing.
When I initially power up then the Ne-2 glows but only for about 10 seconds then it goes out.
Can anyone point me in the right direction? What should I look for?
Thanks in advance,
Freddy

:sam:

 

[cayocosta]

What's your ne-2 voltage Freddy?

 

[AnalogPackrat]

Is your R28 the correct value (470K)? What happens at C7B during the first 10 seconds from power up?

 

[SSLtech]

When you say it sits at zero you mean 0VU? -If so, you have voltage.

If the needle sits off to the left at the rest position, you might have no GR. Check the setting of the two pots on the back panel.

Keith

 

[Freddy G]

OK......I just measured R28 and it is 475k.
The voltage across the Ne-2 is 62V.
C7b reads about 200V when first fired up then it drops to 170V in about ten seconds and stays there....this coincides with when the Ne-2 stops glowing.
Interesting observation.....If I pull out the 12BH7 then the Ne-2 remains glowing. Sounds like a big clue, yes?
Thanks dudes,
Freddy

 

[Freddy G]

Oh yes....Both pots on the back are set fully clockwise :sam:
and yes I meant Ovu

 

[cayocosta]

As the tubes heat up and draw current, the B+ will come down.

But, C7b should be around 275V.

Check your voltages against a schematic and try to find where something is amiss.

 

[Freddy G]

Righto,
Checking voltages......
Be back with my findings tomorrow.
Cheers,
Freddy :sam:

 

[Freddy G]

Hi All,
Here are the measurements with tubes installed:

12AX7a #1:
pin1- 63V
p3- 0.73
p6- 74.6
p8- 0.7

12BH7a:
p1- 61.7
p3- 2.88
p6- 135.1
p7- 48.7
p8- 65.3

12AX7a #2:
p1- 44
p3- 1.98
p6- 44
p8- 0.31

6AQ5a:
p2- 2.96
p5- 112
p6- 56

C7a+ 403V
C7b+ 170
C7c+ 132
C7d+ 56

It looks like everything is too low, can anyone make a guess or recommendation?

Thanks,
Freddy

 

[AnalogPackrat]

Freddy,

C7a is 50V too high and C7B is 100V too low. Is R29 4.7k? If it is, then you are dropping 233V across it which is 49.5mA of current draw going somewhere. You should be seeing about 15-16mA of current through R29. I'm guessing that R29 may be too low or maybe C7b is bad (leakage).

Can you pull the 12BH7 and measure at C7a, C7b, and the plates of 12AX7 #1 and the plates of the 12BH7 socket? That way we can try to figure out whether it's R29/C7b or a wiring problem somewhere else (12BH7?).

AnalogPackrat

 

[AnalogPackrat]

Freddy,
Did you get your meter problem fixed? BTW, I checked out your web page. I think I'm dehydrated from all of the drooling...better have a :guinness:

Analog Packrat

 

[Freddy G]

AnalogPackrat,
Thanks for the help. Right now is the first time in a couple of days that I can look this compressor over. I was hoping to inaugurate this comp as well as a pair of G7 mics (that I have just completed this week) on a recording of cello, violin and piano that I'm doing on Sunday (tomorrow).
Just checked...R29 is indeed 4.7K

thanks for the....drool :green:
Freddy

 

[Freddy G]

AnalogPackrat,
Here are the measurements with 12ax7 #1 and 12BH7 pulled:
12ax7:
pin 6 - 198 v

12BH7:
pin 1 - 201 v

C7a- 407v
C7b- 205v
C7c- 204v
C7d- 68v

cheers,
Freddy

 

[cayocosta]

Freddy,

With those tubes removed C7b is still very low. Try pulling the 6AQ5 also.

 

[Freddy G]

just swapped out C7b for a known good cap. Measured the same voltage..... 205 v
:sad:

 

[soundguy]

[quote author="AnalogPackrat"]I checked out your web page. [/quote]



:shock: :shock: :shock: :shock: :shock: :shock:


check the "building les pauls" link.

damn!

dave

 

[cayocosta]

Freddy,

You're still drawing around 43ma with the 2 tubes pulled.

Put them back in but remove the 6AQ5, then check the voltage at C7a and C7b.

 

[AnalogPackrat]

Freddy,

Sorry you missed your first gig with your new LA2A. Hopefully we can get you up and running for the next one.

OK, so it isn't C7b or R29. Note that at this stage you shouldn't leave it powered on for very long as R29 is dissipating over 8 watts! I'm going to ask a potentially (no pun intended) stupid question, but are your grounds all happy? Next time you've got it powered down and the caps all drained, measure R between your ground points and the ground at the power connection. I did this as I built mine up and had a couple of chassis ground points that were iffy--scrape paint, tighten nuts to make star washers dig in, etc.

So we can infer from the new measurements that the 12BH7 is drawing 5.5-6mA of plate current which is about right. Cayocosta has a good plan. Try either pulling one tube at a time until the voltage at C7b looks more like 275-300V.

If that doesn't lead to a problem area, then you've got something else wired up wrong that is draining current from your B+ supply (R28, R33, R34, R17, R16, R13, R9). Start at C7b and trace each branch of the B+ tree and calculate the current in each resistor. Compare this to the current calculated with the correct voltages. This should lead you to the problem. Do you have a good legible schematic with the voltages labeled? If not, I can email you one that I have.

Hope this helps!

Analog Packrat

 

[AnalogPackrat]

:shock: :shock: :shock: :shock: :shock: :shock:


check the "building les pauls" link.

damn!

dave[/quote]

Yeah, and I'm not a LP kinda guy, but it took me a couple of :sam: to rehydrate after looking at that stuff. Top shelf!

 

[AnalogPackrat]

Freddy,

I just did some quick and dirty checks using your original numbers with all the tubes in and assuming correct plate resistor values. None of the plate resistor currents look out of whack, so the only things left are:

R28 (too low)
R35 (too low)
leaky C7c
Something inadvertently wired to the B+ network that shouldn't be there.

Have you tried pulling all of your tubes and the T4B then measuring the B+ and plate voltages? This might help find the problem without you having to desolder anything (e.g. leaky C7c scenario).

Analog Packrat