moybm

Problem DLa2a, excess gain without signal input.
« on: October 18, 2017, 06:03:28 PM »
Hello, Greetings from Spain.  :D
I write because I have been trying for a long time to figure out what the problem is with a DLa2a compressor that I have set up following the [silent: orts] D-LA2A-rev02 scheme.

When I raise the gain potentiometer to more than 70% in VU mode the meter jumps and reaches the end. All the valves and the TB4 are connected without an input signal. This happens to me equally in both channels.  :-\

If the meter does not pass from this point everything else works correctly, it compresses correctly and amplifies the signal.

I have done many searches and for now I do not find the problem, I have already changed the potentiometer by one of 25k, change channel valves, voltages ...

Components and voltages:
Transformer Input: WSM600 / 15K
Output transformer: WSM 15K / 600

Diode output: 351v
Output R34: 98v and 125v
Output R29: 273v and 277v

V1 (12AX7)
Pin 3: 1v
Pin 6: 121v
Pin 8: 1.16v
Pin 9: 3.3 (alternating current)

V2 (12BH7)
Pin 1: 104v
Pin 3: 3v
Pin 6: 235v
Pin 8: 109v

V3 (12AX7)
Pin 1: 86v
Pin 3: 0.8v
Pin 6: 86v
Pin 8: 0.8v

V4 (6AQ5A)
Pin 2: 4v
Pin 5: 119v
Pin 6: 82v

I do not know what to try anymore, I do not have many components to go testing. Has anyone had something like this? Do you know why it happens?

THANKS!


andyfromdenver

Re: Problem DLa2a, excess gain without signal input.
« Reply #1 on: October 21, 2017, 08:30:39 PM »
Hi! can you link the schematic?
I am wondering if you are experiencing an oscillation.

questions:
your tube pin measurements, are they with both knobs fully counterclockwise?

with no input signal, and your box plugged into a line input. do you get a change in your tube pin measurements, with the gain knob at minimum, then maximum?

disclaimer: I have not built one of these specific pcbs, but I wish I had, lol. Did you follow the build guide exactly and use shielded wire where applicable in the off board wiring?

I could be way off, but at least we bumped your thread.

Thanks!
Andy
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie

moybm

Re: Problem DLa2a, excess gain without signal input.
« Reply #2 on: October 24, 2017, 04:48:06 PM »
Hi! can you link the schematic?
I am wondering if you are experiencing an oscillation.

Hi Andyfromdenver!, thanks for your reply. I was make it from these link, it´s from these foro:
https://groupdiy.com/index.php?topic=32677.0
http://www.silentarts.de/DIY/D-LA2A/D-LA2A_rev02-Schematic.pdf (scheme)

questions:
your tube pin measurements, are they with both knobs fully counterclockwise?
Yes both knobs are counterclockwise.

with no input signal, and your box plugged into a line input. do you get a change in your tube pin measurements, with the gain knob at minimum, then maximum?
Yes,  I remember that I did that test and I noticed that I changed the voltage on pin 5 or 6 of the vacuum valve 12BH7A, which oscillated up voltage, but it gave a measure a little confused, I think error comes from another place, but I could do a video to show what results it gives me.

In fact, when I remove this vacuum valve (V2) the problem disappears but does not amplify the signal

disclaimer: I have not built one of these specific pcbs, but I wish I had, lol. Did you follow the build guide exactly and use shielded wire where applicable in the off board wiring?

I attached the photograph of how I have mounted the equipment, I have looked at many other photos that I look for here in the forum or by goolge and I have seen that the wiring is well mounted, although there may be some cable that is the problem, I have even tried that the entire chassis is properly grounded.

I could be way off, but at least we bumped your thread.
I am very grateful for your reply, I have been truthful for more than 2 months looking for the problem and I still can not find out why. In the end for now I decided to use it without reaching this point. Any help will be appreciated.

Thanks!
Andy

Thanks! :D

More information:

https://youtu.be/3f3ayebQuSs

Today I tried another option, I bought two vacuum tubes 12HB7A and I changed them but I still have the same problem ...

Does anyone have mounted the input and output WSM600 / 15K and WSM 15K / 600?

moybm

Re: Problem DLa2a, excess gain without signal input.
« Reply #3 on: October 24, 2017, 04:49:19 PM »
I try these two tubes but the problem persists.

[silent:arts]

Re: Problem DLa2a, excess gain without signal input.
« Reply #4 on: October 25, 2017, 04:48:01 AM »
Sounds like our standard oscillation "feature".

Quote
I have already changed the potentiometer by one of 25k
this is cool, but did you add the 75k resistor?

moybm

Re: Problem DLa2a, excess gain without signal input.
« Reply #5 on: October 25, 2017, 09:32:27 AM »
Sounds like our standard oscillation "feature".
this is cool, but did you add the 75k resistor?

Yes, I used a potentiometer of 25k and a resistance of 68k which was what I had on hand to see if there was any change but I got exactly the same, I don't notice any change.

I tried it on one of the channels, but what I am now questioning is in which part of the cable I connected the resistor ... the resistance on which cable of the potentiometer should be welded, where it goes to ground or to the other end ?

Thanks :)

Apache5

Re: Problem DLa2a, excess gain without signal input.
« Reply #6 on: October 25, 2017, 10:19:51 AM »
I think to the other end...  potentiometer acts as a divider of audio from ground to 25K, from 25K to 100K is where the 75K resistor goes... If I recall correctly. 

[silent:arts]

Re: Problem DLa2a, excess gain without signal input.
« Reply #7 on: October 25, 2017, 10:36:30 AM »
Not the side that goes to ground, it is the other side.
You will have lower gain, but the LA2A has too much anyway.

moybm

Re: Problem DLa2a, excess gain without signal input.
« Reply #8 on: October 26, 2017, 06:54:07 PM »
Well, tomorrow I will continue to do more tests with the engine, it might be all there is, as I can not look at this anymore, I can think of some component that the position is not yours or it is defective in both channels.

Searching the internet found another person who has the same problem, but nobody gives him solution either.

https://youtu.be/idRbu564NGs

moybm

Re: Problem DLa2a, excess gain without signal input.
« Reply #9 on: October 27, 2017, 07:16:07 PM »
Well, I'm still doing tests and this afternoon I discovered that removing the cable that goes to the limit / com switch has greatly reduced the problem, so I was prepared to keep removing more cables that I do not have screened (but if braided) reducing the problem ... so I deduce that I have a serious problem that the cables interfere ....

... so I will buy new cable I will screen all the cables that there are. The issue is that seeing photos of other assemblies that have made others see that there are cables that do not have shielded.

I will continue to report. THANK YOU


andyfromdenver

Re: Problem DLa2a, excess gain without signal input.
« Reply #10 on: October 28, 2017, 07:55:21 AM »
good plan moybm :-)

i don’t know if it will help in your situation to compare to the handwired original on the recpro site, but their layout will show which connections are shielded two-conductor.
I didn’t get a chance to closely inspect your work vs what the build guide for your project might say, but simply twisting the wires is not a substitution for shielded two conductor :-)
gotta make your box super quiet and super pro audio like it deserves to be! :-)
It will be worth the effort, I know.
es muy importante ;-)
take care,
andy
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie

moybm

Re: Problem DLa2a, excess gain without signal input.
« Reply #11 on: October 28, 2017, 06:12:05 PM »
Well, I can confirm for now that the problem of coupling with the meter has disappeared when I have changed the cables braided by shielded cables. In the stores that I have near where I live do not have parallel armored cable so I made them myself to be able to also give a better shield (braided and lined with layers of aluminum connecting to ground) ... I have to say that now it has been a success.

Tomorrow I will check it with the audio equipment since I am not at home and I will see the results and the dirt that it throws.

I would like to thank those who have written to me and especially to Curiousaudioparts who besides providing me the kit of assembly of the 2 T4B has helped me to calibrate it and to solve the problem that I have been looking for for so long.

andyfromdenver: I have been observing many photos of other projects of other people and I see that they use a lot of twisted cable and that's why I was confident that it would not be the problem ... but I have little experience with vacuum tube assemblies.

Attached photograph of which cables are those that need to sing yes or yes, theirs is that they were all, but specifically these are the fundamentals.


As soon as I have proofs I will upload them to continue polishing and improving it.

THANK YOU ALL!