new la-2 pcb

[guavatone]

[quote author="Svart"]anybody use trafos that weren't the originals? something like edcors?[/quote]

There were people who used cinemag with good results. My next build will have lower ratio if I can find them.

 

[kazper]

I'm almost done with one using edcor transformers. There is someone on here who has used them with there build from a few years back.

I asked him and got the same transformers as he did for this one.

600/10K 10K/600

 

[Svart]

OK so i'm all populated and have the edcor trannies in.

now..

there is no audio.

there is output, I can thump the tubes and get sound, but there is no input whatsoever even with the T4 out of the board. I traced out the input with a meter and find that the gain pot shows .1ohm between all three terminals.. but yet I can turn the gain knob and one side gets hiss the other a form of feedback..

I see that the wiper should go to a grid.. let me check there.

 

[Svart]

ok got the audio to work, my wiring mistake.

but it doesn't seem to be compressing and there is distortion on the signal at all time, sometimes it goes into what sounds like oscillations..

still working on this..

any tips?

 

[Svart]

the unit also has considerable hum too..

 

[Svart]

found part of the problem.. my t4 had a spot on the EL panel that was staying lit. I touched it and it would go out and work normally. as soon as I stopped touching it it would light back up....


I changed the 68k voltage divider network to get more signal from the transformer and the signal/noise ratio improved considerably but it still hums pretty badly.

 

[Svart]

hum is from the pot bodies not being case grounded. Still a little bit of 60hz there but well below the signal if the input level is hot. I think I'll need to get the 600:60k input..

 

[guavatone]

[quote author="Svart"]ok got the audio to work, my wiring mistake.

but it doesn't seem to be compressing and there is distortion on the signal at all time, sometimes it goes into what sounds like oscillations..

any tips?[/quote]

I misswired the freq adj. pot in the back and i was getting HF distortion. did u try turning the pot the opposite way?

 

[Svart]

I was working very late and was tired, I didn't notice that the markings on the PCB are not 123, they are something like 312 or something so I had wires swapped around. I later found that the grounds are VERY important and need to be wired very carefully. The HAX input ground is not connected to the other grounds on the pcb! I needed to ground it to the case to get the distortion to go away. I rewired all the other grounds together with heavy wire and much of the sizzle and hum went away.

While using a 600:10k on the input from true line level I find that the unit's input level is much too low. from a hot preamp this would be just fine but from a line source, it's very low and barely compresses at all. This is why I thought it wasn't compressing. once I opened the t4 I could see the panel lighting faintly. I could turn up the source to just about clipping and the panel only gets slightly brighter. I think the input really needs the 600:60k for line levels

 

[Svart]

any thoughts?

 

[guavatone]

Look for the CJ MOD thread I compiled. I think you need to have the comp kick in sooner right? then make up the gain. My output gain was out of control and I did the 12AY7 mod and changed the 200K B+ resistors to 100K.

I think R11 is for threshold and IIRC I used a 47K or 51K rather than the stock 68K.

 

[CJ]

make sure the T4 socket is wired correctly.
Pull the module and see if thelevel goes up.

 

[Svart]

yessir the socket it wired correctly... at least the others who used the PCB haven't complained about it.. :green:

it's working, it's just that the amount of compression seems to be very low compared to the signal. I can watch the EL panel light a little bit with the peaks when I turn the compression amount to max. I can only attribute this to the fact that I used a 600:10k tranny instead of a 600:60k..

Where is this mod thread?

EDIT: ok I found it. let me read over it..

 

[Svart]

So changing R11 will increase the sensitivity(lower threshold) of the sidechain?

 

[guavatone]

Yup. It did wonders for me. I use v72's whenever possible and they have very little gain comparitively. Try 43-47K and see if u like it.

http://www.groupdiy.com/index.php?t...asc&highlight=threshhold+threshold&start=1440

 

[CJ]

you should be able to shut down any line signal almost completely.
something weird is going on.
what kind of 100 K panel controls?
Audio Taper I hope?

there is a ton of gain in the side chain. follow the ac signal thru with a scope, record p-p values.

EL panel should glow brite at Max Comp.
They are meant for illumination, so...
I hope you have been enlightened.

 

[Svart]

it glows just a little bit, definately with the program material but not what I would consider full brightness.

at minimum peak reduction I get no EL glow at all, at full( all pots wide open) I get a little. I can hear it compressing but it's barely compressing.

So you guys are saying that even using 600:10k transformer on the input I should be able to slam the signal? When I adjusted the resistor R6 and got a little more gain out of the transformer, I also got a little more compression and S/N improvement. Everything tells me that the input level is too low but am I correct?

I used audio taper pots for 100k.

I'm still convinced the thing needs 600:60k to work properly..

 

[guavatone]

I am using an A-10 50K:600 and Sowter output (dunno exact ratio)

My math computes....

50K:600 yields 27dB gain

10K:600 yields 12 dB gain

disclaimer.... my math may be totally off (someone correct if wrong)

on my unit 1/4 to 1/5 up from full CC on output pot puts my signal somewhere decent. what is it for you? 2x that?

What about changing R5 and R6 to lower values? I think R6 is to reflect back the correct impedance to the xfmr and R5 is for the input tube's impedance.

 

[Svart]

output pot about 3/4 way CW(up) puts output signal decent but also brings noisefloor up to an audible level.

I see R5/R6 is likely for impedence but they also form sort of a voltage divider network, which I played with unsuccessfully.

I'll try the R11 change this weekend and see what happens.

worst case I'll just have edcor wind a 600:60k, best case, I change some values and end up using this right away.. :thumb:

 

[guavatone]

I was going to say a divider, but I think R5 would have to be on the other side of R6.... right?

I was going to suggest trying to keep xfmr ratios down a bit, but I realize you are probably sick of experimenting and fixing and want to use the damn thing :grin:

IIRC somone used the cinemage 30K:600 with sucess....