All things LA2a related

[Bluzzi]

The meter moves in amplitude but frequency remains the same. I used a 330pf cap for C4. I'll be checking R11, R21 and R22.

Yes there is continuety and I also checked ohms and ground is not my problem I think as I checked and re-checked all my grounds.

I put a 1khz tone in and listened on output. The tone is there and undistorted along with motorboating. The motorboating disappears when the gain is at max.

I think (can't remember) I did check the resistor values before wiring. There is still possibility of ruining a resistor or cap when soldering because of heat but I always solder using an alligator clip as a heat sink between resistor/cap body and solder point. But you never know.

I'll check my part stock first and then I'll have to unsolder parts to get correct readings I guess.

Might be nothing but last time I measured voltage the motorboating stopped when I probed pin 6 of V1 and pins 2 and 8 of V2. Grounding?

Thanks again AnalogPackrat, I'll focus on the area you mention. Get back to you with findings. (by the way, my T4B is not in).

jim

 

[AnalogPackrat]

[quote author="Bluzzi"]The meter moves in amplitude but frequency remains the same. I used a 330pf cap for C4. I'll be checking R11, R21 and R22.

I put a 1khz tone in and listened on output. The tone is there and undistorted along with motorboating. The motorboating disappears when the gain is at max.

Might be nothing but last time I measured voltage the motorboating stopped when I probed pin 6 of V1 and pins 2 and 8 of V2. Grounding?
[/quote]

Those are good clues. I'll have to ponder that for a bit. You have it set for "compress," right (SW3 shorting R7)? If you set the gain to max and stop the motorboating, does messing with the "peak reduction" cause the motorboating to start again?

Also, let's sanity check your DC measurements on V1 and V2. Now that you can make the motorboating stop, do that (crank the gain up) first. Now remeasure those DC voltages and let's see if anything pops out. I think some of your previous measurements were polluted by the low frequency oscillation.

You might also recheck the wiring on the T4B socket--specifically the "X." Check that it's soldered at the crossing point and that you have continuity to ground at all pins that should be grounded.

A P

 

[Bluzzi]

OK, actually after trying more things the meter does change in frequency as well as amplitude!

I have not plugged in the T4B so I don't think Peak Reduction should (and doesn't) do anything right?

I keep checking my wiring and it always comes up OK! At this point I get the feeling its a bad component or bad solder joint (I checked most and only found one suspect. I re-applied solder but nothing changed so it was OK to begin with).

I am starting to think maybe pictures may be a way to see if anything is amiss. Its harder for me to see my own errors! A lot of wiring has shrink tubing on the joints (some shrunk some not) so it may be hard to actually see solder points.

I can post an album of close shots (I finally got a digital camera) on photobucket. What thinks you?

Thanks again A.P.

Anyone else have any ideas or clues? C.J.?

jim

 

[AnalogPackrat]

Photos might be useful. Did you try stopping the oscillation (by turning down the gain) and then taking new DC measurements on V1 and V2? That might turn up a sneaky wiring error or bad joint (now where's CJ?).

A P

 

[CJ]

Get it motorboating and move wires around with a chop stick. Start with the grid wires of V1 and work your way to the 12BH7.

You should be able to get it to stop, or at least change in frequency.

cj

 

[imo]

I have completed setting up the Bloo version of the LA2A. It is very high end heavy. I have checked all of the connections, and leaning towards possible component failure. I did find that on the hot side of C4, the variable cap, i only have 2-3 volts, and according to one of the schematics i found it should be around 27 volts. I have tried adjusting the cap through its whole range, as well as switching both of the rear adjust pots without any change. I am needing to turn the gain all of the way up to have any gain attenuation on either compress or limit.
Any ideas?

 

[CJ]

what kind of transformers?

check transformer wiring.

could be supersonic oscillation causing gain reduction.

scope, or no?

 

[imo]

CJ,
scope on the way, not yet.
I am using the Sowter input and output transformers 4383 and 8940. I have them wired up per Sowter's schematic on the webpage.
I have found some funky voltages on the 12B7A. All of the other tubes and voltages look pretty good.
Here is what I have got: DC voltages
Pin 1: 15
Pin 2: 0
Pin 3: 0
Pin 6: 196
Pin 7: 15
Pin 8: 24
I used 1k resistors in series on pins 2 and 7 as grid stoppers, which i heard might be necesary with the Sowters
Thanks
Ian

 

[CJ]

Your cathode is shorted out.

How? Bias is gone, grid and cathode look shorted possibly, those sockets are tight, check for a solder bride or small starnd of wire.

I did a voltage chart, but it is burried on what page I do not know.

 

[imo]

CJ,
Thanks so much for the time.
I went in and checked for shorts between cathode and the grids. I had no continuity between any of them. The only continuity i found was between the heaters (pin 4 and 5 and 9) and the center of the tube, which is also part of the ground path. This seems according to the schematics though.
I do have grid stoppers on both pin 2 and pin 7..
Any other ideas on where to look? I have been trying to find another tube to check it with, don't think i have one. The unit seems to be running other than this one area, and i am so close it is a bit maddening/
Thanks
Ian

 

[Bluzzi]

[quote author="CJ"]Get it motorboating and move wires around with a chop stick. Start with the grid wires of V1 and work your way to the 12BH7.

You should be able to get it to stop, or at least change in frequency.

cj[/quote]


See, I knew it! Once CJ bugs you to do something it works!

In my shame I guess it must be my wiring route!

At first I got it to oscillate as much as possible before I started to "chopstick" around. At first nothing but soon I discovered a few wires that changed things but only in a worse way. Others changed the type of oscillation from motorboating to warbles, clicks and squeals.

Eventually I found one that stopped the noise completely! I had to tuck it under C2 (don't ask how I found out). Its the wire that goes from R15 to pin 6 of V1.

Then I plugged in a 1khz tone and listened in case no audio was passing. Yep, my tone was there! Did the gain work? Yep! Time to plug in the T4B (after all this LA2A needs its heart!).
Fired it up, and both speaker and meter confirmed compression was working.

Now time to put music through to hear quality of the audio. The music comes through but I have to turn down the gain a lot! There is distortion but this is such an improvement I'm really happy for now.

I got new voltage readings as follows;


V1
pin

1 = 103.9
3 = 1.1
6 = 108
8 = 1.2


V2
pin

1 = 83.9
3 = 3.8
6 = 212.6
7 = 71.9
8 = 98.9


These seem to be almost there. Next is to tame distortion. I'll come back for that one.

Lesson learned, routing of wire is very important in this circuit and I still have to find out right way instead of above solution.

If anyone has ideas about distortion please speak freely!

I'm not finished yet but big thanks to CJ and AnalogPackrat. Also thanks to Ron for his help and advise as well as a great layout. I felt all alone but you guys came through big time.

Sorry for long message...I'm excited!

jim

 

[AnalogPackrat]

[quote author="Bluzzi"]
See, I knew it! Once CJ bugs you to do something it works!

In my shame I guess it must be my wiring route!

At first I got it to oscillate as much as possible before I started to "chopstick" around. At first nothing but soon I discovered a few wires that changed things but only in a worse way. Others changed the type of oscillation from motorboating to warbles, clicks and squeals.

Eventually I found one that stopped the noise completely! I had to tuck it under C2 (don't ask how I found out). Its the wire that goes from R15 to pin 6 of V1.
[/quote]

Hey hey! Kung Pao Master CJ strikes again! Great news on the oscillation fix, but looking at the schemo, shouldn't R15 go to C2 (th other end of C2 then connecting on to pin6 of V1)? Have you accidentally bypassed C2? If so, big DC on V2's grid could be your source of distortion. Keep at it--almost there!

A P

 

[Bluzzi]

Ooops, sorry my bad. R15 goes to C2 yes but I meant pin 7 of V2. Thats what you get looking at a layout rather than schematic.

I'll be picking up a scope today (have to re-learn how to use it properly) but the probes are supposed to be funky on it so I may not have it functional until I get a set of probes for it.

I'd like to maybe understand why the oscillation occured to get the wiring permanently fixed up. Having a wire tucked in under a component is OK and a last resort but nicer would be proper wire orientation. That may be more of an art than science? I also have a second LA2A ready to be wired up once I am totally happy with this one.

How do the voltages look to you guys? Possible source of distortion?


jim

 

[AnalogPackrat]

Jim,

When you say you have to turn down the gain "a lot" what do you mean? I usually run my Sowter equipped Bloo with the gain around 3/10 or so. The LA-2a is no slouch on gain! If you're using the original iron, you may have even more gain (due to higher turns ratio on the input iron). So I'd bet you are OK if you are only getting distortion above 3/10 or so. Also, what is your input signal source?

Having said all that, CJ has recommended a small mod in the past that you might try. First, pop out V1 and try a 12AY7 which is a better sounding and lower mu dual triode. That will knock your gain down some. If you want to tame it even more, replace R9 and R13 with 100k. This also has the effect of making clipping more symmetrical by biasing the tube closer to the mid-point. I haven't tried these yet, but maybe one day...


:guinness:
A P

 

[Bluzzi]

The max I turn is to 8 or 9 oclock but eeven there I hear some type of harmonic distortion. Not bad. I have been listening through crappy headphones up until now. I'll be doing another fixin' session on the LA2A tonight and trying out better headphones and then speakers (now that I know I won't blow anything up).

I did get the 12AY7 that CJ suggested but had not heard of the 100k mod for R9 and R13. I am using UTC A10 and A24 so that also explains large gain. I'll pop in the 12AY7 tonight and if that doesn't tame it enough then I 'll do the resistor changes.

I feel so close now I am more relaxed about it. Thanks again for your continued support. You guys have no idea how much it means to me to get any type of help. Even if anyone is way off (which you are not) just the fact of writing to try things gets my ass off the floor and into de-bug mode.

I'll be mounting on the case tonight as well hopefully.

jim

 

[imo]

Cj, and anyone else who was watching. I figured out what it was. I had replaced the output cap, the 20u electro, after one of the leads broke while trying to find a good place to fit it in the enclosure. When i replaced it i put it backwards, classic..
Now voltages all look to be in order. I will run some tones through it tonight.
Again, thanks for all of the help. It is so valuable, not only in figuring out the problem, but also in learning about what is going on.
Ian

 

[Bluzzi]

I didn't get a chance to try the 12AY7 sub yet but I have a double question about R7.

This is the resistor marked 1k to 2.7k. So what determines its value and what effect does it have?

jim

Hey, Ian, good to see you found your problem.

 

[imo]

I was trying to search for the mod that CJ suggested somewhere in this thread that raises the threshold a bit. DOes anybody have the info written down? Couldn't find it using the search function I think i got to it at about hour 20 perusing the knowledge base, but at the time didn't write it down as it didn't seem pertinent

 

[John Peacock]

[quote author="AnalogPackrat"][quote author="John Peacock"]It's been a couple days. I did manage to find one other small mistake with the placement of R6 around the input transformer, but this correction has had no impact on the unit's performance. Still lots of HF noise whenever I try to compress. Also using a flashlight to trigger the TB4, I was able to detect gain reduction using an external metering source, though nowhere near the 15-20dB displayed on the LA2A meter.

My A20 voltage is still very low, and I suspect this may be part of my problem, but I don't know. That's why I am asking for your help.

Please!

-Jp[/quote]

If you can pass audio without GR, it sounds like your problem is not related to V1 and V2 circuitry.

Pop the cover off the T4B, hook up a signal of some sort to the input, turn off the lights and look at the EL panel. Does it change brightness with the input signal strength (it's not really all that bright, so make sure you're room is dark)? If it's pulsing in brightness with the music/signal, then your sidechain is probably working. Perhaps the wiring on your T4B or its socket has a small mistake. Did you wire the "X" and solder the point where the two wires that form the "X" cross? Is the X grounded at some point?

A P

A P[/quote]

Thanks for the response AP (and Bluzzi for words of encouragement).

I popped the lid off the TB4 and found that the EL panel does light up (GOOD), but it does not change with the amount of audio I put in (BAD). In fact, it does not even need to have something plugged into the input to light up. All that matters is how much I turn up the GR. GR = 0, no light. GR turned up to 50, lots of light.

I feel like maybe there is a loop somewhere, or perhaps I am bypassing something, or a short?

I can hear a very high pitched wistle when I turn up the GR on the unit (and this comes from the compressor, not the speakers).

-Jp

 

[John Peacock]

Here is an audio sample of what the unit sounds like. This is with gain around 10. I start applying Gain Reduction around 5 seconds in. I sweep the GR until GR = 100 around 15 seconds into the clip and then sweep back down to 0.

http://www.johnpeacock.net/bloo/Bloo_Sample.wav

Thoughts?

Thank you.

-Jp