All things LA2a related

[AnalogPackrat]

OK, I get 0.318VAC at V2 pin 6. Watcha got?

 

[Paul B]

the 1k gridstoppers are between V2pin2 and A9, and between V2pin7 and B8.

The ACV on pin 6 (thats the same as A14, right?) is 1118mV

 

[AnalogPackrat]

Am I getting tired and foggy or what? This will be my last post for tonight, I think.

Yeah, I think you've got something weird on the BH7. Here are some things to try:

1) if you gradually turn down your gain while watching A14, does it suddenly drop off or gradually diminish? Doea the sound of the output also change at the same point?

2) My Bloo never had the oscillation problem so I'm not sure how it looks with just a meter. If you had a scope it would be cake to see it. Have you tried using a wooden pencil or chopstick to move some of your grid and plate wires around while you listen to the thing and/or watch the troublesome point (A14, for instance)?

3) Do you have another BH7 to swap in? Or have you done that already?

4) Recheck all that R21, C4, R22, R11 wiring (and pretty much everything at the BH7).

I really think you are close. It's probably one silly thing somewhere obvious (it's always obvious once you find it).

A P

 

[AnalogPackrat]

BTW, it sounds like your grid stoppers are correct. DId you shrink wrap the bare wires? They aren't touching anything else like the little meter turret board, right?

 

[Paul B]

I appreciate all your help, AP. I'll get on those things in the morning. Yes, I dont care how silly the problem will end up being, just want to nail it.

see ya later
Paul

 

[Paul B]

yeah, I did shrink wrap the bare wires.

 

[AnalogPackrat]

No problem, Paul. Hope we can kill that bug soon...

 

[CJ]

Paul, what are you using for C4? If adjustable, try tightening it down all the way. This might kill any osc. that you have.

Everything looks halfway normal in the GR circuit, which is good news.
It looks like the trouble starts at the BH7 top plate. Before when you had 7 volts here, that would definately cause distortion because you are exeeding the 4 volt bias on the bottom cathode.

Try a 0.01 uF cap at 200 volts if you have one. Put it from ground to the bottom plate of V2 (pin 1) and see if that helps get that pin 6 ac volts reading down.

Also, what are you using for R16? Make sure it is 68K.

Do you have a Fender guitar amp around by any chance? If so, pull the 12AT7 phase inverter tube out and try it in the place of V1 and see what you get.

I am with AP here, since we have it narrowed down to the output stage, and your GR seems to be working good, I will do a new set of voltages with no GR. I will take home the signal generator to get a steady tone instead of music.

cj

 

[Paul B]

C4 is the adjustable cap that Steve includes with the kits. I believe its all the way tightened down I will check again. R16 is a 68k.

Im thinking about replacing caps 1-3. The dc voltages are whacked off of C1 and 2 and the AC voltages are way off on C2 and 3. I think with all the tweaking (not me, the caps) between the 1st and 2nd builds they might have gotten damaged.

I have an 12AY7 and and replacements for V2 & 3 on the way.

 

[CJ]

Cool on the 12AY7!
If you can find a 6.8K 2 watt resistor, try it in place of the 4.7K R29 to bring your B+ down a bit. Sometimes dropping the voltage will cool oscillations.

 

[Scenaria]

hey paul...

you might have answered this before but how would you rate your wiring? is it pretty neat and tidy? I had some oscillations in one that was on my bench that was caused by some untidy wiring..

I also had problems with sowters before I put the grid stoppers in...but I think I remember you saying that you have the grid stoppers in?

 

[AnalogPackrat]

Scenaria,

Paul's got the grid stoppers in on the WCF and from his description it sounds like they wired correctly. I'm curious how you would rate my wiring :roll:

Analog Packrat's Bloo LA2A Build

password is "Bloo"

A P

 

[Paul B]

Well honestly some of the turret board, while all solid and without shorts, could be more tidy. I've just thought that the root of my problem lies in something more obvious, like a wrong connection or component failure. It has always been in the back of my mind to redo a lot of it.

AP, I think your work looks pretty effing pro, for whatever THAT'S worth.
:roll:

 

[Scenaria]

[quote author="AnalogPackrat"]Scenaria,

I'm curious how you would rate my wiring :roll:
[/quote]

does it work?

if so then I would rate it to be excellent haha

im staying away from that question.. :green:

 

[AnalogPackrat]

Smartass! Yes, it works. Quite well. If you thought it was better than good you would have said so, I guess I got my answer (C, maybe). Hey, I can take it. At least I used different colors for the wire and components! :razz:

A P

 

[AnalogPackrat]

[quote author="Paul B"]
AP, I think your work looks pretty effing pro, for whatever THAT'S worth.
:roll:[/quote]

Hey, no need to butter me up! Look at some of the other photos posted around here. There are some really neat wiring jobs being done by this crowd. I'm always loathe to neatly bundle up a bunch of wires with tie wraps for fear of crosstalk/coupling/oscillations/bad dreams. That's my excuse and I'm sticking to it!

A P

 

[Scenaria]

seriously... if it works then I would avoid waking the "sleeping dog" cause you never know what can happen once you start poking around in there..

BUT

whenever someone tells me they are having induced hum or oscillation issues I always ask how tidy their work is.. if they say its kinda messy I usualy suggest cleaning it up a bit...

if anything it atleast makes it easier to troubleshoot and spot potential problems

 

[AnalogPackrat]

Don't get me wrong--it works and I'm not changing it. I was just looking for feedback for improving future builds. Fire away!

 

[Scenaria]

im not sure how you went about wiring the turret...

but what I found easier is to actually put 12 inch leads on every turret where needed before putting the board into the chassis...

then I would simply run the wire to correct tube or whatever and trim to length... it was easier than trying to work in the box with the turret board in there

 

[AnalogPackrat]

Yeah, that's what I did. You had that table of approximate wire lengths in the manual, too, so I just added an inch to that length and tacked them on before bolting the turret board in. That chassis looks big until you get near the end and then it's hard to get a couple of fingers AND a soldering iron tip anywhere near some parts. I think my wiring inside the chassis (the REAL point to point stuff) is a little messy. I tried to keep grid wires away from, or at least perpendicular to, plate wires. But then I ended up having longish loops of wire instead of short and neat wiring. Oh well.