All things LA2a related

[Sister Ray]

Hi A P, thanks a lot for the extensive reply!
I get the way the schematic vs. the Bloo kit works, and will take care of posting as you described in the future.

Here are the results of the questions you asked me, thanks once again for
taking the time!

[quote author="AnalogPackrat"]1) All tubes in and filaments happily aglow?[/quote]
Yes

2) Neon lit?

Yes

3) Did you measure each resistor prior to soldering it in?

Yes

4) What values did you use for R34 and R36?

R34: 22K/2W
R36: 1K

5) Do you have an R38 [22k] and a C113 [510pF]?

R38 Yes
C13 Yes (I assume you meant C13 here?)

6) What brand/model of input and output transformers did you use?

Input: Sowter 4383-f
Output: Sowter 8940a

7) Are you in the US or elsewhere?

I'm in Ireland, and use the Hammond 369JX power transformer, have set it up as per this post, for 220V:
http://www.groupdiy.com/index.php?topic=234353#234353

What voltage is at the node between R16 and C7-C?

238V

What voltage is at the node between R17 and C3?

268V

What voltage is at pin 1 (or 6) of V5 (the plate of the paralleled triodes)?

96V (I assume you meant V3 here?)

Kind regards,

Sister Ray

 

[AnalogPackrat]

No problem on the help. I think you're almost there. Yes, you correctly interpreted my several typos.

A quick check indicates that V1, V2, and V3 are drawing the expected range of B+ current, but V4 does look like the problem. V4 is only drawing about 270uA at the plate. It sounds like you don't have a spare 6AQ5, so before ordering another recheck all the wiring related to V4 (C11, R34, R35, C13, R38, C7-D, C10, R36, and all connections to and from these components). It could be a simple wiring fault. Also make sure C7-D and C10 are installed with the correct polarity ("-" to ground).

Now for a bit of complexity. You've got a mix of old and new versions of the circuit on V4. The laster version which added C13 and R38 also changed R34 from 22k to 10k and R36 from 1k to 470 ohms. I doubt this will fix your main problem, but this is a common thing with the Bloo build. IIRC, the instructions and schematic don't match up on this point. It may not matter, but why wonder? I used 10k and 470R on mine, which, BTW, you can take a gander at here:

My Bloo LA-2a

Excuse my somewhat sloppy wiring. Hey, it works!

Another thing you should consider doing is adding grid stoppers to V2. Several people have had oscillation issues at the WCF when using Sowter output transformers and this fixes the problem. I used Sowter and installed grid stoppers from the start, so I don't know if I would have had the problem or not. Search this thread (nearer the first 20-30% of its massive length) and you'll find info about how to do this simple mod.

Good luck!

A P

 

[ruairioflaherty]

[quote author="Sister Ray"]I'm in Ireland[/quote]

Sorry to derail the thread but surprised to see a fellow Paddy here, welcome!. Where are you based? I'm in Killarney. Shelves stacked with projects, boards and transformers but zero time to get stuck in.

Nice to see you are getting to finish the Bloo.

Cheers,
Ruairi

 

[Sister Ray]

[quote author="AnalogPackrat"]No problem on the help. I think you're almost there. [/quote]

Well, thanks again

I've checked all the wiring in the unit, and it seems all is wired fine.
I'm going to the shop tomorrow, and will get some replacement resistors for R34 and R36, some 1k resistors for the gridstoppers and see what that will do.
In the meantime however, I've also ordered another pair of 6aq5a's, hope that helps!

I'll keep you up to date of my progress.

Regards,

Sister Ray

 

[Sister Ray]

[quote author="ruairioflaherty"]

Sorry to derail the thread but surprised to see a fellow Paddy here... [/quote]

Hey Ruairi,

I'm actually not a true Paddy, moved here from The Netherlands about 10 years ago...
Lived for 9 years in the outskirts of Dublin, but last year I decided to move to the countryside of beautiful Meath (near Slane/Newgrange), much better

Since the move was completed and the studio wired up again, I'm slowly able to pick up some projects again.
After this, I have a shelf full of vintage stuff to attack too, so I feel your 'pain' .

Good to see I'm not the only one living on this island being interested in this kind of stuff, maybe we can hook up one day (I still haven't properly done Kerry before...).

Feel free to drop me a PM any time!

Regards,

Sister Ray

 

[ruairioflaherty]

After 9 years in the Irish rain I think you deserve honorary citizenship! You're welcome to drop by to my place in Killarney anytime, we have very famous mountins and lakes just minutes from my home. I have a studio in my place here in the countryside which was my fulltime gig for many years but I went straight and got a day job last year. Are you operating a commercial studio or is it just a hobby?

Cheers,
Ruairi

 

[RAM]

Hey I'm incredibly new here. Just wanted to say: I'm from Ireland too!From Dublin! Killarney is great though and I'll be nice and not start listing a bunch of stupid kerry-man jokes :grin:

 

[Sister Ray]

[quote author="AnalogPackrat"]No problem on the help. I think you're almost there.[/quote]

The tubes I ordered arrived today, put in a new 6AQ5A, and the voltages are much better now.
Still want to see if changing R34 and R36 to the values you proposed will bring it even closer, but from the readings that I just took, it's clear that the tube was indeed the culprit!
Thanks once again, I can now start making space in my rack for this baby :grin:

[quote author="RAM"]I'm from Ireland too!From Dublin![/quote]
In Dutch, we have a saying from which the litteral translation reads:"The more souls, the more joy!" Hope that makes sense :thumb:

Regards,

Sister Ray

 

[Nadege]

Hi everybody,

I've finished My La2a today, it was a very long time project... (As you see I bought the PCB to drip a long time ago;-) )

But I have a strange noise isssue. The compressor is working great, the noise is the only problem I have.

Here is a picture of the unit:

http://twin-x.com/groupdiy/displayimage.php?pos=-2535
http://twin-x.com/groupdiy/displayimage.php?pos=-2534

and here is a MP3 of the noise I have

http://twin-x.com/groupdiy/displayimage.php?pos=-2538

The noise vari with the gain knob, and is oscillating strangely. On the MP3, I didn't touch any of the control, the noise oscilate by himself.

I made the grid stopper mod as matta explained it in a pevious post, but it didn't change anything.

Any idea????? :sad:

 

[Nadege]

might it be a problem due to the fact that my heater connection aren't really thightly turned together?

 

[Nadege]

bump???

 

[Nadege]

any idea.....

 

[API]

Hi.

I would try to careully move some cables around, please use a insulated tool or similar, not your fingers!
I had a problem in my point-to-point version where the output cap was to close to the input circuit and got alot of noise.
You would be amazed how much the position of parts and cabling will change the noise.

Regards,
Dennis

 

[emrr]

Agreed; think you will have to hunt for poor wiring or component placement. Sounds like induced noise that correct grounding / placement / shielding would cure. Can't be any more specific than that without seeing patient in person.

 

[bernbrue]

Hi,
I can see at least 4 different connections to chassis ground. I would try to connect all grounds to a single spot in your chassis. Use shielded wires whenever you can.
Regards
Bernd

 

[dripelectronics]

thats a nasty sound ,

try moving some of the wireing around ,

or download the opto 3 atoms manual and apply the

dc refrence and the ground lifts ,

http://lomayesva.com/la2a_pcb_ver3.pdf

that noise sound more like the gain wiring / shielding interference problem ,

so try moving the gain pot wires around ,

the ver 1 was prone to occilations , but that doesnt sound like them ,

try the gnd to one point , say next to the input xlr socket .

LEAVE the power GND to the base of the power transformer .

other wise ,

if its killing you ,

i have a ver 3 (atoms ) 1oz copper pcb laying around here , that
i got when i was having the production ver proto typed ,.
they sent me two , so it's yours if you want .
(sound so good , it will make your eyes bleed)


just pay for shipping .

email me , g.

 

[Mr. Dave]

I have 59vdc on pin 5 of the t4b, all the tube voltages seem ok. I can't find any obvious wiring mistakes or cold solder joints on the pcb.

Compared side by side with my other working unit, everything appears to be the same except for the t4b voltage and the transformers. ( the working one has utc ha101x and triad hs-54, the one with the problem has sowter and grid resistors) I'm using Drip version 1 boards.

I haven't plugged the t4b in, as I'm afraid I'll fry it.

I've searched the forum, but can't seem to find anything except for this, which isn't the problem:

"On the T4 pin 5 I've seen a number of people saying that they have a reading of around 8vdc but it's not clear if that's with the T4 in or out. I have a reading of 55vdc there without the T4 installed. Is this right?

Strapping the faceplate to the chassis ground fixed that."

I'm stumped, any suggestions?

Dave.

 

[Nadege]

Hi,

I found the problem With this weird sound.. I rewire the heater connection with new really tight twisted cable and the weird noise is gone. :razz:

What I still have now is a pretty loud HUM varying with the gain of the unit.

I've try the star ground connection at one point and i can't notice any improvement.

I'll try the gnd lift mod as suggested by drip on the input and output xlr to see what's happening.

If you have any idea of what to do.... :?: :?:

 

[Nadege]

Just a last question.

I noticed that the 6aq5 and 12BH7 tube become very hot,

I'm not very familiar with tube build but I assume this is normal. Isn't it :?:

 

[emrr]

[quote author="Nadege"]
I noticed that the 6aq5 and 12BH7 tube become very hot,

I'm not very familiar with tube build but I assume this is normal. Isn't it :?:[/quote]

yes, those tubes should be very hot.