All things LA2a related

[emrr]

Thanks for the input, really, everyone.

regulated PSU is obviously one higher level of complexity, and one more thing to fail.

Some people get religious about tube rectifiers in low level low power gear. I like the idea of them, and the slow start B+. But that seems to be a bit superstition and romance, more than anything. I wouldn't expect rectifier sag in an LA-2A, though you might get some when driven into total distortion.

Some people get religious about battery powered direct heated triodes also; I haven't listened to that to say for myself.

I've got too many boxes of too many parts; all options are on the table, and possible without additional expenditure.

I just haven't sourced T4's yet.

I keep yammering on mainly since posts just drop on down the LA-2A list quickly, and probably aren't seen by the occasional viewer. Trying to get maximum observation.

I'll be sure to show whatever crazy version I build.

 

[guitarbill55]

In case you haven't found your T4's yet; as of a few weeks ago drip was offering some custom built ones for $90 which is just a little more than what I paid JBL for the tent sale ones I bought several years ago. I haven't tried drip's yet but from what he says they use parts that are as close to the originals as possible. They should be quite good. Use e-mail to contact him- he'll get back to you eventually.

Cheers

gb

 

[emrr]

Thanks, order is in as of two days ago, so hope to have final parts sourced in the very near future.

 

[Bluzzi]

If you are interested I have 2 A-24 I will be putting up on eBay soon.

jim

 

[pH]

I've been struggling for a while with my new LA-2A thinking there's something wrong, but maybe I'm chasing my own tail here.

I hit it with a normal amount of level (for a Digi 002 anyway) with no GR and the LA-2A distorts really easily (most noticeable on bass). I back WAY off signal from the mic pre or DI and it cleans up. Kicking in GR can clean up a slightly distorted signal. I know there's been a lot of talk about how sensitive the Gain knob is, but this is happening regardless of makeup gain level.

I even did the 12AY7 + 100k plate resistors mod on V1 and...it still distorts easily. I used Sowters for the transformers, nice PEC pots. All voltages good, all grounds and wiring quadruple checked. Even tried a vintage T4 with no change.

Is this normal? Should I just take my ridiculously high input gain like a man?!?

 

[dripelectronics]

is your input transformer backwards ?

the gain is supper touchy , often i find i can only use about a quarter turn of
the knob if using pec lin taper.

if using the alps blue velvet , i get alot more turn for the burn .

a hot signal into the la2a is needed for the compression

to become active .

stereo adjust and limiter response , depending how you wired them ,

should be both full clock , or full counter clock wise ,

the only time to hear distortion besides overloading the input

or the output,

is when turning the reduction pot and the stero adj / lim res is not in the proper position , ( i find it a pleasing 808 style distortion not the overload type of distortion )


lack of ground in the t4 area will give more of a high pass effect ,
same with the output transformer .

and loss of t4 reduction use .

if your using a DRIP board , and you have chased and double checked every thing ,

i would buy a raido shack de-soldering iron and check if your input xformer is backwards , (take a marker and mark the present position
first)

if it is a drip board and your at wits end , i would gladly make it
work for you .

stuff like this happens , it could be something as simple as a mis wired
switch ,pot or xlr connector or even the shielded wire shorting out the
gain pot etc.

best ,

g.

 

[pH]

I don't think it's the gain pot sensitivity at issue, as I can dial it down to silence and still hear the fuzzzz unchanged as I turn down.

I already pulled up the input tx and it's lined up correctly. Stereo pot was wired right, but I took it out and put a 1Meg and a jumper to eliminate possibility of a bad pot. Response is set correctly, but a little scratchy - maybe I should check for DC on the pot?

Can I completely disconnect the side chain amplifier to try to isolate stages?

Yep, it's board. Funny thing is I've always done tube circuits PTP but thought I'd save a little time with this one using a pcb :?

 

[CJ]

papa roach used that rig, the xfmr's are cooked.

check C3.

 

[pH]

C3? Brand new orange drop, solder shiny and 230vdc in and not leakin' out.

CJ, transformer Jedi, is there a chance that a bad xformer could make this noise?

 

[CJ]

maybe, i dont know.

but oinethingi justfigured out, remember when people started using the Sowter/Drip tag layout and everyone was getting F'ed over with oscillatioons and feedback?

Well, somebody figured out that if you messes with the shielsds, ie. inner electric shiekds, pieces or copper foil shunted to ground, which prevent the pri and sec affecting each other in a capacitance way, well, as soon as they messed with that, the osc went away, but guess what, so does fidelity!

youare incresing pri-sec capacitance by dis connnn3ctibng 6the Sowter shield, so it is a bad fix, like foam on a shuttle tank, to F with the orig Sowtwer plan, re lay out the LA 2 tag bord , watch the grid wires, put the Sowter shields bak in place, and notice the high end response.

I Lovce Joe The Mechanic.

I want Ian Gillan's crusty schlong.,
And TRonni I ommi also.

my ex looks exactly like Amy Wiine traub, and acts like her also, so if any bnody has any complaints as to why I am so f ed up, ask Amy/Caledonia.

Dylan is so dipressing, I had to chill him out.

 

[dripelectronics]

apart from a lighting strike penatrating your house and landing ontop of
the transformer , i think your transformers are ok.

their are hundreds of people very happy with the performance of their
opto pcb.

i have put it side by side with re-issues / vintage la2a /
vintage la2s with equal results.

what ever cj is talking about , is news to me concerning the ver 2 opto

.,:roll:

hating or fear mongering on the board isnt going to help this person with their problem.

the bottom line is it's a great pcb for advanced and beginners .

and please dont take your transformer apart as implied.

i didnt design the board so you had to modify your transformer.

most projects require a little troubleshooting.


ver1 pcb had no grid resistors , so occilations could occur when used
with sowters.

but i think matta solved this .
(the ver 1 was designed for the original xformers p.s)

ver2 was designed for sowters . period !
(but happy to say works with most transformers very well.)

believe me , ive done some weird brutal things to these transformers
during expirimentation of both this pcb and the new opto72 pcb ,
and they keep on keeping on ,

in fact i have only bought 1 pair of transformers from sowter
and have done all proto typing and expirimentation with this magic pair.
for both the opto and the opto72.

you have to understand , if you have all the correct resistors in all the
correct places ,
and if you have all the correct caps in all the correct places .

the circuit is exact.

there is no need for mods to solve your problem,

99% of the time it is wiring.


a bad tube would cause your problem ,
but i see you have already tried this .

improper gnd connection could do this ,
but i assume youve checked this .

are these x formers the new 68/versions ? or the ones stated in the
manual.

i havent tried the 1968 version , nor have i checked with sowter on
their pin out , if it matches the 8940b (out) or the 4383c (in) that the
board was designed around .

a.) it is not the pcb , you have done something wrong
b.) faulty part.
c.) your set up is causing the problem . (mixer/patchbay/etc.)
d.) your overloading the input .

keep calm , were gonna work this out .

p.s we might resume this conversation in the PCB thread.

g.

 

[pH]

Didn't mean to open a can o' worms here.

I was just asking if it's possible that a transformer could be delivered faulty, and what it might sound like if it's only slightly defective. (and I don't mean to inflame Sowter by that question!)

Plus it's easier to blame a manufacturer than to own up to your own stupidity
:grin:


I'm just trying to learn, and troubleshooting has always taught me way more than connecting dots. And believe me, I'm really getting to know this circuit.

 

[dripelectronics]

ah , no worries .

par for the course.

g.

 

[cannikin]

Hey pH,

I had the same issue last week, check 2 things, cold solder joint around either input tube or 12BH7 and grid stoppers, of coarse.

 

[ytrehalf]

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?

 

[ytrehalf]

Strapping the faceplate to the chassis ground fixed that.

 

[emrr]

Notice R25 on Drip 2.0 board is labeled 33K, while various schematics list it as variable 18K-100K. Meant to be trimmed for T4 cell matching.

Has anyone delved into this at all? Have any comments on the advantage of trimming versus going with 33K and walking away? Greg, are your T4's matched during build in such a way that 33K is specific to your version?

Related: SSLtech posted a method for trimming related comp/limit resistor R7 somewhere in the hundreds of pages, which I wish I could point out easily. Keith, anything you can add about R25?

Thanks

--------EDIT---found SSLtech quote--------

You adjust R25 until it matches. Without calibrating R25, they don't track accuratley on ANY LA-2a.

Replace R25 with a trimpot of about 1.5 or 2 times the stated value of R25 then start it halfway and NEVER take it all the way to either extreme...

Somewhere in there will be a setting which makes the meter agree when switching between GR and +4, after setting a tone to 0VU output in +4 mode with no GR present. Increase the GR and set it so that it matches at around -5dB (5dB gain reduction). It will be maybe a little off at -3dB, maybe a little off at -8dB... a little off all over the scale, but I find that matching the meter at-5dB is usually an excellent compromise.

The only way to get them to track correctly is to MATCH the optos in your T4b. Then you re-adjust R25 and try again.

Read the comments by David Kulka and myself in the T4B threads, and you should be able to find out a lot. T4B matching and behaviour research is something that I've done a LOT of...

If you buy a NOS T4B from David and plug that in, you should find that there is a better match between optos, since they DID select and match the optos. Setting R25 will then give you about the closest match that you can get.

However, if you've not adjusted and set the value of R25, you'll NEVER getting them to track would be like winning the lottery, and if you put a different T4 in there, it wouldn't track again, until you adjusted R25 again.

 

[Sister Ray]

Hi,

New posting member to the forums here, but have been lurking for ages.

A few years back, I bought a Bloo LA2A kit from Scenaria, and due to circumstances, I never managed to complete it.
A couple of weeks back, I decided to pick up the work again (after all, having an expensive kit like that just sitting around is a shame).

So, the unit is now finished, but I have a couple of strange DC voltage readings after sticking the tubes into the unit.
Obviously, I wouldn't like to fry the T4B, so I would be interested in getting some feedback.

Here are the voltages, the first value for each row is the 'advised' voltage in the building manual from Scenaria, and the second value is my measurement.
There are two voltages that are really off (bold red), and another two seem a bit far out too (indigo)...

B21: 360V - 382V
B20: 275V - 332V
A20: 131V - 324V
A14: 216V - 273V
A17: 90V - 96V
A19: 105V - 273V

My first suspicion is that the 6AQ5A tube might be faulty, as the two values that are really off are both turret board pins that are directly connected to that tube... But before purchasing another tube, I'd like some input from here.

Also, while measuring A19, it takes about 3 minutes before it settles to a stable voltage, dropping down about 50 volts during that period until the final voltage has been reached.
Not sure if that's of any importance, but thought it might be useful information.

By the way, without the tubes in, the 'advised' values on B21 and B20 were pretty much spot on.

Any help is really appreciated!

Regards,

Sister Ray

 

[Sister Ray]

Fixed a wiring problem, voltage measurements are now in the post above.

Regards,

Sister Ray

 

[AnalogPackrat]

Well, let's see. A couple of questions to start with:

1) All tubes in and filaments happily aglow?
2) Neon lit?
3) Did you measure each resistor prior to soldering it in?
4) What values did you use for R34 and R36? (there are two versions of the circuit and these resistors changed)
5) Do you have an R38 [22k] and a C113 [510pF]? (these were also part of the change made to the later version of the circuit)
6) What brand/model of input and output transformers did you use?
7) Are you in the US or elsewhere?

The first thing to note is that your voltage drop across R29 (which should be a 4k7 resistor) is only 50V. All of the B+ current flows through this resistor so we can start to get a handle on the problem by looking to see where the B+ current is going in your build. You've got 50V across a 4k7 resistor which means you are drawing just over 10mA of B+ current. This is a few mA short of what we should be seeing. Normally this would be 14-16mA giving you a 65-75V drop across R29.

The 12BH7 should draw 4-5mA, the 6AQ5 10-12mA, and the 12AX7s about 1mA each. If something is wired incorrectly, often you can determine where it is by how it affects the DC operating point of one or more of the tubes.

What voltage is at the node between R16 and C7-C?
What voltage is at the node between R17 and C3? (a.k.a. A14)
What voltage is at pin 1 (or 6) of V5 (the plate of the paralleled triodes)?

A P

p.s. One reason you may not be getting a lot of help is that you are referring to Bloo specific instructions when you ask about "A" and "B" points. These are only in the Bloo manual. I happen to have built a Bloo and have the manual, but most people don't. If, instead, you refer to nodes on the schematic (which many people have or can find) more people can relate to your description. For instance, "B21" is the node between the rectifiers and R29. Or "A14" is the node between R17 and C3 (or alternately pin 6 of the 12BH7). Get it?