All things LA2a related

[EarthTone]

Turret board. Halfway between p2p and pcb maybe.
First time for tube gear for me, so it’s a bit scary to be honest!

Same. Maybe get a friend with tube gear building experience to go over it with you before powering on? I’ve built a lot of mics & preamps but all solid state & I’m a bit scared to get into tube stuff as well. I have a friend who’s well versed in tube gear who is encouraging me to get into it and can help with safety & stuff. Mics though. I know your La2a will have higher voltage than mics do! Best of luck with the build. Looks cool.

 

[Toobme]

Good idea! One can fry oneself if not careful.

In the meantime, I’m getting glow on my heaters, but think I must have wired the 3-position switch incorrectly.
The meter gets pinned with nothing going in.
The switch is doing the wrong things - so i need to go back to the schematic and rewire it properly.
But the pinned meter and huge hum have me worried.
Noooooooo!

EDIT: I have ordered another switch (a 3P2T non-shorting) to replace the one I was using (3P3T shorting).
Waiting for it to arrive, but also noting that I did not ground the front panel at all.

I'm at least going to have to grind down the paint on the inside for each pot and switch, and put star washers in there, correct? Then tie those points somehow back to my main ground near the IEC?

Argh - this hum is driving me nuts. I thought I had a good layout going, 90 degree crossings, tight windings, not going around the sockets. Some ground is looping somewhere.
Does anyone have a pre-flight checklist for these units?
I'm not putting in the T4 until the buzz goes away, but I'm wondering if anyone has a standard checklist they go through - to help my troubleshooting.

Thanks in advance for any wisdom, folks!

 

[sr1200]

i didnt know dan did the LA2A, ill have to check that out. i've made 4 of the LA2A's (two for me and a pair for a friend) Another friend is opening up a new studio in a couple months and expressed interest in me building one for him. Thanks for the lead.

 

[Toobme]

OK, tracking this down a bit.
I chose not to use the variable cap at C14 (despite having one). Just thought it’d be one less component.
Is that doable, or is it best to have it there — or the 150pf silver mica?
Thanks, folks!

 

[Toobme]

Still only getting 251V between chassis ground and positive of C7.
I’ve double checked the caps and resoldered them, although the joints looked good.
Still the same 251V.
I’m starting to go nuts now. But still smiling at the object that is for now silent.
Anyone have a clue as to why low voltage?

 

[beatnik]

IIRC the variable capacitor are in flat position towards the maximum. I would fit 390pF in the sidechain. For the feedback network tuning the value makes sense as the high frequency bump is depending on what output transformer is fitted. You can still substitute the variable cap with a fixed type once you have dialed in the optimal value.

250V at the second filter capacitor should be ok. What voltages do you have on each tube anode - girds - cathodes ?
Post a complete chart and we might be able to understand if there actually is a problem somewhere.

 

[Toobme]

Thank you, @beatnik for taking the time to explain that.
I guess it was silly of me to hope that I could just leave out the variable cap (or any cap) in that place.
Will post a complete set of readings to follow.
Again, very much appreciated - this forum is a gem, populated by stars!
Thanks

 

[TDJ]

I don't think you want to do that. The 12BH7 is setup as a white cathode follower. The voltage at the plate of the lower half (100v on the schematic) is what is feeding the DC voltage at R21/C4. You see that R21 and R22 form a voltage divider from the plate voltage - I'm guessing your lower plate voltage is low, maybe ~81v. So the DC voltage at R21/C4 doesn't matter at all - that is a feedback path to the cathode of the gain stage before it. The amount of feedback is controlled by the ratio of R21 and R22, R11 and R10. Changing R21 will change the amount of feedback, which you don't want to do.
So why is the DC voltage different? Assuming the resistors around the WCF are all correct (R17, R20 etc) then the tube is just biasing a little different based on it's characteristics. I would not tweak the circuit, I'd try different 12BH7. Compare the top plate voltage, bottom plate voltage, and bottom cathode voltage and you'll see different tubes bias at different points.
And there is explanation for a WCF online you can look into if you want to know more about the bias point.

thanks! i actually ended up leaving it - the unit sounds amazing as is - and performs as it should, so i’m not gonna worry about the difference in voltage on the schematic!

 

[Toobme]

Anyone used the Sowter 1449e for input?
Where did you attach the white primary ct?
I put it like gambaholic’s layout, but that third post on the terminal strip connects to nothing with the Sowters. Or so i can fathom.
I’m just trying to get the thing to pass audio at this point.
The second i plug in the output from the la2a into my recorder, without the unit powered up, i hear a buzz so loud that at zero gain, everything but the gain redux mode on the meter is pinned. Turning input or reduction knobs is doing nothing.

I’m keeping on, not giving up, but saying prayers at this point and hoping someone else has been down this road before 9and was clever enough to solve it!).

 

[beatnik]

I suggest again a voltage chart of each tube socket. It would tell if there is a specific section with problems, wiring mistake or perhaps wrong component value.

Check there is close to 0 ohm resistance between each ground points and earth.
Make sure all exposed metal parts have a good connection to earth.

Painted chassis from diy suppliers usually require a fair bit of paint scraping as they don't bother with masking tape.

This is also true for components on the front panel. You should probably remove paint behind each hole, or at least use toothed washers

Looking at the photo you posted it seems you are not using toothed washers.
Not only they improve reliability by locking the fastener in place, they also bite into the paint and increase chances that metal parts make good contact with earth.

You should also check your wiring for ground loops.
I don't know the layout you are using but the one from recproaudio has been used countless of times and I think is a copy of the original La-2a layout. You cannot go wrong with that one.

 

[Toobme]

Thanks again, @beatnik - it must be the paint - my switches and knobs on front panel are just floating! Will grind it out, grind paint off, and use star washers!
All ground points were showing almost nil resistance - now to ground the panel.
Appreciate your guidance!

 

[Toobme]

Sorry, the last couple weeks have been nutty, as I’m sure they are for most folks!
I finally got around to doing the measurements.
I have yet to redo the grounds with toothed washers (they’re on their way here still!), but here’s my chart so far:

After the 1n4007 pair: 275.1 V
After the 4.7K 2W Resistor: 275.1 V

6AQ5A (V4)
Pin 2: 4.22 V
Pin 5: 111.5 V
Pin 6: 40.02 V

12AX7 (V3)
Pin 1: 58 V
Pin 3: 0.706 V
Pin 6: 58 V
Pin 7: 0.5 mV
Pin 8: 0.69 V

12BH7 (V2)
Pin 1: 54.1 V
Pin 2: -20.4 V (yes neg 20 V!) This must be messed up, correct?
Pin 6: 133.3 V
Pin 8: 60.3 V

12AX7 (V1)
Pin 1: 70.6 V
Pin 3: 0.673 V
Pin 6: 160.1 V

At the Neon Bulb: 39.83 V

I see that things are way off and in one case, negative.
Tempted to rip her all apart and start again from scratch.
*continued hair pulls and groans*
But still loving the challenge, just hoping for a clue as to what area sounds the most messed up.
Are turret boards (first one for me) harder to achieve good joints than pcb - I checked every joint for (at least) continuity and in some cases resistance, but I’m wondering about “reflowing” my joints first before tearing it all down.

 

[beatnik]

maybe something wrong around the 12BH7.
instead of ripping all apart you should print the schematic and with some marker pens follow along the schematic while referencing it to your build. also check component values while you are doing that.
It's time consuming but it will make sure you don't have mistakes in the build.

 

[Toobme]

Right - will do. Time to dust off the printer.

 

[paulcheeba]

I had my ADL 1000 modded some years ago. Adding the UTC i/o transformers was by far the best sonic upgrade. Obviously the HA input transformer is ridiculously overpriced BUT for exactly this reason. It is the sound of the LA2 imho.
Second was the grounding path and power transformer.
Then cc resistors and pio caps.
I had to sell that ADL a few years ago to a local producer who said he wouldn’t think sell it back to me. Not even for twice what he paid for it.
The sound is in the iron.

Wiring for UTC's? A16 & A24

I used to have an old LA2A but I wanted stereo so I bought an ADL 1500 after reading good things. It didn't have that lovely sweet spot, break up that the LA2A has on bass etc and I'm trying to upgrade it. Thanks for your advice I have bought 2 UTC A24 and 2 A16 and I need to know how to wire in the NOS UTC and what other brand components I should aim for? I've given up on the opto attenuators as they don't plug in. All the best Paul!

 

[Toobme]

Ok, some progress here!
Passes audio now, gain reduction working (audibly and metered) but output metering nothing in either 10 or 4.
I can hear my voice, but it’s very fuzzy and not very clear.

After triple checking everything, i found i had connected ground from gain control to pin 6 of the t4b!
Fixed that and now have audio.

Poked around with chopstick and moved everything but no change.

I THINK I may have something suspicious here on the Sowter input trans.
See my pic, I’m wondering what the white wire should be connected to.
I just put it on the third post of the input strip, but it’s connected to nothing.
That would seem to me to be an issue, as it’s just floating around, no?

Thanks for any suggestion!

 

[dmp]

White on the Sowter 1449 is the primary center tap, which you don't typically use for connecting to a line level input source. Connected how you have it is fine. Floating CT is not a problem as any induced noise would be rejected by the transformer as common mode. The blue & red however should stay close together, tightly twisted.

I'd suggest printing out the schematic and following every connection with a highlighter in comparison to your build, and doublechecking component values along the way. It sounds like you have multiple mistakes. You can pull out the T4B and just focus on fixing the intput / output section first. It should pass clean audio. You must have a mistake in the SW2 area if you aren't seeing signal on the VU.

 

[fallout]

My guess is the white wire is the center tap and should be connected to pin 3 of the terminal strip.

You're saying that your meter is working in GR mode but not Output? Check your switch connections.

 

[Toobme]

Thanks @dmp and @fallout - I appreciate the quick response!
I’ll check on this and report back.

 

[Toobme]

Hiya @Biasrocks - I’m wondering about the technique you used to ground out the front panel. Did you mean to solder a lead from the case of the pot to the turret ground? Thanks so much, if you’re still around (i know that was a few years ago now!!!)!

I’m using the Canare starquad.

Quad cable is fine, slightly more capacitance but the runs are short.

You want to minimize ground points if possible, attaching to R10/R20 gives you a proper ground.

Looking at my build you can see a black wire running from the back of the GAIN pot to the turret which is connected to chassis ground, you should do that if you haven't already.

Regards,
Mark

Update: found a cold solder on pin 3 of the 12bh7. Fixed that and now hear my voice coming through! It’s there but seems to have too much gain at 15 or 20 on the gain knob. It starts saturating much sooner than i’d expect, so some tuning is going to be necessary. But this is exciting! First tube project that is passing audio for me and I gotta say - turret board is a beautiful thing, but I think I underestimated the soldering on it. PCB it seems easier to tell if you have a good solder. This way needs ohm measuring and continuity to find the errors - wish I could change my handle to ToobNoob!

Thanks, @beatnik, @dmp, and @fallout — I’m well on my way now and inspired to get to the finish line. I’m not even all the way through rechecking everything, and will probably find more little errors in my build.

Tracing thru the schematic with a highlighter is actually what’s made me find my mistakes!

Now if only I could find A PROPER SWITCH for this VU selector. Mine is a 3 sections of 4 posts. I want to find a 2 pole 3 position to be like the old units, correct? Anyone know a good fave part for that?