All things LA2a related

[Bowie]

thanks for the help!

i'm getting 7.1v now at the heaters, which is a touch high. the 6aq5 is getting 7.1 fine, however the 2nd 12ax7 (v3) isn't. Yea I finally found a location of my problem. Continuity tests fine across all the heater stuff. i'm going to get pictures tomorrow with some light and get them up. would this be a problem if i blew out this tube on initial power up? i caught some grounding problems and fixed them after first power up.

as for the center tap, on the hammond 369jx transformer i'm using there is a center tap on the heater wiring that i'm sending to ground. that's correct, right?

If a tube went out upon firing up the unit, that's not a good sign.  As far as your heaters go, I just used a 269jx in my most recent build and got the same voltage.  See my thread in the Drawing Board section about dropping it closer to 6.3.
If you have a CT on the PT, yes, ground it at the power ground.

 

[peterc]

Not sure if this has been covered, but does anyone know what the lowest B+ is practical for the LA2?

Thanks, Peter

 

[Le Roux]

Hi Guys,

Looking for a bit of help. Hours into trouble shooting my Drip Opto4.
Whitmore tubes, Drip slow T4B, A-10 input, Sowter output.

All voltages on board are within range.

1khz 0 db signal into input transformer. 
Pin 7 to board.  As soon as it connected, the signal gets cut to barely anything.
Pulled R5 off of ground and the issue goes away.
Obviously I cannot power the unit this way.
Totally confused why, and what to do?

Tried a utc ha-100x and same thing.

If I bypass input transformer and inject signal right into board, signal is good, and gain, but still not  compressing.

Thanks
Neil

 

[wimmy7986]

Hey guys,

I just finished up a build with the cayocosta layout, and I can't seem to get the peak reduction pot to change the compression.  With R3 and R37 fully CW, I get a nicee beautiful compression, but I can't seem to change it at all with R2.  The only thing that seems to change is that when i have R2 fully turned up, I get a little "hairiness" added to the signal.  I've triple checked all wiring, voltages, and grounding and can't figure this out.  Has anyone had this happen before?

Thanks!

 

[wimmy7986]

Never mind....  I got it figured out.

The lug with the shields connected to it wasn't making it to ground.  I ran an extra wire to ground and then it worked as it should.  Beautiful!

 

[Gambaholic]

And in answer to your earler question...

You can power up and test an LA2a signal path with ONLY two tubes (12AX7 and 12BH7), the two transformers, and NO T4.

Everything else is sidechain. When the 'Peak Reduction' knob is all the way down, they do nothing.

The BEST way to learn, is to start with something working, then find out WHY ot works. -Even start with something that DOESN'T work, then figure out how to MAKE it work, then figure out WHY it works...

Look at my recent Bloo LA2a build...

It has swappable transformers, mounted on sub-plates. So far, I've tried Jensens (some people LOVE 'em) I've tried Sowters, (some people LOVE 'em) I've tried UTCs (the originals, AND some different UTC options) I've tried EDCORs (some people LOVE 'em) and I've tried Lundahls. (some people LOVE 'em).

Guess what... they're all different characters. They're ALL loved by different people.

Here on THIS forum, you'll encounter some people who've built several different versions of the LA-2a. They've used different options, and they've resulted in different sonic characters. Some have tried "night-light-and-Vactrol" home made T4's, others have swapped tubes. Sometimes they've swapped BACK.

You'd think that if everything was so clear-cut, we could ALL just read up and use our experiences in collaboration with what we read of others' experiences... And yet we ALL tweak and twiddle until we settle with what we like.

Elsewhere, you may read a post by someone who bought the original UTCs for about $650 (the cost of an HA-100X and an A-24 on eBay.. recent sale prices) and they've decided that the LA-2a sounds fantastic with them fitted. -I'm not going to argue... I've got TWO LA-2a's with UTCs in them (one with an HA-100 and the other with an A-10 as the respective input transformers) and they DO INDEED sound fantastic!

...However, that other person (on Gearslutz or wherever) won't have the 'baseline reference' of other options... They just know that their home-build LA2a sounds fantastic. -But LOTS of LA-2a's sond fantastic... probably MOST of them do. -They then start to profess that theirs is the best because it makes them feel better to tell people that theirs is wonderful, and you VERY rarely (if EVER!) hear people saying: "Yeah, my gear's not really all that good... there's better stuff out there!" (-If you did, Behringer wouldn't be making any sales!!!)

So I say that you'll learn more by just making something that WORKS first, then making something that works BETTER... -Why? -Beacuse you'll have HEARD the difference, not just read that there (apparently) is one. You'll have HEARD what the difference SOUNDS like... not just some person's (biased) adjectives.

Here's how far Scenaria and I went in terms of comarison...

JUST DO IT. -You can always fiddle later, and there is NO "best" one... I've built countless versions, and they're ALL great. (something which the gearslutz alphas with their one cherished compressor don't have the luxury of reference, in order to appreciate!)

Build it. Test it. Love it. -Adapt it.. LEARN from it.

Then build another. Some times you'll prefer one, some times the other.

Keith

Hi Keith,
I've tried to PM you but your box is always full so I'm going to try quoting you in hopes you respond.

Because you have probably built more LA2A's than most here I was wondering if you could tell me your thoughts on a couple common changes people make in an LA2A build.

What are your thoughts on 'metal film vs carbon comp' and also what, if any, differences you noticed when using alternatives to the ceramic disc caps in an LA2A?  Is polypro for all ceramics in the LA2A a good swap or not really any meaningful difference?

I'm very curious to know what you discovered with your experimenting. Any insights you could offer would be greatly appreciated.

Thanks,
John

 

[Deepdark]

Carbon comp. resistor tend to be a little noisier (hissy) than Metal film, and especially the Plate résistors. Concerning the polypro, I always use Panasonic polypro film caps. Love them. But replacing the ceramic caps for film, not sure. Why not using some good Mica, or good quality Vishay ceramic cap?

 

[Gambaholic]

Carbon comp. resistor tend to be a little noisier (hissy) than Metal film, and especially the Plate résistors. Concerning the polypro, I always use Panasonic polypro film caps. Love them. But replacing the ceramic caps for film, not sure. Why not using some good Mica, or good quality Vishay ceramic cap?

Just to be clear it's not that I'm looking for recommendations for my build so much as I'm very interested to hear what conclusions Keith (or anyone who has intentionally built at least 2 different flavors of LA2A) has come to. I'm currently building 2 with 'more or less' vintage components and I'm planning on building 2 more by the end of the year that I will 'experiment' with. I'd really like to hear what conclusions others have come to that have done the same thing. 

Have you tried swapping out the polypropylenes for ceramics to hear the difference? If so what did you notice? Also, I've read that if using ceramics you should try to get ceramics made with NP0 or C0G dielectric. All the LA2A's I've seen use Y5U or Z5U discs so that's another question I'd like to answer with some experiments. It's just a real pain in the neck to keep soldering and desoldering components in a small chassis so the process is slow and annoying. 

 

[dapolk]

Hello, I'm doing a p2p build and have a ? about the PT - I'm using a Triad R-4A - the cayocosta layout, '68 schematic, and UA reissue all show a pair of white/blue and white/blk wires joined to the AC neutral via turret board 3...my R-4A doesn't have this pair of wires.  I have the high voltage CT and the filament CT grounded as shown...what do I do in regards to the AC neutral?

Thanks!

 

[59flame]

I'm working on a la2a build. I need to test my input transformer.  It is an old black ha100x bought used.  I don't have a scope at the moment but can hook it up to an ipod which will deliver about .300 mv to pin 1 and 6. I assuming this is the primary. There is a strap between 3&4  and 8&9. so do i look for a voltage somewhere 10 times as large on pin 7 and 10 which i guess would be 3.3 volts? Never have done this so i need some expert advice.

 

[ilfungo]

Hi
the termination resistor of 600 ohms is only required to connect directly to the converter or LA2A it is also important when the LA2A you insert into a mixing console ?
Thanks!!!

 

[Gambaholic]

Hello, I'm doing a p2p build and have a ? about the PT - I'm using a Triad R-4A - the cayocosta layout, '68 schematic, and UA reissue all show a pair of white/blue and white/blk wires joined to the AC neutral via turret board 3...my R-4A doesn't have this pair of wires.  I have the high voltage CT and the filament CT grounded as shown...what do I do in regards to the AC neutral?

Thanks!

I'm using original R-4A's also. See the picture below. Both black are the same, both red are the same, both green are the same. The center taps go to ground. Reds go to B+. Greens go to heaters. With the blacks I went...
1. Black to small 'AC turret board' turret 1.
2. Turret 1 to turret 2 via buswire.
3. Black hookup wire to frontside main turret board turret 24.
4. Turret 24 to power switch IN via white hookup wire (twisted).
5. Out of power switch to frontside main turret board turret 25.
6. Turret 25 via black hookup wire to IEC socket L pin.
7. Neutral pin to small AC turret board pin 4.
8. Pin 4 to pin 5 via bus wire.
9. Pin 5 to black R-4A wire.

I hope that helps.

 

[deveng]

Just finished a p2p LA2A.  Everything works well except one strange thing.  The unit is 3.5dB down at 20kHz.  I tracked the issue down to the input circuit and found some very odd behavior.  I pulled the T4B and all tubes so the GR and Gain pots are simple resistances to ground.  Then I removed the ground leads from each of these pots (so they're open circuit).  With no grounds attached the frequency response is flat to 20kHz.  Measure point is at post connection of R6 & R7.    If I connect one of the ground wires, 20kHz drops about 1.7dB, when I connect the second ground wire, it drops another 1.75dB  (for the total of 3.5dB rolloff).  Weird.  Wires are good Mogami mini balanced or Canare GS-4 instrument wire.  I can even remove the Jensen 11p4 input transformer,  feed the signal directly to R5 input termination resistor and the problem remains the same ruling out any transfomer issue.  This should be essentially a resistive circuit yet I have some mystery shunt(s) to ground, which work out to be about 140pf per pot.  I believe it may be within the PEC pots.  At first I thought it might be the conductive inner coating on the GS-4 wire.  I didn't know it existed until I visited the Canare site and checked the specs.  I removed that coating but no change. 

I ran out of debug time today, but tomorrow I'll test these pots to see if they in fact have some shunt capacitance. I may just try some cheap Alpha 100k's to see if the issue goes away.  Has anyone ever had this experience with PEC pots? 

Regards,
Jeff

 

[Deepdark]

what transformers did you use?

 

[deveng]

I went for the Jensen 11p4  and 10k61 output.  I also used all 1% metal film resistors, NPO ceramics, poly caps and good low impedance electrolytics.    Filament wire was vintage style waxed push back wire (great stuff).  Most of the other wire was teflon.  So it going to be a "modern" version.  Since I have the HF rolloff I haven't listened to it yet.  THD+noise and IMD distortion measure were good.  No appreciable 60Hz in measures either.  If they sound too "clean" I might change them later.    I've got to get the response rolloff fixed!  Its driving me crazy not being able to run audio through it.  Gotta get it right on the bench first.

My math was close.  I simulated the input circuit with 2 phantom shunt caps.  With a 140pf cap in across each control pot, the simulated response shows the same -3.5dB down at 20kHz.    If its not the pots I'll be very surprised. 

By the way, Martin's case it beautiful.  He really did a nice job on them.  The only thing is its tight working on the wiring inside the case.  I tried to wire as much as possible outside,  then mount and finish but inevitably you have to wire within the case.    Looks great when done though!

Regards,
Jeff

 

[deveng]

Update on frequency response issue.  Its not the pots.  I thought bad pots was highly unusual anyway.  So that left only the wire.  Found that shielded wire no good in this application.  Replaced with tightly twisted 24 awg hookup wires and I get flat response to 30kHz. 

After changing the wire, I read back into this super long thread.  It is mentioned that these wires are not shielded wire, just 3 wire or 2 wire in a single jacket.  If I would have read that first, I would have saved my debug headache.  All good now.

Regards
Jeff

 

[deveng]

I dug around my stash of parts and found 3 vintage 7025 tubes.  One is an RCA long plate, another is a GE and the last is a Amperex.
I initially tested the unit with a Sovtek 12ax7wb (pulled in working condition from my DR Z when I retubed recently).  Today I popped in the RCA.  The Sovtek was better by 0.5dB at 30kHz and had 0.01% less distortion but the RCA was better by about a 0.5dB in noise.  Negligible spec differences.  I'll test the GE and the Amperex, then do some listening tests with each in V1 location.  May the best tube win.

Regards,
Jeff

 

[ubxf]

as a side note yesterday i tried a 6072 (12AY7) in V1 position thinking that it would reduce the gain. it did help a little for the gain but i thought i sounded worse in terms of tone. Sorry not a scientific test but a very notable difference.

 

[gridcurrent]

as a side note yesterday i tried a 6072 (12AY7) in V1 position thinking that it would reduce the gain. it did help a little for the gain but i thought i sounded worse in terms of tone.

it will have little effect on gain due to feedback.
any arrangement of a 6072 in that circuit will manifest greater harmonic distortion.
(full disclosure:  I am a 6072 lover).
the chooser of the 12AX7a knew what he was doing.

 

[ubxf]

Thanks for the feedback it makes sense i read earlier in the thread a suggestion to try 6072 so i did. I use the LA2A as an insert on a DiGI 192 interface and i have to keep the gain way down  (10 or 15) . The sound is fine it just looks odd .