All things LA2a related

[bobine]

Figured it out, never mind.

share!

 

[sircletus]

Figured it out, never mind.

share!

You bet!

The LA-2A schematic is ever-so-helpfully full of voltage reference points, so figuring out total current draw for the circuit (as well as current draw for individual tubes) is actually pretty easy.  It just took a slap to the forehead to realize it.

The circuit's total current (excluding heaters, VU lamps, etc.) is pulled through R29 (4k7/2W).  Voltage reference before R29 is 350VDC.  Voltage after R29 is 215VDC.  Voltage drop across R29 is therefore 135V.  Using Ohm's Law (in this case, I=E/R), total current draw through the circuit is about 29mA (135/4700 = .0287A).  To bear this out, I pulled R29, put my meter across where R29 was and, sure enough, measured about 30mA on one of our LA-2As.

As another forum poster pointed out, the plate current figures on tube data sheets are "show-off" numbers, showcasing how much current a given tube is capable of delivering.  So if I were to spec a power transformer by adding up all of the plate current specs given on tubes' data sheets, I'd definitely be safe when it comes to picking out a power trafo, but it would a.) likely be overkill and b.) probably require changing a few resistors due to increased voltage coming out of the power trafo if my circuit isn't pulling close to the trafo's specified max current.  This is why voltage vs. current curves would be SO helpful from power transformer manufacturers...

As a side note, Hammond makes a couple of power transformers that will readily accommodate a pair of LA-2A's based on the above calculations (including +20% current "wiggle room"):

The 370CAX is spec'd for 500V @ 81mA, with two filament secondaries, the usual 6.3V at 2.5A (adequate for driving two LA-2As) and an additional 5V @ 3A.  Radio Daze carries this model for $77, though they spec the high voltage secondary at 71mA.  Still enough to supply two LA-2As.

I'm thinking about picking up the 370JX (a new model) to use for a "lab" power supply, as it can deliver 161mA @ 500V and a whopping 3A on the 6.3V winding...

 

[Biasrocks]

I've got an original Beede VU meter here that I intend to use in my LA2A build.

I've got 3 5/8" wide by 2 1/8" tall measuring the meter.

Does anyone know the actual cutout for this meter or the original LA2a panel?

Mark

 

[Biasrocks]

Got her up and running for the first time today, passing signal nicely.

Two small wiring errors, forgot one wire to ground and had the overall phase reversed 180 degrees, both easy fixes.

Getting a good healthy control voltage on Pin 3 of the T4B socket and all the other voltages seem within tolerance.

Other than that everything is looking and more importantly sounding good!

Waiting on a drip T4b and Pete's custom 3U front panel and this guy is good to go!

All in all a really easy project, I was surprised at how quickly it went together.

Mark

 

[millzners]

I heard back from David Geren from Cinemag:

The CMLI-15/15B (wire leads) is $45.02.  The CM-9589H (high-nickel core) is $92.77 while the CM-9589L (50% high-nickel + 50% steel) is $74.04.  See AN-104. 

UPS Ground for one of each will be $9.00.

That really beats the Sowter pricing, even if I use a CM-9589L for the input and output.  I'm going to go for it.

BTW is there a difference in THD/coloration with the high-nickel core vs. the 50/50?  If I buy one for input and one for the output, maybe I'll get one of each just to play around with it.

 

[Biasrocks]

Finished her up today, I'm really happy with how she looks and sounds.

Original UTC & Triad Iron, A-10, A-24, R4A. Found the Beede meter on EBay.

Wired up with the help of David J's excellent How to build an LA2A book.

Front panel from Pete at Grandmaster Audio.

She's dead silent and sounds fantastic.

All in all a very satisfying project.  8)

Thank you for the help!!

Mark

 

[pacemaker]

Hi guys,
i am finishing a D-LA2A with sowter inputs and edcor wsm 10k/600 output,
i'm trying to calibrate the vu meter but when i send a 1V 0DBV from my generator,
and when i set the gain so as i get 0db on the vu meter, i got 2,80v output
is this the normal behaviour of the LA2A, ie can't get unity gain ?
or do i have a problem ?
if there's no problem, is there a way i can increase the meter resistor to get 0db in 0db out on the meter ?
regards,
Francois

 

[pH]

I've been chasing a problem in my new PTP LA-2A. Unit works fine in normal operation, but I've noticed that as I dial up the Peak Reduction past around 2 o'clock the meter starts to show gain reduction, even with no input. Swapped different transformers and tubes with no change. After reading a post on Kenetek's website saying that the newer LSI EL panels are sometimes too sensitive and light up too early I've added series resistance before the panel and the mysterious GR went away. Has anyone else experienced this?

On the original schematic there's a 2.2M resistor (R8) shown within the gain reduction cell, but it's not listed on either Drip's or Silent Arts' boards nor is it inside the Drip T4B that I'm using. Was this resistor in the original T4?

 

[bdubya]

Finished her up today, I'm really happy with how she looks and sounds.

Original UTC & Triad Iron, A-10, A-24, R4A. Found the Beede meter on EBay.

Wired up with the help of David J's excellent How to build an LA2A book.

Front panel from Pete at Grandmaster Audio.

She's dead silent and sounds fantastic.

All in all a very satisfying project.  8)

Thank you for the help!!

Mark

Nice!!  Way to go on the all point to point thing.  Bet it sounds awesome. 

 

[ruckus328]

Hey guys, I'm finally getting around to building up my D-LA2A.  Just curious what type of electrolytics you guys are using here.  My usual go-to's "Panasonic FC/FM, Elnas, etc) don't go up to 450V.  I was thinking of going with the Nichicon VZ or PS.  Just want to see what the rest of you recommended.

 

[Biasrocks]

I went with Xicon 450V's, nothing special.

If you want original go with Mallory TC72 for the 10uf output cap.

I doubt if there would be much of a difference aside from the lightness in your pocket.

$30.06!

http://www.newark.com/cornell-dubilier/tc72/capacitor-alum-elect-10uf-450v/dp/14F178


I went with an Xicon for the output cap and it sounds great.

Here's some notes I had on capacitor choices.

capacitors :

1.) 2  | .1 @ 400v (or 600v orange drops)
2.) 3  |.01 @ 500v ceramic is used in original (Z5U type 1kv) (or 600v orange drops)
3.) 3  |.02 @ 500v (or 600v orange drops)
4.) 1  |.001@ 500v (or 600v orange drops) (reissue is silver mica)
5.) 2  |500 pf @ 500 (for place of 510pf and 470pf caps)(mica caps work here as well)
6.) 2  | 10 uf@ 450v (mallory tc72)
7.) 1  |100 uf@25v (reissue uses 47uf@25v)
8.) 2  |33 uf @450v (reissue uses 22uf@450v in place of)
9.) 2  |47 uf @450v
10) 2  |50-380 pf | arco 465 variable capacitors (surplus sales)
       (or used fixed value 150 pf is a good area for this)

Mark

 

[kazper]

http://www.alliedelec.com/search/productdetail.aspx?SKU=8529089

Save you $10

I used Orange Drops... and for the low voltage Electros FC.

 

[Biasrocks]

Cool, it would be interesting to hear the difference between a generic Xicon and the Mallory part.

Mark

 

[ruckus328]

Thanks guys, it was mostly the 22uF/33uF 450V electrolytics I was talking about.  All I ever use are Elna's or FC/FM so I don't pay attention to what else is good.  I had planned to use orange drops for the films (where they will fit), probably wima polypros for the smaller ones.  Might go with that mallory for the 10uF.  I don't like to get too stupid with audiophile grade stuff, but $20 I could live with.  Now if there were 10 of them, it'd be a different story 

 

[Kingston]

Cool, it would be interesting to hear the difference between a generic Xicon and the Mallory part.

I've done some head to head with different caps in LA2A. The output stage is a great testbed for it: easy to hear the differences.

That TC72 played like a great quality electrolytic should. Oddly enough I could hear very little differences between that and several others I tried, like Rifa's and some random PSU Nichicons I had around. I don't quite understand the Mallory prices to be honest. They are not "audiophile" caps by any stretch of imagination, just oddly expensive but quite generic electrolytics. I'd go for Rifa or similar trusted modern brand who still supply axial caps for an LA2A build. All electrolytics sound a bit constrained and "dull", which is a good thing since you're looking for that sound for LA2A. Film caps sound obviously more detailed - perhaps cold - in comparison, perhaps not a good thing for LA2A.

About arco 465 trimmer... careful with that one in the main feedback loop. It pretty much defines how LA2A top frequencies sound like. I don't know what the material is, but it's very low quality. You can use these for your "historically accurate" but lofi LA2A sound, but I didn't like it one bit. I used them to tune the frequency response only, to find out the exact pF range cap I needed, and then replaced with an equal silver mica.

 

[pH]

If you want original go with Mallory TC72 for the 10uf output cap.

I doubt if there would be much of a difference aside from the lightness in your pocket.

$30.06!

http://www.newark.com/cornell-dubilier/tc72/capacitor-alum-elect-10uf-450v/dp/14F178

FYI, it's quite a bit cheaper here:
http://www.alliedelec.com/search/productdetail.aspx?SKU=8529089

*edit* Kazpar beat me to it!

I've been swapping and comparing components on a few recent LA-2A builds. Like Kingston, I compared some different 10uf electros which to my ears achieved the same end as the TC72 - kinda muffled. So if that's what you want, you can find other electros for cheaper.

In one unit with vintage transformers I'm trying to pull a little more clarity out, so a Solen seemed like a nice fit on that. But the electro might help tame highs with a much clearer sounding transformer like the Jensen or Sowter.

 

[Biasrocks]

Here's a couple of tracks run through my LA2A, original iron, xicon elco's, ptp.

7-10db of gain reduction.

The first note distorts slightly on this track, that's from the original track.

http://sharktankpro.com/GDIY/LA2/Ballad-Bass-Before.wav
http://sharktankpro.com/GDIY/LA2/Ballad-Bass-After.wav

The before track is stereo, after is mono

http://sharktankpro.com/GDIY/LA2/Ballad-Piano-Before.wav
http://sharktankpro.com/GDIY/LA2/Ballad-Piano-After.wav

They aren't level matched, but you'll get the idea

I wouldn't describe this limiter as sounding muffled, I would simply use the word character to describe it.   8)

I suspect that the 100VDC biasing the output elco has something to do with minimizing the sound differences between parts.

Mark

 

[Kingston]

sound differences between parts.

between electrolytics. There were bigger differences to be found between film caps.

Of course it was finally no contest for my build when I discovered russian PIO's. They are a different class. In physical size as well, unfortunately.

 

[pH]

Kingston, which type of the russian caps have you used for C5 (10uf)? I've tried some PIO caps for C2/C3/C4 and really like them, especially with the brighter transformers.

 

[Matthew Jacobs]

sound differences between parts.

between electrolytics. There were bigger differences to be found between film caps.

Of course it was finally no contest for my build when I discovered russian PIO's. They are a different class. In physical size as well, unfortunately.

Kingston, can you tell me where I can get some. I've googled it but not much results.

Could you recomend a reputable European distributer? or do you get these locally?

Regards
JD