Old LA2a Information- (Dave Jahnsen writings)

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Unbound

Well-known member
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Jun 21, 2004
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53
Location
Union City, NJ (NYC area)
Hi there,

I started my LA2a project (or at least collecting parts) about 3-4 years ago. Finally have some time to sit down and finish it but....

I had gotten most of my parts based on the Dave Jahnsen manual. I remember at TT someone (a few people) had posted some revisions to his writings and parts. I'm really not going to build it according to his plan anymore, but I thought maybe having the revisions posted here might be good. I still have the Jahnsen manual around here and would like to correct it anyway.

Thanks
Aaron
audiocowboy.com
 
I think these are all of CJ's notes:

This is the first draft of a page by page list of things that might help make David Jahnsen's book on "how to build a Tube Compressor Limiter" a little easier to work with. This is my first take, so expect additions and subtractions. Any input is greatly appreciated!


Page 11 In the fourth paraqgraph down, he says "C12 and C14 are mica caps...." He meant C12 and C13. Also note , these are the "color dot" types shown in the illustration.

Page 12 Diodes: I would add that "Freds" (fast reacting diodes) could be used for the diodes in the LA2a. They generate a little less switching noise than regular diodes.


Page 36 Forth Paragraph: He mentions upgrading some ceramic caps here. I believe that and C8 should remain ceramic. Changing to a film cap might slow down the attack time. Also note that C8 is really two caps. A .01 and .02 connected in paralell.

Stereo coaxial cable is what he is really talking about. And it certainly comes in rolls smaller than 1000 feet!

Second Paragraph: He mention's using XLR jacks instead of barrier strips, but he never tells you where to locate them. Universal Audio uses XLR's that are located on the back of the chassis near the input and output transformers. If you locate your's here, you will have to alter Jahnsen's layout to make room for them. Think this out first!

Pinout info for wiring XLR's to the barrier strips:
Input: Pin 2 of XLR to barrier strip pin 1
Pin 3 odf XLR to barrier strip pin 5

Output: Pin 2 of XLR to barrier strip pin 10
Pin 3 of XLR to barrier strip pin 8

This is for 500/600 ohm input and output.

Pin 1 of XLR's to barrier strip pin 7 which is chassis ground.


Page 42 There should be a listing for a 470 ohm 1/4 watt resistor in the Resistors section.
There should be one less (cnt 4, not 5) 470k 1/4 watt listed.

As Dave mentions earlier, you might want to consider an upgrade to the 10uf/450 volt cap, as it passes audio to the output transformer.

Again, consider "Fred's" for the diodes.

Page 43 Consider Sovtek as a source for 12AX7a vacuum tubes. Their earlier ones were bad, but their latest ones are getting rave reviews.

Page 45 He lists 5 knobs in the Misc. parts list. This quanity will surely vary from person to person. I only used 2. Universal Audio will sell you the originals that they use. Call Erica or Sophia at (831) 466-3737 (USA) . They are $7 each.

Page 46: JBL Professional/UREI Parts Dept. is listed as a source for the T4b. They might tell you where the nearest dealer is, but they won't sell to you direct.

Page 47: Magic Parts has moved. Their new address is:

Magic Parts
1221 N. Mcdowell Blvd
Petaluma,CA 94954
(707) 769-5100

Tech America I believe was bought by Radio Shack. That's where the listed phone number will take you.

The phone number for Apex Electronics has changed. The new number is:

(323) 875-1309

Page 48: Pacific Radio Electronics has moved. Their new address is:

Pacific Radio Electronics Inc
969 N La Brea Ave
West Hollywood, CA
(323) 969-2035

Yale Electronics is no longer listed in the Yellow Pages

Page 52: There is a pinout for the input transformer hiding in the Appendix on page 86. Here is a link that has pinouts for most UTC transformers:

http://www.waltzingbear.com/Schematics/UTC.html

Page 55 The Schematic! Errors that I know about:
R3: The top of the pot should connect to the end of C6 that connects to the barrier strip. It is shown in error to connect to the wrong side of C6.

C13 should connect to the junction of R38 and the control grid of V4. It is shown in error to connect to the junction of R38 and C7d.

There was also some talk on a resistor value in the plate supply being in error. R16 is shown to be 68K, but many people have found this to be too high to give the correct plate voltage on V1. Experiment with this til you get a value close to around 220 volts.

Here is a link to a LA2a manual and schematic which I believe to be correct. It also has resistors values and voltages shown, so print it out if your working on the LA2a. The schematic is on the last page of the manual, page 14.

http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/

Page 58: The layout line on the left side of the page that reads 2.25" is in error. It should read 3.25".

Also, the holes for the pots are shown to be in line,
where on the original units they are actually offset.
The bottom pot is closer to being 2" away from the tube, not 1.5" YOU SHOULD CHECK ALL CLEARANCES ON YOUR PARTS BEFORE YOU PUNCH AND DRILL! Especially between the pots and tube sockets. Punch all your tube and transformer holes before you mount your terminal strips.

NOTE: IF BUILDING AN ORIGINAL CHASSIS, YOU MIGHT WANT TO MAKE IT A LITTLE DEEPER THAN THE STOCK 2 1/2 INCHES. THIS IS BECAUSE THE VU METER YOU BUY MIGHT BE DEEPER THAN THE ORIGINAL , WHCH MEANS IT WON'T CLEAR SOME OF THE COPMONENTS DIRECTLY BEHIND IT!
I learned this the hard way, right at the end of the project when I went to close the front panel!

Also, get your T4 module before you mount it's octal socket. This way you can align the socket and T4 module properly.

Page 59 Most people locate the compress/limit switch to the front panel.

Page 60 Twisted Pair just means stereo coax.
I would insulate the buss wire connections on V2. (pins 1 to 6, 3 to 8)

Page 61 The wires leading to the power switch are shown to be brown. They should be black.

The star ground connections are not shown in the wiring illustration.

Shielded Wire Connections: This should read Hookup Wire Connections, since no shield connections are shown in the illustrations. There is such a thing as shielded wire, but it's use is very rare. It looks just like normal gauge color striped wire, not like black coax. It has a thin shield between the insulation layers.

The blue wire that goes from post 2 center to SW3 pin one is in error. It should go from poat 1 center to SW3 pin 1.

Page 63: The can style filter cap terminals are not labeled.

The filament wires are shown as being red and yellow. They should be white and black.(see page 62 notes)Also, one end of the shield should be grounded to the chassis if you use coax.

There is no coax shown going to the VU meter lamp.

Page 64: R16 should probably be a 1 watt resistor instead of 1/4 or 1/2 watt.

Page 65 The following color codes on the resistors in the illustration are in error:

R5 68k shows brown black orange, s/b (should be) blue gray orange

R6 68k shows brown black orange, s/b blue gray orange

R7 1.2k should show brown red red

R38 is shown to make a long stretch between it's two connecting points. An extra mounting terminal might be needed here.

Page 66 C8 is listed as a .02uf cap. It is actually two caps, a .01 and .02 ceramic cap that are connected in paralell to give the .03 value needed. Most people will probably use just one .03uf cap. Listing C8 as C8a and C8b would have helped clarify things.

Page 67 C8 marked on for two caps. See above.

C7-a,b,c,d are now shown as four discrete components rather than a can type filter cap. I guess this was his way of showing an alternate way of doing things, as most people will have an easier time sourcing the discretes over the can.

Page 71 Star Ground: It might not be pratical to use buss wire for the star ground, as the ground wires will be coming through a lot of high voltage areas. Insulated wire works just as good.

Page 72 If you use regular wire for ther filaments, twist them together to cancel out the AC hum. Also elevate them above the tube socket. Also, the first thing I would wire would be the filaments, since doing it last would make it very difficult.

Shielded Wire Connections: Again, this should just read hookup wire.

Page 73 Fourth paragraph: Most people won't use tube sockets with a ground pin in the middle. Plan accordingly. This means using extra terminal strips for the grounds for these resistors. I like to connect them to the star ground via hookup wire.


LA2a date should read 1969, not 1979.
 
Thanks cayocosta!

It's good to see that info again-maybe this can go in the la2a thread? :green: :guinness:

Joel

PS does anybody have a link to that fancy full color poster/layout that someone made a while back in TT? Sorry-I forget who it was.
 
[quote author="Mbira"]Thanks cayocosta!

does anybody have a link to that fancy full color poster/layout that someone made a while back in TT? Sorry-I forget who it was.[/quote]

I'll give you a clue...

..You just thanked him!!! :green:

Keith
 
I built an LA2a a while back using Daves book, and I remember having corrections and updates written all over it.

That 470 ohm thing was tricky, if I remember right it was a cathode resistor, I had accidently installed a 470K and that sure threw my voltages out of whack. Lucky for me Anthony Demaria was more than happy to talk me through my layout and find all the little problems. He is a really cool guy I met him face to face last week at AES, he was at the Presonus booth. I guess he designed this crazy tube mic pre for them. I looked to be like his version of a VIPre, with the variable impedance feature.
 
> What is that cap numbered 50-380p?

Throw together two transformers and several high impedance nodes, and your 20KHz response can end up -2dB from stray capacitance or +2dB from transformer resonance. And then put this box in line with a whole studio full of similarly sloppy boxes, your overall response at 20KHz may be down (or up) many dB. And in radio the FCC required an annual test proving 15KHz pretty close to flat. Trying to meet that with all them boxes in line was tough. So a lot of radio gear has trimmers so you can tweak-out errors (or "correct" errors in other boxes that can't be tweaked so the FCC sees a good response).

But actually the errors were usually quite small, the trimmer was more of a just-in-case thing. Putting in a fixed value like the suggested 100pFd will probably be flat enough for rock-n-roll, certainly flatter than your mikes or speakers.

If you are a perfectionist, run a response sweep. If 15KHz-20KHz isn't close enough to thrill you, get a handful of 47pFd caps and parallel them until the response is flat enough to play roulette on. This is slightly less convenient than a trimmer, but maybe more stable in the face of vibration. Once you find a "flat" compensation, it should not change even if you change tubes. Changing LDR could have a very-very slight effect, but I doubt it. Changing transformers does suggest re-checking the compensation, but in that case you already have the iron hot.

A too-modern alternative would be a DIP switch with 22, 47, 100, 220, and 330pFd caps that can be switched in or out to get any value. 1957 meets 1972.

You CAN still find trimmers. Actually, this situation can use the main tuning cap out of an old AM radio. As you turn the tuning knob, you see some sheet-metal plates rotate in and out of mesh. Pull that whole part. Problem is: one side of an AM radio tuning-cap is the case, which really wants to be grounded, and I think the LA-2's compensation cap wants neither side grounded. You can, like most wein-bridges, mount the cap on insulators (or PCB-stuff) and use an insulated knob so you don't knock-out the signal while trying to adjust it. Or find an old pocket-radio: they used plastic-case tuner caps.
 
The amount of info I'm getting on this site is amazing. Thanks for sharing the wealth guys much appreciated. Hopefully in time I can give some back.
 
This is about as flat as I can get mine. The top line is zero gain reduction.

la2a_sweeps.jpg
 
[quote author="cayocosta"]This is about as flat as I can get mine.[/quote]

[quote author="slash14"]http://www.audiotester.de/[/quote]

Damn, I gotta get me a PC! :roll:

Peace,
Al.
 

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