All things LA2a related

[amishsixstringer]

I believe even the original La2as are quite high gain.  Rarely is the gain knob ever past the 12:00 position. 

Neil

 

[stitch-o]

I believe even the original La2as are quite high gain.  Rarely is the gain knob ever past the 12:00 position. 

Neil

Thats still 5 out of 10 and pretty normal to me.

The one I'm building, I can't even get the gain knob past 1.5.
I've read about this problem too, especially between A-10 and something like the Sowter 4383.
CJ suggests: 12AY7 at V1 and 100k for R9 and R13
Volker suggests: 25k pot with 75k resistor as opposed to 100k pot for gain down.
I still think change valuse at R5 and R6...but I very well could be wrong  ;D

Why all the gain down mods if these designs we are doing are part for part the originals?

just curious...

cheers!

 

[amishsixstringer]

After doing some reading, I've found that my meter, in order to read 0dBVU, needs to see 1.228VAC across the terminals with a 3.6K resistor inline. 

I can measure DC voltage and see this voltage move in relation to R1's adjustment, however ACV is extremely low.  9mVAC is all I'm able to get across the terminals at max adjustment. 

I reheated every joint in the meter circuit.  I've tried reversing + and = - just for kicks, with no results.  Checked all the resistors about 10 times.  I took the switch out of the circuit all together.  I'm running out of ideas. 

It's Easter Sunday, so I'm sure that you're all with family and out of town, but I'm posting this anyhow.  Hope to hear from someone soon.  I'm sure it's something stupid, but I'm getting frustrated. 

Thanks a lot guys,

Neil

 

[AnalogPackrat]

The spec you are reading is for signal response (AC input--the music) of the meter.  What the adjustment on you LA-2a is doing is providing a variable DC bias to the meter to offset its position when it is switched to read GR.  Why is this necessary?  Because without it you meter will sit at the left end of the scale and (if you've wired everything up correctly) when switched to read GR it will twitch further left (nowhere).  To set it up, switch to GR and adjust the thing with no signal so that it reads 0dB.  Now when you start smashing some audio with it the needle will swing left and read GR on the negative side of the scale which makes some sense if you think about it (first, it's gain reduction and second there's a lot more scale to the left of 0dB). 

Hook up your volt meter across the VU meter terminals and set it to read DCV.  What do you see?  Does the voltage change when you adjust R1?

A P

 

[amishsixstringer]

Yes, that makes perfect sense. 

With R1 turned all the way counter clockwise, the neon turns off, and I get 17.4VDC across the meter.

With R1 turned all the way up, neon light is on, meter has 55.9VDC across it.

This is the SAME voltage that is across the neon (A1) at any position of R1.

Thanks,

neil

 

[AnalogPackrat]

Yet the meter never moves?  Something's not right.  Maybe the meter is toast.  Can you post a few pics of your build?  Maybe a close-up around the meter switch area?

A P

 

[amishsixstringer]

Nothing moves at all.  I called IMS, which is where I bought the meter.  The guy told me he's never seen a bad meter in 3 years working there.  I thought this seemed kinda silly.  He then told me that checking DC voltage on the meter to set it zero was not correct and that I had to look for 1.228volts AC.  Even when I told him the application and everything.  I felt like an ass to say the least. 

Anyhow, here's a pic I took with my cell phone.  Sorry for the fuzziness.  Thanks for helping.

 

[amishsixstringer]

Is there a way I can test this meter for function?  How difficult would it be to take it apart and fix it?  (assuming it can be)  I've read that some VU meters have some sort of rectifier/diode circuit in them.  Is the Sifam Al39 one of these?  Also, the needle is free to move around when I handle the meter.  It's not mechanically "stuck".  I'm not looking forward to spending another 90 bucks on a meter. 


Neil

 

[AnalogPackrat]

I'm assuming you don't have much test gear around (signal generator, etc.).

Just disconnect the leads from the LA-2a and jumper it to some audio source (mic pre output, CD player, whatever) with a 3k6 resistor in series with one of the leads.  Actually if you just pop off the red wire in your build you can connect one end of the audio input there.  Just make sure you've actually severed the connection to the LA-2a and turn it off--you don't want to accidentally apply 50VDC to the output of your CD player...

You should see the meter wiggle with the music if you've got a decent signal.

A P

 

[amishsixstringer]

I just bastardized a cable and disconnected the red wire and the wire at the negative pole of the VU.  I turned my cd player volume all the way down and soldered the wires up with the 3.9K resistor on the positive leg of the vu.  I turned the volume up a tiny bit and no meter movement.  Before I cranked the volume, I measured about .6VAC across the poles of the meter.  (This measurement was fluctuating fast due to the music being played through).  I increased the volume a bit, still no meter movement.  Then I gave it some juice.  Nothing.  I measured again and was getting spikes of up to 3VAC across the meter.  I'm assuming this means the thing is garbage.  Should I try something else before ordering a new one, or taking it apart?

Neil

(Edit: I tried reversing the poles of the wires from the CD player with no luck.)

 

[AnalogPackrat]

Start a new (hopefully short) thread topic asking about your possibly broken meter.  Most people don't read this thread and won't see your question buried in here.  Hopefully someone will have an idea.  My inclination would be to carefully disassemble and check the diode bridge.  You may be able to fix it if there is a broken connection or bad diode.  I believe these are going to be Ge types (1N270 or 1N34 or similar) which have 0.2V forward voltage.

A P

 

[amishsixstringer]

Thank you for the great advice.

Neil

 

[amishsixstringer]

Alright.  Meter is a go.  Zeros out perfectly.  I checked voltages and all looks good.  Popped some tubes in.  I'm still waiting on a 6aq5a to get here, but I put the other 3 in.  I can get the meter to dance with a mic plugged into it and some gain.  I haven't heard anything, but will throw it on the pro tools rig tonight.  I will be posting some photos up soon.  Thanks a ton guys!

Neil

 

[AnalogPackrat]

Good deal.  The 6aq5 drives the EL panel.  Without it you won't get any compression action, but the audio path should work fine.  So you can at least pass audio through the thing and play with the gain (you won't need much!).  Hope your last tube arrives soon.

It's now time for you to begin your next project...so, what's it gonna be?  1272?  API312?  1176?

A P

 

[amishsixstringer]

Thanks a lot analog.

I was thinking about jumping on that 500 series rack that seems to be happening.  Pultec looks fun too.  What do you suggest?  This was my first diy project that I built from scratch.  I've done a lot of modding and wiring before, but nothing like this.

Neil

 

[amishsixstringer]

The plate voltages on V3 are still high (316V).  I'm assuming this is because V4 is not installed.  These 2 tubes seem to work together in the sidechain.  Should I be worried right now?

Neil

 

[AnalogPackrat]

No problem.  Glad you got it sorted out.  I can't tell you what you should build--it depends on what you have, what you're doing with it, and what you think you want.  I can tell you what I did and why...

I have a DAW setup using a Presonus Firepod and Reaper (used to use Sonar 3.x).  So I had some basic mic pres, but no other outboard.  I also started out with an LA-2a (Bloo kit).  Then I built a pair of 312s and a pair of 1272s to get some old school mic pre flavor.  Then I stalled.  I have a ton of parts for other projects, some of which are partly built: 2x WHAT compressors with DOAs and transformer I/O, 2x 1272 with Carnhill iron, 1x 1176 Rev D, 1x Telefunken V76 with CineMag iron, 2x NYDave one bottle pre, 2x NYDave passive EQ, 2x FETBoy pre, an assortment of line amps for makeup gain duty on passive EQs and summing, scads of DOA boards, and piles of other junque.  I was hoping to end up with a nice set of relatively flexible pres and outboard for small projects that I would then mix in the box.

I think my cat has somehow passed along some genes--I can't stay focused on...hey, look at that ball!...what were we talking about?  I also like naps on weekends.  I keep getting distracted by other projects ranging from home improvement to guitar/bass upgrades/mods to building effect pedals to fixing tube amps to whatever.  So I've not kept up with the latest projects around here--I've got plenty of backlog already.

A P

 

[AnalogPackrat]

B+ will be pretty high until V4 is installed.  It will draw more current than all the rest of the tubes combined which will drop the B+ quite a bit.  Nothing to worry about (yet). 

 

[amishsixstringer]

Just passed audio.  AMAZING.  Very thick just with the amp.  I see why  people track through them even with no compression.  I have a TINY bit of hum with the gain on FULL.  Otherwise it's quiet as hell.  ROCK

Neil

 

[AnalogPackrat]

You will never have the gain anywhere near full in any real situation.  The thing has gobs on tap.  Enjoy your new toy and please take some pics of the finished product.

A P

p.s.  You should call the guy at IMS and give him some sh*t about the broken lead you found.