All things LA2a related

[Ad0lescnts]

ok I just did this and the buzzing is significantly less. However, there is still some hum and the VU meter still goes nuts when I turn the gain above 6.

what next?

Thank you!
T

 

[CJ]

I have seen this on my LA2 also. Probably ultrasonic oscillation. You might need to tweak your wiring a little. Get the meter to peg then start moving your wiring around with a chop stick. And make sure your front panel is grounded to the chassis if you yhave one of the hinged chassis.
I subbed in an 12AY7 for the AX7a first stage amplifier. Less gain which helped the oscillation, plus you are likely to find a 12AY7 that sounds better than your typical AX7a. More of a vintage sounding tube.

 

[Ad0lescnts]

AWESOME!
you were right, it was oscillating frequencies. I found that the problem was the two wires for the limit/compress switch. It got rid of my squeals at higher gains.

The only problem is that there is sitll some hum, and i found when i put my finger close (not touching) the two posts for the limit/compress switch, the humming goes away. How do i fix this?

Thanks so much for all the help,
T

 

[Ethan]

[quote author="Ad0lescnts"]
The only problem is that there is sitll some hum, and i found when i put my finger close (not touching) the two posts for the limit/compress switch, the humming goes away. How do i fix this?
[/quote]

Make sure the metal of the switch is making good contact with your chassis. If the chassis is painted or anodized you may have to scape off a bit around the hole for the switch to do this.

Yeah CJ's awesome. I bugged the crap outta him when I was building my LA2. I owe the man a few dozen rounds of beer! :sam: :guinness:

 

[CJ]

So you were the guy!

:guinness: :razz:
FYI: keep your grid wires low, next to the chassis, and keep your B+ lines high. Also, try to make the B+ wires cross any grid wires at a 90 degree angle. jeeeez, giving away all my top secret stuff. oh well, getting old. time to download the hard drive!

 

[Ad0lescnts]

awesome!
I looked into all those things and i got rid of all the buzzing. My only concern is that it does get pretty noisy when i turn the gain up all the way, but I was playing with it today and it seems i only really need to have the gain set at about 2 to get the volume where i need it running back into my interface. At this level there are no noise problems. Is this standard practice for these things?

Man this thing is awesome, it's amazing how many different sounds you can get just out of the one peak reduction knob!

thanks,
T

 

[Cainester]

Should the Sowter trannies be mounted inside or outside the chassis?
Thanks,
Caine

 

[Scenaria]

you can mount em on the outside... I dont think theres room on the inside for the output one anyhow

 

[Cainester]

When the circuit goes to ground, does it matter where the ground is tied?
i.e. T2 ground is much closer to the xlr ground than the ground near T4B.

 

[CJ]

I would stick to the stock ground layout. Jahnsens book or see
http://www.vacuumbrain.com/docs/la2.html

 

[Cainester]

Just a couple of solders before completion.. :grin:
Question:
Is C13 required if there will be no stereo couple for now?
I ask because I musta not ordered it..
Caine

 

[CJ]

if thats the cap that goes to the barrier strip, forget about it.

 

[Scenaria]

is that the one going from the stereo trim to the 6aq5a?

I dont have the schem in front of me..

its a picofarad value...

 

[Cainester]

Yeah,
It goes to pin 6 of the 6aq5 and its value is 510pf.
Thats my very last solder point.

 

[Scenaria]

You should put that value in there...

the earlier one used a single 30uf

then they changed it to a 30uf in series with a 1k and strapped the 510pf across em...

I know keef has a theory as to that change and im sure others in here do too...

I think you will be ok powering up to check for proper voltage levels etc.. but dont take my word for it

you should be able to pick that cap value up down at radio shack

 

[Cainester]

I powered the unit without any tubes or T4 and am getting some pretty high voltages.
275 unrectified, and 405 after.
Is this ok?

 

[Scenaria]

a few things for you to check out...

the two turrets on the rear right hand side... be sure you have approx 6.3 volts AC there..



to ground R29 should be around 360V DC (measure the front turret)
to ground R34 should be around 275V DC (measure the front turret)


check across your neon lamp... it should be around 54-60V

you shouldnt be getting 405V anywhere :shock:

 

[Cainester]

I have 405vdc in a lot of spots.

heaters = 7.28
R29 and R34 = 405
Neon = 25.6

The rectifiers both go the same direction right?

 

[Scenaria]

ok power it off...

something is not correct



and yes... the cathodes (the part with the line on it) should be on the turret closest to you... the anode should be in the rear...

 

[cayocosta]

Without tubes installed the voltage will be very high - there's no load.