All things LA2a related

[CJ]

OK, put in just the line amp tubes in (V1 and the 12BH7) and try some audio thru there. Crank it up and see if you like the distortion a little better.

Measure V1 pins 1 and 3, 6 and 8.

 

[AnalogPackrat]

I wouldn't worry too much about CJ's advice until he starts telling you to peel the covers off of your transformers :shock:

I have a hunch what he's up to with the 100k's but I'm not going to risk guessing. You'll have to wait for him to get back here...

 

[Paul B]

With V1 and 2 only, no t4b, the hum is there. sounds about a 120hz. The voltages at V1 are pin1 180 VDC, pin3- 1.57VDC, pin6- 188 VDC and pin 8 - 1.74VDC. The distortion seems a little less harsh maybe.

 

[CJ]

We will kill the hum later.

The 100 K's are just to lower the gain a bit and make the clipping more symetrical. The 220K's clip one half way before the other.

Voltages look cool on V1.

Measure the voltage on R16, both sides.

Thanks.

 

[Paul B]

Youre thanking me!?! Well, you're seriously welcome.

Measuring R16 at B2 on the turret board (the R13,R9,C7c side) I got 255VDC, other side 347 VDC

 

[CJ]

OK, good.

B+ is high because side chain tubes not in yet.

I think were done with the line amp.
Check Pin 3, 1 and 8 on the 12BH7 just to be sure.

Was your VU meter working fine on the +4 setting?
We will get that dialed in next for level monitoring, then thae last thing left is the pesky side chain.

Just curious, where did you get the filter caps C7 * * * *?

What kind of transformers are you using?
(I/O)

Thanks!

 

[Scenaria]

he's using sowters for the audio and an allied for the power

sowters are the reason for the grid stoppers on the 12BH7a.... otherwise theres a lot of unstable oscillation on the output

 

[CJ]

Yeah, I know that, hairbrain!
Tell me somthing I Don't know, alright?
And pass the Hookah. :grin:

 

[Scenaria]

here ya go

:green:

 

[CJ]

That looks like Chappelle's crackpipe!

 

[SSLtech]

That's me in the mirror in the picture, to the left of the camera flash, red sweat-shirt.

I'd just had a hit myself, and had to stand up and walk it off... :wink:

Keef

 

[Paul B]

C7a&b are Xicons, c& d are KyOEI or something like that.

V2 is measuring 129VAC at pin1, 5.26 pin3, 136 pin8

the meter on +4 was the same: barely register from 0-40 or so, need to turn to 70 and above to get to 0db.

 

[CJ]

OK, line amp looks good.

Lets see if we can get that meter to read your output level on the plus 4 db setting.

You have a 8940 with grey hot , pink cold on the 600 ohm out.

Straight wire the meter plus to the grey transformer lead connection.

Minus meter goes to a 3.9k and straight into the pink lead/connection.

Try that with some music running and see how the meter works. We don't care about gain reduction. If your switch already works good in the +4 setting, forgot the above.

We can move to fixing the sidechain.

 

[Paul B]

Something occured to me while I was starting the above instruction. I have one hole and grommet for the output tranny wires. The hole is on the opposite side of the grey, pink and black wires. These wires are wrapped pretty closely around half the tranny before they get inside the box. I imagine its not ideal to have the ouput wires wrapped around the tranny :?

I'm thinking about yanking the tranny, drilling another hole for the output wires and insulating them. Let me know what you think.

 

[CJ]

That would be a good idea. There is not a problem electrically, but if something were to catch those leads, you don't want them ripped out of the transformer.

Will make hookup a bit easier I imagine.

 

[Paul B]

Ill do that later, then. I hooked the output tranny straight into the VU as you said and am getting the same results. Have to turn gain near 80 to get the VU up to 0db on the peaks.

 

[CJ]

What kind of meter?

 

[Paul B]

Its the Beede meter that Steve ships standard (I think) with the kit.

 

[AnalogPackrat]

Scenaria provides a nice Beede VU meter with the Bloo kits...standard A scale.

<edit>

 

[CJ]

Paul, check the ohms on the resistor that is in series with the vu meter.
Disconnect one end of the meter so you don't slam the needle with the ohm meter. (it uses a battery inside for ohms)

Do you have a CD player handy?
Wire one output into the LA2. Might have to hack up an RCA cable.