All things LA2a related

[khstudio]

Obviously we would all like an original to compare to.

CJ kind of backs up what I said in my post. All the variations are cool for something. Mellow vocal & bass was definitely what I got WITH C4 set with a -10 db roll off at 20k & C4 installed with a value of aprox. 330pf. In some ways I liked it better...On other material it was too dark.

But I think this is where the switch idea comes in.

CJ, for those of us who want to step outside and add a mod switch to these caps, what would be the best way? What you said was cool but you really didn't answer the big question. Come on man...I know you got it in ya
:green:

 

[CJ]

If you really wanna have some fun, put a pot in place of R11 68 k feedback resistor.

 

[khstudio]

Out of the 10 guitar amps I've built, I put a negative feedback control in 4 of them. I like having control of the neg feedback, especially in my deluxe reverb. Its more touch sensitive & responsive but also noisier & overdrives faster...but nicer & smoother.

With that said, are you pulling my leg or is it something cool in the LA-2A?

Will it respond like my Guitar amps did?

Also, could you please help with the wiring type for my D-esser (type) switch. I'm not having any noise problems & don't want to.

The bottom line is I like it both ways and would love to have a switch on it.
I really what to make sure before I drill into the front of this thing.

 

[keithcamilleri]

hi there,

I tried passing audio trough the la2a ...sounds really good did not put in the the t4b still since i m not very happy with my voltages is there anyone that could point me in the right directon...

a20 is way too high its 240v
b20 is around 300v
and pin 5 on the t4 is around 52v

is there something wrong?

regards

 

[dissonantstring]

hi keith,
i'm a newbie so these are just suggestions of what i would check.
first, IIRC CJ suggested in this thread that tube cirucuit voltages can be within +/-20%. i'm not sure if you are using 10K/2W for R34, but if so, maybe use 22K/2W as suggested early on in this thread. but before that maybe you should check to see if your diodes (CR1 & 2) are in the correct orientation and that they are doing their job. please remember to power off, unplug and discharge PS caps for safety's sake before doing anything.
sorry, like i stated i'm new at this so i don't know if this will actually lead you to the real problem.
hopefully our resident LA-2A geniuses (CJ, SSL TECH and Scenaria) will chime in. let us know how it goes.
regards,
grant

 

[CJ]

Keith, if no smoke, you are half way there.

I still do not have Steve's wiring diagram, so you will have to relate the A20 thing to an actual point on the schematic.

Pin 5 on the T4 will drop when you plug that thing in.

 

[keithcamilleri]

thanks dissonantstring I 've checked the diodes and they were all ok I appreciate your help a lot.

The back row of the turret is A and the front row is B. The neon is connected to A1.

Thanks CJ , A20 is where the 10k/2W resistor on the turret board is placed the point I am measuring is the point where there is a wire connected with pin 5 of 6aq5a. the voltage is 240V. instead of 10k I am using 22k and before the resistor I am measuring 320V.

I've turned SW2 to Gain Reduction and the meter went into full scale output +10 and output +4 did nothing...

Do you think there is something wrong with unit?

regards

keith

 

[CJ]

Looks OK.
Measure pin 2 on 6AQ5 which is bias which tells you the health of the circuit.
s/b around 7.5 dc.

 

[keithcamilleri]

thanks cj for yor reply what is the expected voltage for pin 2 ?

 

[CJ]

7.5 vdc

 

[CJ]

he is a link to a voltage chart - your mileage may vary 20 percent:

http://vacuumbrain.com/The_Lab/TA/LA2/la2_voltages.jpg

 

[keithcamilleri]

hey cj not looking good the voltage is measuring 2.2v dc is there any test u might suggest so that i would not fry the t4b

 

[CJ]

pull the 6aq5 out.
read the label to make sure it is a 6aq5.
is it new?

unplug unit.

set meter to ohms.
check ohms of r34, r35 and r36


if ok, fire up unit without 6aq5

measure dc volts on pins 2, 5 and 6

report back if still alive

 

[khstudio]

HELP.

Is it ok to use ceramic-wirewounds in the PS divider circuit?
I read that these can cause oscilation or other problems.

I originaly installed the 2w metal oxides recommended in the parts list but the tech I took it to said they aren't rated for that high of voltage & when he checked it it measured 5.5k instead of the 4.7 I measured when I installed it. He said it couldn't handle it and was caused by the heat.

Whats the story here.
2watts:
4.7k
10k

Also, should R38 - the 22k feeding the 6AQ5 wattege be increased a little above the recomended 1w metal film?

 

[keithcamilleri]

still alive :grin:

well the 6aq5a is brand new

pin 2 is 0v
pin 5 is 324v
pin 6 is somewhere around 48v

i am in pain i heard the a24 and a10 working now and i need this baby to start working

 

[khstudio]

CJ,
What should I do about the C14 switch?

 

[CJ]

Keith, your screen voltage is too low. This is why the tube is drawing so few ma.
Increase the screen voltage to 135 volts somehow. Which screen circuit are you using? There were several different versions over the years.

If you increase the screen, you lower the internal resistance of the tube, which will increase currrent which will bring your cathode bias point up, as well as bring the voltage measured dirrectly of of the 6AQ5 plate down to a more nominal value.


Sandblock resistors are fine for the power supply. Thats what they were designed for. If your caps are in good shape, what could those resistors possibly do to cause trouble?
Not enough inductance to resonate correctly with the caps and power transformer.

What is the C12 switch? Are you building some sort of eastern troublemaking device?


Hmmm, maybe I f'ed up. You should measure the screen voltage with the tube installed.
Otherwise, the internal resistance of your volt meter will just be forming a voltage divider with the plate and screen resistors.
So plug in the tube and see if it changes.

 

[rove]

How would I go about adding a light to the beede meter on my bloo?
thanks.

 

[Rafita]

Hi first post asking help.

im doing the basic testing on the bloo and A19 and A20 read diferent

im doing the test with tubes in

A24 -A25 = 6.72 V
B21 = 359 V
B20 = 281 V
A1 = 54 V
A20 = 92 V * Should be around 131v *
A14 = 227 V
A17 = 88 V
A19 = 122 V


R34 = 22k /2w
R35 = 220 k

thank's

cheer's from mexico

 

[Greg]

[quote author="rove"]How would I go about adding a light to the beede meter on my bloo?[/quote]
What I've seen is to get a 6VAC lamp (or close to that) and just hook it up straight up to the 6.3VAC coming from the secondary of the power trafo. This is what I'm planning to do, but haven't found the lamp yet. Or the lamp holder as well if I can find one. Gonna have to search around.