LA2A building thread for newbies

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Arrigotti

Well-known member
Joined
Jun 3, 2004
Messages
187
Location
California
Since I am a Pro Audio DIY newbie, and am building an LA2A, I figured I'd start a thread. There should be some folks building these since Scenaria just finished his first batch of kits.

This of course needn't be the official thread for building these, but since one doesn't yet exist here, I shall start it.

First question: Has anyone tried adding a bleeder resistor in the power supply to drain the caps? This has been a standard feature on the DIY guitar amps I've built. I was thinking of adding a 240K/1W resistor to ground just after the 2nd Filter cap. Any thoughts?

Second question: I am using the Allied $18 power transformer. It has a grey "shield wire". I haven't seen one of these yet. I will guess that is should be grounded to the chasis and is connected to an internal shield of the Power transformer: Yes or No?

Any and all input is appreciated.....Including good jokes.
 
Thanks!

That is what I was looking for.

With regard to the bleeder resistor, I was only thinking of adding it for safety reasons. A cap with 340+ volts stored in it does not exactly make me feel all safe and happy inside.

As for jokes: "I poured spot remover on my dog, and he disappeared." -Steven Wright
 
I am using a .01uF and .02uf ceramic capacitor for the cathode bypass caps of the 2nd 12AX7 to equal .03uf.

The darn things are so tiny, I broke the .02uF cap. I don't have anymore, so I was wondering whether or not I could just use THREE .01uF caps in parallel instead of the .01 and the .02 in parallel.

BTW I am using ceramic caps because I believe I read that other types of caps in this position would mess with the attack or release time.


Thanks
 
Scenario
I just wondered and i think many others too, can you supply the la2a chassis only, it will do in Bloo. I can't see any point in ordering a kit and paying import duty at the front door when i can obtain most of the parts over here.
Vikki(uk) :roll:
 
Hey Vikki,

the hard part is that I never know how much of each to order if I part them out... for every chassis I would sell I would end up stuck with a $100 meter, $50 transformer, T4b etc....

the other part that is difficult is that the chassis is already prepunched to certain dimensions... meaning you couldnt use any other meter in it other than the one its made for... same with some of the other stuff on the back.. the turret has to be the correct size to line up I had to have all of the insulators custom cut and punched in order to line up...
 
I am using a star grounding scheme in my LA2A and was wondering if it would be wise to not connect the A10 and A24 grounding pin to pin #10 since the ground pin is connected to the chasis and this would ruin the star grounding scheme.

Instead I would run a ground wire from pin 10 of each transformer to a local star and then to the main star grounding point (the input xlr ground)

I know Winton mentioned doing a star grounding scheme with success.
 
Here's another one for you.

I have been using Ron's layout (the one with the gray background) and it has helped tremendously. I have also been using a few schematics as well.

There is one part of the layout that I either just don't get, or it could possibly be incorrect: On the limiter response pot, the middle pin is connected to the top pin. It seems from the schematics that the middle pin should be connected to the bottom pin of the potentiometer.

I am probably just all screwed up on this, but does anyone have an input on this?

Thanks!
 
Hi,

The top and middle lugs of the pot should be connected, but I do have a mistake - the 02 cap (c12) from the tube should go to the bottom lug of the pot, not the top lug.

Ron
 
I got my LA2A all together, and no smoke yet. I have checked all the voltages and they all seem within 10% of the chart in Dan Jansen's book except for pin 5 of the 6AQ5a. I have 171 volts as opposed to 100 volts listed in Dan's chart.

I've been checking all my resistors and they all seem correct.

Any thoughts on this?

Thanks.
 
[quote author="Arrigotti"]I got my LA2A all together, and no smoke yet. I have checked all the voltages and they all seem within 10% of the chart in Dan Jansen's book except for pin 5 of the 6AQ5a. I have 171 volts as opposed to 100 volts listed in Dan's chart.

I've been checking all my resistors and they all seem correct.

Any thoughts on this?

Thanks.[/quote]


Ha! That's about the voltage I got from a Spice simulation! I threw away the model because it was so much off from "reality".
Now I'm curious, too ... maybe the Spice model wasn't that bad ?!

JH.
 
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