All things LA2a related

[kruz]

not much time today to find the solution.. but i did another freq test comparing both C4 fully CW and fully CCW, i can't see any dramatic difference, is this normal??

Also, i've checked the freq rensponse of the A24 as noted in an UTC catalogue, it doesn't look like my bloo output imho

 

[AnalogPackrat]

Something definitely looks wrong. What cap did you use for output (C5)? Can you do another sweep and try to look at the node between C5 and the output iron (and then work your way backwards until you find out where the HF loss happens)?

A P

 

[kruz]

[quote author="AnalogPackrat"]Something definitely looks wrong[/quote]

What cap did you use for output (C5)?

Sprague Atom 10uF 600V (TVA1963)

Can you do another sweep and try to look at the node between C5 and the output iron (and then work your way backwards until you find out where the HF loss happens)?

Will try to do.. but probably on monday

Thank you!

 

[Guest]

heres the sweep i did of an la-2a reissue
a bit low on the volume .

hope this helps

 

[CJ]

Kruz, that cap curve looks about right.

You can put ther probe on the V1 output cap, grid side of V1b and sweep it.

Try it with the neg feedback disconnected also.

Lower your gain pot first.

 

[Guest]

the c4 vari cap seems to do magic on the frequency response

here are some curves of two la-2s

top curve is using arco 465(open aprox 3/4) (universial audio reissue factory setting)

bottom curve is using 390 pf cap

 

[Guest]

and both using arco 465 (3/4 aprox open(eye balled))

frq response snaps back into place :shock:

 

[Scenaria]

you need to mess with that trim cap

take it completely out and see if the curve goes back up..

 

[pahstah]

i just finished my la2a bloo ( for the meter i used sifal al29 but i'm waiting for the beade meter to arrive), adn i took some readigs of voltages, there's some difference with those recommended by scenaria:

with tubes on

b21 318v (sugg. 360v)
b20 252v ( 275v)
a1 48v ( 60v)
a20 77v (131v)
a14 206v (216v)
a17 80v (90v)
a19 109v (105v)

can i put 4b in or should i try to change voltages?

thank you all

paolo

 

[CJ]

Everything looks cool but A20. Might be the tube, might be a component value, might be the schematic, might be reading error, might be wiring error,

you don't need the T4 to listen to the line amp. Put some music threw there.
then, if it sounds fine, plug in the t4.

 

[pahstah]

thank you cj, i did as you counselled and everything is okay: it passes music and it compress as an la2a should, i think; looking at different schematics i've found that r34 is 22k 2w in some and 10k 2w in others so maybe that's the reason for a difference in voltage at A20?
however the bloo kit is really really nice! i'm only waiting for the meter to arrive and then it will be completed!
THANKS!

 

[CJ]

Congrats!

Rock over London.
Rock on Chicago.
Weston-Better Meters For Better Reading

:thumb:

 

[khstudio]

I just finished building an LA-2A with the Jenson mods & transformers & NEED some help.
I'm following the Cayocosta layout & noticed that pin 1 of the 6aq5 isn't connected(to pin 7, like on the scematic). I didn't think too much of it until tonight when I took it to a local tech that helps me out & he checked it for noise, oscilation, etc...
Anywho, the scope kept showing a spike that didn't look normal. We checked the schematic & found the missing connection (6aq5), fixed it and the spike went away. :shock:

Is this a missed error?

Also, could everyone/anyone PLEASE post what they use, or don't use for caps:
C4 & C14?

I also asked if there were any boards left?
& what caps were used (C4 & C14) in the one that "MATCHED THE REISSUE".

??????????????????????


Just to add, I have recieved some feedback on the caps...just trying to gather a little more.

 

[Bluzzi]

Looks like you are right about the 1 an 7 pin on the 6AQ5! I started my first wiring (turret boards) last night 'till 5:30AM. It felt so good to again be constructing instead of buying parts. The turret boards are so simple to do! The circuit boards were looking good but I think I really like wiring point to point. There is something about stripping wire, wrapping it on posts and soldering that I enjoy (man I better get a life!)

I was planning on going point by point comparing schematic to Cayacosta layout. Mine has some parts that are not marked (C5 which I now marked) and there is a 470pf cap going from pin 10 of A10 (Ground) to R28 on the main turret board which is not named on the schematic or the Cayacosta layout (so I think I didn't order damn!).

Can someone else comment on these discrepancies. Once I have finished my LA2A's I'll try and create a new layout and parts list from scratch.

Jim

 

[kruz]

i haven't noticed this 6AQ5 pin 1-7 discrepancy...
mine works without the link.. i've also checked the hi-res photoes at the UA website.. and i can clearly see that pin1 is not connected to anything.
now.. dont' ask me why cause i don't know :green:

 

[Bluzzi]

I just verified and all of the schematics I have which is about 3 variations all show pins 1 and 7 linked going to R3.

Does anyone have an LA2A working with these pins linked?

Anyone making only from Cayacosta's layout would not link but others using schematics may have caught this.

It may be that both work! Would be nice to have a definitive schematic/layout though.


Jim

 

[CJ]

Look up the tube diagram for a 6AQ5.

Pin 1 and 7 are both tied to the same element.

Probably done for mechanical reasons since they had a pin open.

Or maybe to give the P to P guy more layout options.
Or both. People in the old days usually had two or three good reasons for everything. Now were down to one or two.

So use either/or pin 1 and 7. I would use both. Provides more integrity to the tube socket/tube system. Seems to thghten things up, which is much needed for the weaker and smaller 7 pin socket.

Some old Joe's, probably with RF burns on their hands, like to tie down all loose metal so it dosen't float in space and pick up stuff.

But that does not apply in audio land.

 

[Bluzzi]

Thanks CJ, that clears that one up.

Back to finishing my turrets tonight. I can't go further until I get some metal work done and I can't until mid Jan so I will probably solder all wires coming from turrets now and just cut to correct length when I have the metal for mounting the trannies, tubes and T4B.

Any wiring tips? I mean things like keep certain wires away from certain components to minimize hum?

I already know about the twisting the filament feed wires together.

Thanks.

Jim

 

[kruz]

thank you CJ.. as always :thumb:
:guinness:

 

[CJ]

Mechanical tip:

Get all your transformers and tube sockets first. Then lay them on the chassis for a realty check before drilling and punching. And do not drill holes for the T4 octal socket until you have the T4. It is in a square can and looks real silly when sitting at a 45 degree angle. Line the socket up with the can in it, then mark your mounting holes.

IF you use A10 and A24 type transformers, be prepared for some close tolerance aggrevation, the mounting holes are very close to the bolt circle.

I have some templates floating around here for the UTC stuff if interested.