All things LA2a related

[dripelectronics]

I havent tried the edcors yet , taking a look at the diagram now.

funny thing is edcore is here in New Mexico ,
i should support my local economy
maybe ill order some to test them out.

those ct ohm resistors on heaters

could help.


g,

 

[CJ]

Isn't Pin One Ground?

BTW, I went thru the whole thread, here are some pages that might be useful:

Differences between Revs: Page 15
Close up of 6AQ5 stage: Page 55
Voltage and Resistance Charts: Pages 68, 81, and 102.
C4 Trimmer: Pages 78 and 85
Sowter Wiring: Page 102

cj

 

[jho-audio]

yeah, pin 1 is gnd.. the image is incorrect. i have pin 1 to gnd and pin 3 is going to the transformers in place of pin 1.

I spent the time to check out this thread, entirely.. its quite a read. I think I may have retained 1% of what I read at most.

Nice job pulling the page numbers out. Might be helpful to keep that info on the first page or in a meta somewhere as this page is likely to get buried as well.

I'm quite sure that I didn't get my Edcor hookups right when I look at the UTC, Jensen and Sowter connections. Just can't seem to make sense of where the wires should go.

 

[jho-audio]

100 ohm resistors from heater secondary to gnd fixed my hum issue entirely... still getting boat loads of oscillation. using the chopstick to move wires around doesn't seem to be changing the oscillation either. I'm going to re-wire my rats nest la2a and see if I cant figure this out.

Still looking for someone to verify my edcor wiring job.. any insight is appreciated.

Cheers

 

[King_Cruiser]

Decrease R11, 68k on the UREI diagram.
Try 62k.

 

[jho-audio]

Thanks for the tip.. tried a 100k pot to dial in the feedback.. changing this value (ended up at 56k) helped ever so slightly. Still have lots of oscillation. Re-wired and tidied up the compressor, tried the chopsticks again, still didnt help.

Do either of the transformers need to be isolated from the chassis? both are bolted straight to the chassis providing a ground connection.

Not too sure what else to try. Anyone have input on edcor connections? I ended up with the following scheme so far:

input (WSM10k/10k)
- pri #1 to XLR pin 2
- pri #2 to XLR pin 3
- pri CT n/c
- sec #1 to HAX pin 7 connection
- sec #2 to chassis gnd beside transformer
- sec CT n/c
- HAX GND to chassis gnd beside transformer

output (WSM10k/600)
- pri #1 to A24 pin 7
- pri #2 to A24 GND (on board)
- pri CT n/c
- sec #1 to XLR pin 2 and A24 pin 6
- sec #2 to XLR pin 3 and A24 pin 1
- sec CT n/c

also changed R5 to 11k as recommended by Jensen..

Double and triple checked my ground connections and front panel to ground..

Not sure what else to do at this point.

 

[jho-audio]

dont know if this piece of info helps troubleshoot.. the oscillation seems to go away temporarily when a loud source is sent in. As soon as the source lets up the oscillation comes back. Switching from GR to +4db also changes the oscillation. In GR the oscillation is slow and almost at one constant frequency, in +4 it oscillates with shorter wave lengths and a higher pitch.

 

[King_Cruiser]

C4

 

[jho-audio]

Still no go. tried 90, 180 and 360 here.. all have the same oscillation.

 

[King_Cruiser]

LEVEL ONE>

1) Input

A) Short Out XLR Pins 2 abd 3
B) Short Out B Side of Input Magnetics>
-1 Before UREI Sch. R6 68k
-2 After 68k

C) Short the grid of V1 at Pin 2.

2) Amplifier

A) Short Out Pin 7 Of V1

3) Output

A) Strap 610 Ohm Resistor Across Output Magnetics B Side
B) Strap A Side Of Output Magnetics To Ground And Turn Up Volume To 11-Listen For Mechanical Osc.

C) UREI Sch. C5

-1 Outide Foil
-2 Proximity Effect

4) Grounds

A) Resistance
B) Control Panel to Main Chassis

 

[jho-audio]

nice.. thanks! I'll be checking through this list tonight when I leave work.

 

[jho-audio]

1) Input

A) Short Out XLR Pins 2 abd 3

Signal cut.. still oscillates.

B) Short Out B Side of Input Magnetics>
-1 Before UREI Sch. R6 68k
-2 After 68k

Not really sure where to make these connections? short one secondary to gnd? short one secondary to after R6?

C) Short the grid of V1 at Pin 2.

No audio passes with this short in place.

3) Output

A) Strap 610 Ohm Resistor Across Output Magnetics B Side

Same oscillation with minor motor boating.

B) Strap A Side Of Output Magnetics To Ground And Turn Up Volume To 11-Listen For Mechanical Osc.

No audio passes.. Couldn't identify any oscillations.

C) UREI Sch. C5

-1 Outide Foil
-2 Proximity Effect

No changes by moving this cap.. cap foil seems just fine. Cap is new Sprauge Atom.

4) Grounds

A) Resistance
B) Control Panel to Main Chassis

Going to start checking resistance at my gnd points.. ill look through this thread again to find these measurements unless someone has them readily available?

Control panel to main chassis gnd is good.

pin 1 of 12bh7 is 124v, should be 100v.. going to also look into this to see what could be causing it.

Thanks again for the help!![/quote]

 

[jho-audio]

just added a 1k resistor in series to r29 raising the value from 4.7k to 5.7k. This brought my 12bh7 pin 1 value down 10v from 135v to 125v and my pin 6 value down to 260v from 290v.. drastically cut down the oscillations.

I have way too much b+ (316v at r29) and this seems to be the issue. Im going to pick up a bunch of 2w resistors tomorrow between 6.8 and 9k and see if I can get r29 down to 275v from its current 316v output.

 

[jho-audio]

yay.. success!

A 10k 2w resistor at r29 brought my b+ to 270v and my 12bh7 pin 1 to 105v... swapped my rca 12bh7a tube for a new ei 12bh7 and oscillation seems to now be gone.

I feel like a tool for this.. forgot that the edcor power transformer had 550v secondary rather than the 500v expected.

the Edcor in/out transformer hookup is real simple.. ignore the ct's, ground the input to its own ground post and you are good to go.. same connection as in my text post about hookup, ignore the picture. They sound great and are cheap, highly recommend giving them a shot!

Thanks for all the help and to those who made this project possible.

 

[King_Cruiser]

Fantastic

 

[dripelectronics]

GJ :thumb:

 

[emx]

I've got a question about R5. I've noticed that The jensen trans have a different value for r5 when they are used and was wondering if I'm using a similar 1:1 ratio transformer(the cinemag CMLI 15/15/b http://cinemag.biz/line_input/CMLI-15-15B.pdf) if I should be changing the value of R5?

 

[emx]

I guess a better question would be......

What specifically does the Jensen mod address?

 

[emrr]

[quote author="drpat"]What is everbody using for the 10µF 450v output cap? Has anybody played around with different ones to see which sounds best???[/quote]


has anyone tried putting the output cap between the transformer and ground rather than between the tube and transformer? This is rarely seen, but regarded by some to sound superior in some circuits. Just a thought.

 

[dripelectronics]

interesting .

after grounding that cap ,

then what ?

that wouldnt pop the fuse ?

what happens to the output x former , how does it do it's job ?

and since your sending the audio into the case , how do you get it out ?
:grin:

g.