All things LA2a related

[cayocosta]

My layout had the labels for those two resistors switched, although the placement and values were correct. Sorry if that caused your problem.

I've revised it and uploaded the new revision to CJ's site. It's also here now:

http://home.earthlink.net/~cayocosta/diy_files/la2a/la2a_layout.jpg

 

[sleepingtiger]

Thanks Cayocosta!

Yeah, the instructions are backwards in the Bloo manual for this (page 9) Anyone else doing this kit might want to make a note of this. After I got a little further along in construction I started referring to the schematic more regularly which is what I should have done all along. My bad...

Thanks again Steve, Cayocosta, and everyone else for all of your patient help...

 

[Scenaria]

just to make sure...

on page 9 and 10 in the manual.. do you have this table?



Mount R12 (470k) between pin 8 of V1 and the center terminal of V1.

Mount R14 (2.7 k) between pin 7 of V1 and the center terminal of V1.




Component: Value: Connections: Check
R38 22k V4 (6), V4 (center)
C13 510p V4 (6), R3 (top)
C9 0.02µ V3 (1), R37 (bottom)
C12 0.001µ R37 (top), R37 (bottom)
Link Link R37 (top), R37 (middle)
R20 1k V2 (3), V2 (center)
R12 470k V1 (8), V1 (center)
R14 2.7k V1 (7), V1 (center)
C1 0.02µ V1 (7), V1 (1)

Cause this one has R12 and R14 correct (I think :\)

I think im starting to see whats going on... Out of respect for Ron I never intended on his layout being used in addition to the manual (I know.. his layout rocks!) but I think whats happend a little is that in the bloo manual I used one schematic revision while his layout is using the other... this is causing some confusion for those who are using both the maual and his layout..

I'll re-adjust the manual to follow the layouts schematic if that makes things easier..

 

[sleepingtiger]

Yeah,

I'm more than confused at this point...

[/quote]on page 9 and 10 in the manual.. do you have this table?



Mount R12 (470k) between pin 8 of V1 and the center terminal of V1.

Mount R14 (2.7 k) between pin 7 of V1 and the center terminal of V1.




Component: Value: Connections: Check
R38 22k V4 (6), V4 (center)
C13 510p V4 (6), R3 (top)
C9 0.02µ V3 (1), R37 (bottom)
C12 0.001µ R37 (top), R37 (bottom)
Link Link R37 (top), R37 (middle)
R20 1k V2 (3), V2 (center)
R12 470k V1 (8), V1 (center)
R14 2.7k V1 (7), V1 (center)
C1 0.02µ V1 (7), V1 (1)[/quote]


this is exactly what I have but this doesn't agree with the schematic...

Regardless, I changed the values around and the results were not favorable... it increased the output but distorted it

 

[cayocosta]

"it increased the output but distorted it"

What was your +4 meter reading as it was distorting.

These things will distort if you push them.

 

[sleepingtiger]

It was peaking at about -5 I believe... it wasn't near zero... With the gain knob at about 8o'clock it sounded quiet and awful...

I've still got about 113VDC at pin 8 of the 12bh7a. I figure that's the biggest clue. The other voltages around the tubes are close in line with what they should be now. I'm looking only at the schemo in the back of the Bloo manual at this point. The one that you sent is illegible on my mac.

 

[cayocosta]

The changes are correct, so don't go back to the way it was.

Yes, you still have a problem with the 12bh7. Should be around 1 volt on pin 8.

Trace your wiring coming into the grid - pin2, from R17, also make sure that R19 is grounded with the other end connected to the grid. Check your pin 3 1k to ground.

If nothing turns up, review all the other 12bh7 connections.

For testing, make sure your input signal is not too hot, or too weak. Gain reduction set to off. Gain set to about 10:00. Pots on the back turned full clockwise. Set this way, a good input signal should output around 0vu, in +4 mode.

 

[Scenaria]

hmmm

im looking at the 1968 schematic and it has R12 as 470k and R14 as 2.7k

so does the older one with the jumper plug on the front end...

oh I see.. you mean the pin assignments were reversed

R12 goes to pin 7 and R14 goes to pin 8

 

[cayocosta]

Hold on.

Now that I look at the older schematics, I think there's an error on both.

Pin 8 should be around 100 volts, not 1 volt.

There's no DC source for pin 1 to be 100 volts other than the cathode pin 8, which connects to pin 1 through the 1k resistor. Therefore, if pin 8 is 1 volt, pin 1 cannot be 100 volts.

(By the way, the '68 schematic does not show a voltage reading for pin 8.)

Jahnsen's book also confirms the Pin 1 and 8 voltages both at ~100.

So, your 12bh7 is probably okay.

Check out that distortion though.

 

[cayocosta]

Yes Steve, the layout was accurate - the resistor values and placement were correct - with the exception of the labels (R12, R14) having been flipped on the two resistors. So, if anyone assembled according to the layout's R12 and R14 labels (rather than the resistance values depicted on the resistors) you will need to swap the resistors - otherwise you're okay.

You might want to give your people a heads-up anyway.

Again, the layout's been corrected and revised as of 05/15/05.

 

[sleepingtiger]

This is bizarre...

I'm real confused but I feel like I ought to let you know that I can't figure out any way to keep R17 from smoking at this point...

 

[cayocosta]

12BH7 Voltages:

Pin 6: 220V
Pin 1: 100V
Pin 8: 100V
Pin 3: 4V

These are approximate.

What do you have for these?

 

[sleepingtiger]

I can't tell you. Whenever I turn it on R17 begins to catch fire. From hat I remember the voltages were close to what you just gave before...

 

[Scenaria]

pull out the 12BH7A

does it still smoke?

also double check your values for R15 and R19, R21

 

[sleepingtiger]

Yes, it smokes tube or no...

 

[cayocosta]

Sounds like you've got a wire or two crossed somewhere.

Replace R17.

Before you power up again, really check your 12bh7 wiring against the layout and schematics.

On the 12bh7, you should be able to trace:

Pin 6 - .1 cap - Pin 2
Pin 2 - 470k - Ground
Pin 3 - 1k - Ground
Pin 7 - 470k - Pin 1
Pin 8 - 1k Pin 1

 

[cayocosta]

The 10uf cap coming off pin 1 - is the polarity correct? Plus side goes to pin 1.

 

[Scenaria]

umm

your using the sowters correct?

my guess is you have the grid stoppers messed up.. (maybe)


if you used the 1k's for grid stoppers on the 12BH7A they should be put in series between pin 7 and B8

and pin 2 and A9

just tack the 1k resistor to the pin and then solder a lead on the other side of the resistor over to the appropriate turret on the turret board

if you want.. pull the grid stoppers out until you get i working.. then put the grid stoppers back in (all they do is stabalize the 12BH7A) only needed if you are using sowters...

 

[sleepingtiger]

Yeah...

R15 - 470k
R19 - 470k
R21 - 100k

 

[sleepingtiger]

10uf cap is correct
grid stoppers are correct