All things LA2a related

[Paul B]

By the way, (with the same gain setting as above) on either side of C11 I am getting 29 VAC with PR set at 50, 102VAC peak with PR at 100

 

[CJ]

OK, thats promising.

Lets take a few measurements along the way just so we have a referance for future DIY'ers with trouble.

Measure DC volts on the plate of V3, pins 1 and/or 6 (they are strapped) with no GR.

Without any GR, inject a signal and turn up the gain to give a 0 db output on the VU meter set at +4 db.

Then turn up the GR full blast and measure the AC volts on both sides of the V3 plate cap, C9 on my schematic, 0.02 connected to pins 1 and 6 on the 12AX7a.
The 12AX has a gain of about 100, so I expect to see about 10 volts ac on that cap.


BTW, I find it helpful to pencil in these voltage measurements on a copy of the schematic for future use. I circle AC volts and underline DC volts. You got to keep em seperated, like the song says.

 

[Paul B]

V3 pin 1 getting 99VDC. with PR at 100, am getting an average of approx. 2.5VAC on tube side of C11, 4.1V as a peak. on the R37 side of C11, am getting what looks like an average of about 1.5 VAC, with peaks at about 3V.

 

[CJ]

Plate volts look good, 0.88 bias divided by 1000 ohm cathode resistor means 0.88 milliamps flowing thru V3.

0.88 ma times 220k plate resistor is 0.00088 amps times 220,000 ohms equals 193 volts across R33. So 193 plus 99 measured across the tube equals 292, which is I believe close to your suply volts to V3.

Just for the heck of it, swap V1 with V3 and see if you get the same numbers on the plate and cathode (dc) and on C9 (ac).

.

 

[Paul B]

I got 95 VDC at pin 1, .87 at pin 3 and nothing changed at C9.

 

[CJ]

OK, unhook one end of C9, the end that does not go to the plates of V3, and measure the ac volts again.

You might want to hook a 1 meg resistor from the cap to ground to give V3 an ac load during measurements.

 

[CJ]

Paul, I am going to be either on a river somewhere, or sleeping in a tent in the woods for the next three days. So try by passing the Lim Response circuit by connecting C9 cap directl;y to the top of the R3 stereo adj pot. Disconnect c^ also. I think you are losing GR signal inbetween C9 and the output of C11 somewhere, so trace it out with the hi liter.

cj

 

[Jim50hertz]

Hi Guys

A while back i got my Bloo2 up and running but discovered some voltages were off, particularly at A20 and A19

I've had a good look at the circuit, checked all my component values and swapped out the R3 and R37, the 1 Meg pots in case that was the problem...

With the tubes in but no T4B l've still got wrong voltages at A20 (76V - nearly 60V out) and A19 (84 - 20V out)...

B20 reads 275V and I'm using 22k at R34..

Any ideas what may be wrong?

My other voltages are as follows, still out but not as far

B21 - 353V
A1 - 56V
A14 -225V
A17 - 86V

Thanks in advance!

Jim

 

[Scenaria]

Hi Jim,

look this scan over

and tell us which one you wired yours up like...

a lot of guys get different revisions mixed together which sometimes causes odd voltages.

 

[Jim50hertz]

Hi Steve

Ok, this is making a bit more sense now...

I'm using the A10 so i guess i'm looking at the 1979 LA2A but i've got

R7 = 2.7 K
R34 = 22K
R38 = 22K

I'll try swapping these values out tomorrow and see what difference that makes to the voltages, thanks for posting that steve!

Cheers

Jim

 

[Jim50hertz]

Hi Steve

I decided to go for it tonight and swapped out the values, chaining resistors in series to get the right values....

Hallelujah i think we're nearly there!

Got

B21 - 360V
B20 - 267V
A1 - 56V
A20 - 140V
A14 - 219V
A17 - 119V (the only one significantly out)
A19 - 109V

I'm thinking of chucking in the T4B and seeing if she'll pass audio...

Here goes

 

[Scenaria]

dont put the T4b in yet...


just check to see if you have clean gain first


(you dont need the T4b to pass audio) that way if anything is wacked you wont destroy the t4b

by the way your voltages are looking right on target... what were the resistor values you changed?

 

[Jim50hertz]

Ok, it passes audio but i have a weak, buzzy signal...

Also, when i put the meter position to gain reduction it whacks right up to the red... Hasn't been adjusted yet tho

The values i changed were

R34 from 22k to 10k
R38 from 22k to 220k
R7 from 2.7k to 1.2K

Reckon there might be grounding issues, closer listening sounds almost like mains hum, gonna swap out this gain pot cause it sounds noisy too

 

[Scenaria]

triple check your rotary switch wiring..

a lot of people mess up in this area..

becareful with that meter.. they are very expensive to replace

 

[Jim50hertz]

That may be it, i appear to have wired it correctly but positioned upside down

How do i know which pole is Pa and which is PB as i may also have referenced them incorrectly?

Cheers

Jim

 

[Paul B]

I know everyone is on the edge of their seat regarding my situation, so here's what's new:
I havent disconnected C9 yet, but I took new readings with a more controlled input source. With a 1k sine wave, I turned the Gain until reading 0db at +4 setting.
Source at input jack: 7.72 VAC
Output signal: grey (pin 3 of output jack) 105mVAC, pink (pin 2) 1344 mVAC :?
C11, tube side: 11.1 to 75.1 VAC as PR is swept from 0 to 100
t4b side: 11.0 - 74.8 as PR is swept from 0 to 100

C9: tube side 0-2.21 VAC as PR is swept 0 to 100
R37 side: 0-2.08 VAC as PR is swept.

The voltage at the 68k-2.7k juncture at the input tranny (insulated turret) AND at the top of R2, is 223mVAC

 

[AnalogPackrat]

Paul,

What's your new voltage source? Try reducing the input to 1-2VAC. What AC voltage reading do you get at R5-R6 node?

A P

 

[Paul B]

Hi AP,

The signal source is the same 1k tone I was using before. Most of the past readings were with a CD input source.

When I measure straight off the input source I am getting 1.42VAC, which is strong enough to give me a 0db reading with the gain at 44. I get the 7.72 when I measure off the pins of the input jack to ground. That is after going thru a DI box. I'm using the DI box for the XLR out to get into the Bloo, by the way. In any case, I am getting 446mV at R5-6 junction and 213 mV at R6-7

thanks again,
Paul

 

[CJ]

Voltages look good on your 6AQ5 output!

Something is fishy on you input circuit. The Sowter has a 1:4 ratio, so you should be seeing an increase in ac volts as your source goes thru the input transformer. Also, if using an XLR jack, you measure your signal between pins 2 and 3. Pin one is usually the shield.

Try ditching the DI box and just hook up a cd signal straight into the LA2. Measure input vs output on bpth sides of the input transformer. Also make sure your lim/comp sw is wired right.

cj

 

[rove]

Hey I just got my bloo to the testing stage. Plugged it in the other night and powered up. nothing caught fire. Measured voltages (w/o tubes) and they were closer to 400 for both b21 and b20, but I figure that is ballpark. A1 is 59V (IIRC) and I had 7+ V btwn A25 and a24. so I power down throw in the tubes, and power back up, tubes heat up, but then the fuse goes and that is it.
Today I got some new fuses and they blew up ifrst try with out the tubes in. any ideas where to start?
thanks.