All things LA2a related

[emx]

I just picked up a new multimeter and it's getting close to the same readings as the Rat Shack one. Is it normal for Allied power Transformers to be this much higher than the label states?

 

[dripelectronics]

are you using the allied 6k88vg ?

you should show about 278v DC after r29. (4.7k 2 watt)


it's not the tranny , somethings not getting fed it's diet of power.

give us some tube voltages , and perhaps i can help you .

i dont speak bloo , but if you could just shout out the tube pin numbers ,
that would be cool.

g,

 

[nielsk]

I am building a mono only LA2, anyone know what I can leave out? I am looking at R30, C6, C12.....Could R3 be replaced with a fixed value? How much is R37 actually used?

Thanks!

 

[emx]

What if I were to be getting too much current from my AC directly off the wall socket? I'm getting 122/123 V, how much would this affect things?
My power Tran, the 6k88vg, is rated at 117 V and only supposed to put out 500v and 6.3v even though both of my trans I've bought put off closer to 600v 6.8v heater when removed from the circuit? I'll have to get back to you on the tube voltages when I have a minute.

 

[Bluzzi]

I may be mistaken but it seems you may be measuring both windings. Measure from the center tap and see what you get.

jim

 

[AnalogPackrat]

The transformer is rated "at load." The voltage will drop once it has to work a bit. An LA-2a should draw about 15-20mA from the B+ supply. Hook it up and turn it on (with tubes) long enough to get a measurement on both sides of R29 (which will allow us to calculate the current draw).

A P

 

[emx]

Ok before r29 is 398 after is 395v. Also I've just noticed that my neon is only lighting up on one side and not both the bars inside the bulb. I might have broken one side of the neon connection when I moved the turret board. I'm going to check this as well.

 

[AnalogPackrat]

Don't sweat the neon yet. Mine only partially lights up and it still works. Neon is just providing a voltage reference for the GR mode of the meter (sort of like a zener diode works). Anywho...you are not drawing the correct current from the PSU which is why your voltage is high. This is with all tubes in? If so:

1) Verify that R29 is the correct value. With the power off and the big caps COMPLETELY discharged, measure R29 (I think you can do this in circuit as R29 has no parallel path around it).

2) Check your sidechain wiring (V3 and V4). V4 should draw about 12-14mA. I suspect something is wrong in that area as that is the approximate amount of "missing current" if R29 is correct.

A P

 

[emx]

Ok R29=4.6k

I'm not exactly sure how to measure the MA or V3 and V4, which pins am I measuring?

 

[AnalogPackrat]

OK, if R29 is 4.6K (4k6 for the euros), we can use...wait for it....Ohm's Law to compute the current. We know the voltage at both ends of R29, so the voltage ACROSS R29 is 398-395 = 3V or so (your meter may not be very accurate for HV).

V = IR, or I = V/R

I = 3V / 4600ohms = 650uA (less than a milliAmp).

Your neon lights, so we know it is passing current. Looking at the schemo and indicated nominal voltages and using Ohm's (oh, so useful) Law again, the neon needs about 500uA to run. My guess is that you have not completely connected your B+ to the rest of the circuit. There should be a lot of things connected to the node between R28 and R29. Are there? Can you trace along wires from that point to the several plate resistors feeding the tubes (R9, R13, R17, R33, R34)?

A P

 

[AnalogPackrat]

Another possibility is that you have not connected the tube grounds. If the plate Rs are connected to the B+, make sure your have all the ground connections. I think you are missing a single wire somewhere (B+ or ground).

A P

 

[emx]

Ok, I had to pull out the turret board on my unit as it was obstructing my view of the pins on the valves. In doing so I noticed that one wire in particular came off of (I think it was) pin 3 on V3 very easily like it was almost not even soldered on completely, this was connected to one of the turrets on the board that I noticed was a lot lower on voltage than it should have been unfortunately I couldn't correct this without removing the turret board. I'm going to get back to the forum when I have completed putting it back in shortly.

 

[Bluzzi]

I've been using my first LA2A a lot lately and although I sub'd a 12AY7 for a softer gain its still touchy.

A.P. suggested using 100K for R9 and R13. I haven't tried that yet and its the easiest thing so I probably will do that first.

The other thing in my mind is to use a lower gain Input Xfr. I believe the A10 is 1:10 right? Wouldn't using something like a 1:3 or 1:4 reduce the gain substantially and make the Gain knob's travel useful throughout its range?

The Jensen transformer JT-11P-1 is a 1:1. Would that be a drastic change?

I love my LA2A when its tame. However with a singer that has poor mike technique the high level at input causes distortion. So far I've been lucky as its a Blues project I'm doing and it actually suits the singer and style.

I guess what I mean is how do I reduce the distortion at input. I'd still like to have it available but only when I twist the Gain knob past 2 oclock maybe.

Just trying to find that perfect blend before I undertake finishing my second LA2A.

jim

 

[dripelectronics]

i found that using an alps blue velvet really helps ,
it has a nice slow log.

found that i could dial in with more precision .

 

[NewYorkDave]

With a line-level input signal, there's really no good reason to use an input xfmr with such a high ratio. Use a lower ratio--even 1:1--and you'll be happier, I'm sure.

 

[Bluzzi]

Thanks Dave. Thats what I figured since some have used the Jensen ones at 1:1 and I didn't see any complaints from them. I'll have to get one and try it out.

If you notice though most of us using the originals find the input too strong.

I'll be renting a new LA2A during the holidays to try and see any differences.

jim

 

[emx]

Ok Repatched my turret board back into the unit and I caught several things as I did. Initially I had opted for a more functional mounting of the turret where it was facing directly out of the unit facing forward, this mounting proved to be not so much of a good idea and I then mounted it like everyone else here does. In the process of moving it I guess 2 wires came loose, I fixed this. Also I just found out why some things weren't grounding properly, the turret's I bought to use as grounding posts didn't conduct well I ohmed them out at anywhere from 22 ohms to several hundred in the batch as I just now checked them. Also some of my pots didn't quite ground out right even though I dremel tooled out the inner hole(Next time I'm not screwing around, it's star washers everywhere. All my voltages are now roughly in check give or take about 15 or 20 % -------except , not quite sure about my neon. My neon doesn't light unless I turn R4 up almost half-way and I'm getting anywhere from 17 to 57 volts off of it depending on how R4 is set, is this normal?

 

[dripelectronics]

your neon is ok , it does that .

your voltages can fluctuate alot , 15-20% can be ok ,
but best idea would be to list all voltages of each tube ,
the orginal resistors were plus or minus 10%

given the carbon resistors , i imagine 20% is ok too.

we can go over them with you.

but it does sound like your on the right track.

best.

g.

 

[emx]

Ok here is my turret voltages, I'll get right back with the specific tube voltages.
..............................................................................

A1-17 to 57 volts
A2-266
A3-108
A4-113
A5-1
A6-0
A7-109
A8-109
A9-.002
A10-107
A11-217
A13-0
A14-217
A15-.003
A16-7.5
A17-82
A18-0.834
A19-133
A20-116
A21-275AC
A22-279AC
A23-279
A24-3.2AC
A25-3.2AC

B1-17 to 57V(depending on setting of R4)
B2,B3,B4-266
B5,B6-18
B7-111
B8,B9,B10-70
B11-2.5
B13-0
B14-265
B16-0
B17-265
B18-0
B19,B20-265
B21,22,23-370
B24,B25-123AC

I'm doing the Cayocosta Layout with the 10K R34's and 4.7K R29's.

 

[emx]

Ok here is the tube voltages.....

V1
____

#1-108V
#3-1V
#6-113

V2
____

#1-109V
#2-0.02
#3(through 1kResistor to TurretA6)-0V
#6-220
#7-70
#8-111

V3
______

#1 and #6-82V
#3 and #8- 0.834v
#7-0.003


V4
_____

#2-7.3 to 7.5V
#5-116
#6-133