All things LA2a related

[Scenaria]

if you are using the 22k/1k config then this...

I just checked one of mine that has a 22k/2w

9.05v

89.6v

and

127v on the grid


remember its an approximation

 

[wilebee]

so I want 127vdc on my grid and 90vdc on the plate ?

 

[Scenaria]

yes... at or about those values...

remember most of those resistors are 5%+ tolerance so the numbers will shift a little....not even counting diff tubes

 

[wilebee]

I guess my confusion is that I have more voltage on the grids that I do on the plate if I have the 22k feeding my plates and the 220k feeding grids then maybe I have tube wired wrong

 

[cayocosta]

Jahnsen's La2a Book references 2 different schematics and shows: 125V/104V, and 100V/70V. Both are plate/screen voltage.

The '68 JBL schematics shows: 133V/100V.

3 different schematics indicating that the plate voltage is above the screen (grid) and the voltage range.

Wilibee, I'd look for 100V - 140V on the plate and 70V - 100V on the screen.

I'd have to remove my La2a from the rack to check the voltage - and that's a real pain so I won't - but my notes indicate that my voltages were:

123V/86V plate/screen.

(That's with the 10k plate and 470R cathode resistor from the 1968 schematic.)

 

[cayocosta]

Schemo Info:

The '68 schematic is the latest one I am aware of. (If anyone has a more recent version, please make it available.)

You can spot an older schematic because it will feature the jumper block on the input.

(This seems to indicate that the 10K/470R 6AQ5 plate/cathode combination is the last revision.)

If the new reissue units are different - and if anyone is interested - maybe someone can have a look and document (and post) the B+ rail details as well as any other significant differences with regard to the '68. We'd then have another viable point of reference.

 

[electronaut]

Has anyone ever done the Jensen LA2A mods explained here? Any comments?

 

[wilebee]

well if i'm understanding scenaria correctly he is saying that I should have higher voltage on my grid then on my plate

 

[Scenaria]

I'll re-check the numbers tonight..

 

[CJ]

There were some stability problems with the 6AQ5, that's why all the fidling with the screen circuit, cathode resistor, etc.

That tube was designed to drive a voice coil, so the EL panel is probably quite a mismatch.

 

[fum]

Ok, and limping across the finish line last in the Great PNW LA2A Baja, here's my voltages:

R29 to ground: 367 V
R34 to ground: 266 V
Across the neon me gots 52.2 V

This is sans T4B. These look ok to me, thought I'd check in, as R34 could be considered a little low (275 was stated V)

Regards

ju

 

[cayocosta]

Ju, voltages look great. My B+ was around 266 also - it will fluctuate with your AC mains voltage.

 

[fum]

Ok, plugged in T4B, powered up. No joy.

Passes audio, meter zeros out, but no meter activity in GR mode. Will have a look see.

Should the EL panel be on all the time, or does it light up when a signal is applied. Presently, we gots no lights in the EL panel when T4B is installed?

Regards

ju

 

[electronaut]

[quote author="fum"]Ok, plugged in T4B, powered up. No joy.

Passes audio, meter zeros out, but no meter activity in GR mode. Will have a look see.

Should the EL panel be on all the time, or does it light up when a signal is applied. Presently, we gots no lights in the EL panel when T4B is installed?

Regards

ju[/quote]

When you fiddle with the Zero Adjust, does your neon light?

 

[CJ]

JU, the EL should flicker with the music during GR. Follow the ac volts from the 6AQ5 coupling cap to the T4 socket. You need about 100 vac to get that thing lit.

Make sure your T4 socket has a good ground. I think it's 2-4-6-8 get grounded?

 

[Scenaria]

this might sound stupid... and please dont take it the wrong way but you did turn up the peak reduction pot right?

I just had to ask as im known to make this ******* mistakes on occassion... usualy following a few pints of guiness

oh and make sure your R37 is turned at the midway point..the same with R3

 

[fum]

[quote author="Scenaria"]this might sound stupid... and please dont take it the wrong way but you did turn up the peak reduction pot right?

I just had to ask as im known to make this ******* mistakes on occassion... usualy following a few pints of guiness

oh and make sure your R37 is turned at the midway point..the same with R3[/quote]

Oh, no. Wouldn't take it wrong. I've been known to do stupid things like that on occasion too. But in this case, I did execute it's full range to see if it would effect the VU. R3 and R37 are at midway. I'll check some of the above after the evening ritual( pajamas, teeth brushin, story readin. X3) :grin:

Another thought. I've got a flourescent on above this when I'm lookin at the T4 panel. Is this a "day glo" kind of light, or should it be more faint?

I know that the T4 socket has a good solid ground, as I was using it to test some of the other grounding (one lead on the T4).

Regards

ju

 

[SSLtech]

It's faint-ish, but visible except possibly in mid-day Florida summertime outdoors blazing sunlight...

You gots a scope to go huntin' with?

Keith

 

[fum]

Yup to the scope, would need directions to the favorite huntin spot tho =)

Look for audio signal being fed to the T4?

ju

 

[CJ]

Yes, and you have to have the cover on to get good GR. Otherwise cells saturtated with light.

Ohm out pin 7 of T4, should go to the 2.7 k/pot jumction. And pin 8 goes to ground. Did you wire up the T4 yourself?

Also, might wanna use a coupling cap on scope probe to prevent killing scope.