All things LA2a related

[Paul B]

Back panel trim pots are full clockwise. Theyre wired correctly. the voltages at V4 are pin 2, 9V, pin 5- 105V, pin 6- 140V

 

[Paul B]

Posted: Wed Jan 19, 2005 1:44 am Post subject:



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.

Here's an old post from CJ. Can you tell me exactly where to measure this?

Paul

 

[AnalogPackrat]

Houston, we have a problem. V4 pin 5 is the plate and this should be around 135V while the screen grid (pin 6) should be lower at around 100V. V4 won't work properly with the voltages you have. Earlier you said:

"Using Sowters, grid stoppers in. Using the 1k, 22k 4.7k config."

This is the older config with R34 = 22k and R36 = 1k, right? You're not using C13 and R38 which were added in a later rev, right? Recheck R34 and R36 as well as R35. Something isn't right with the wiring around V4.

BTW, I built the newer config with the extra stuff on the screen grid of V4 and 10k R34, 470R R36.

As for CJ's comment about tracing the EL panel drive... The AC signal from V4 is on the side of C11 that is away from the tube. Put your meter in ACV, clip the back to ground and probe that end of C11 which should be soldered to pin 3 of the T4B socket. There should be a wire from pin 5 of V4 over to the insulated lug where the other end of C11 is soldered. The thing is, I don't think V4 is wired correctly, so there's no point in tracing the output voltage yet.

Analog Packrat

 

[Paul B]

Analog takes hammer, hits nail on head.

I have been using the 1k 22k 4.7 config WITH C13 and R38 in place!

Are you then saying I either need to remove R38 and C13 or get the 470 and 10k config? If that is the case which would you recommend, or does it matter. I hope this is it... :razz:

 

[AnalogPackrat]

Aha! Yes, you either use the R34 and R36 you chose and leave out C13 and R38, or you swap our R34 and R36 for 10k and 470R and leave in C13 and R38. I built mine with C13 and R38 figuring there was a reason they did the rev. Other folks ahve built them the other way. I'd say whichever is easiest to do at this point... You're almost there!

 

[Paul B]

maybe this is insanity but I have to ask it:

my voltages for pin 5 and 6 on V4 are exactly reversed of what they should be - what if I just conected pin 5 to A19 and pin 6 to A20 - or is that idea completely stupid??

 

[AnalogPackrat]

Nope. Can't do that. V=IR. Do you have parts for the R34 & R36 change? If not, I believe you can just bypass R38 and leave C13 in place to get the thing working. Theres a wire that runs from C7D (lower left one) to R38. Remove the end at R38 and solder it to pin 6 of V4. You can leave R38 in there, but it will be shorted and won't affect anything. Now you've essentially got the "old rev" wiring.

 

[AnalogPackrat]

To clarify my V=IR comment... The DC voltages at those pins are defined by the DC currents flowing through the resistors at each pin (plus a bunch of fun thermionic stuff int he tube itself). Swapping the leads around will result in even wackier voltages and may even damage the tube.

 

[Paul B]

I removed R38 and C13 and I still have the same voltages - 105 at pin 5 and 139-40 at pin 6. :?

 

[AnalogPackrat]

Recheck R35. Make sure you don't have a 22k in there instead of a 220k. For fun, measure R38 which you just removed.

 

[Paul B]

R35 is 220k. R38 measured at 22K

 

[AnalogPackrat]

Strange. With the cover open and the power on, carefully probe DCV at the other ends of R34 and R35. You should get the same voltage (roughly 275V). Have you listened to it since you removed R38 and C13?

 

[Paul B]

they measure exactly the same, 288V. It sounds the same as before - need a horribly strong and distorted input signal to get any reaction out of the VU meter in either mode - with PR at 100 can get the meter to dip 7db, thats all.

 

[AnalogPackrat]

Well, I'm stumped. If I think of anything else, I'll post again. :?

 

[Paul B]

thanks for all your help Analog. :thumb:

anyone else? :sad:

 

[CJ]

too lazy to backpaddle, where are you guys with this?

your signal input level should be like a cd player plugged in. a mic will not give you much gr.

 

[AnalogPackrat]

Paul,

Well, I had a couple of potential ideas. Looking at the currents in your 6AQ5 and comparing to those on the schematic, it seems like R34 may be a problem. If you have a spare 22k 2W resistor (verify its value with your DMM before using it), remove your existing R34 and put in the new one. The other possibility is a bad tube. Do you have another 6AQ5? Another possibility is a weak EL panel, but I know nothing about these things.

Analog Packrat

 

[AnalogPackrat]

Welcome back, CJ. Maybe you can bail us out. Paul has resoldered his T4B pins to eliminate cold joints and intermittents. He also accidentally had a combo of old and new circtuitry around the 6AQ5. Now he's back to the old version with no R38 or C13 (his choice). What we see for DC voltages on the 6AQ5 look wrong to me. The screen grid is at 140V and the plate is at 105V.

Paul has also taken the cover off of his T4B with the unit in place and the power on. Shining a light into the thing made his meter (in GR mode) move much more than his input signal would even with max peak reduction setting. Maybe you're right and he's just got too weak of a signal.

Analog Packrat

 

[AnalogPackrat]

I was just looking back through this thread for clues and it looks like Paul has the same problem that wilebee had back in Feb. Anyone remember what the solution was?

 

[Paul B]

Thanks guys,

I have another 22k2w and will try that. I will also get my hands on another 6AQ5.

As far as input levels go I have used a mic, guitar, (preamped and not, )and CD player. They have all needed a horribly strong and unusable input signal to register anything on the VU.