All things LA2a related

[college101]

OK..I just set my meter up on Vdc of R28...and then turned R3(the stereo adjust 1M pot) and geuss what...

I get dc voltage adjustment control over 2 volts of dc...I measured from 320 to 318...and then back up from 318 to 320...Is this correct guys?

Nothing happened with the needle either..still pegged at 20!And I also checked with adjusting R37...no change.... Is it my bad wiring job?

 

[college101]

I think I need to turn my power transformer...Every pic I see online has the PT core set vertically, where I have mine set horizontally... MAYBE this is causing some inductance on to the heater wires and causing the problem?

The next geuss after that would be, my NOS tubes are bad...Even though they are showing current passing, some of the annode and cathode readings are WAY off, or maybe even Not functioning...age is a factor in a tube's life, but it may just be some bad tubes!

I rechecked continuity all over the circuit, and everything seems to flow correctly...where to go from here?

 

[Sredna]

The positioning of your power transformer could affect the amount of hum you'll hear but
this is not your main problem at the moment so leave it for now.

Try to exchange the tubes just to exclude that possible problem.

Watching the pictures it's very hard to follow the connections. I think it would be a good idea to
redo the wiring and try to follow Cayacostas layout more closely. Neat wiring helps a lot when trouble shooting...

Did I understand you right, when you turn the unit the neon light goes off?

 

[college101]

Yes Sredna...you are correct.. the Ne-2(neon) does turn on...and adjust's its brightness when I turn the zero adj pot R4.. I can adjust the Neon from all the way OFF to all the way ON.. BUT-when I adjust R4, the neon adjust's , but the needle does not adjust...? Im still trying to figure it out!

I will re-do the wiring to make things "neat"  but if you look at my post a couple pages back...so of those readings on the tubes are not even at 1 volt...which is what makes me think, the tube is crapping out...

 

[Sredna]

Hmm, I misunderstood you. I thought that the neon goes off when you turned the whole unit!  ;D

The voltage reading you got by R36 (junction r36/C10/V4 cathode) is way too high.
Are you shure that you wired v4 correctly?

 

[college101]

I will check that later tonight once I get home...I followed cayacosta's layout in terms of set up...

The light seems to be in normal operation...but when in GR metering on sw#2-the needle does not move...

I had a 470K resis in there at first, but then i realized that was wrong...bought a new 470 ohm resis- BUT Im not sure if my voltage reading was from before or after the change...

On page 104...I have PIN2 of V4 with a dc reading of 85vdc

 

[Biasrocks]

It's really hard to follow anything in your pictures, it's quite literally a rats nest. If it were me, I would rip out all the wiring and start again. All your heater wires should be twisted together to minimize hum. Try to use like color wiring for each function. One color for B+, one for heaters, one for ground, etc. Make's it much easier to follow and troubleshoot.

Here's my build, while the wire dressing is not perfect it powered up and worked the first time.

Here's a nice example of PTP wiring in this thread.

http://www.groupdiy.com/index.php?topic=43574.msg0;topicseen#new

Go slow and check every connection and resistor value as you go. If something isn't clear while you're building, ask the question here before proceeding. I have a copy of David J's how to book which is very good, if you PM me your email address I will forward a copy to you.

Mark

 

[college101]

Ok..Im gonna re-do the wiring over the next few days for sure

I want be clear though Mark...I am not having any HUM in the unit...none at all...My heater wires are twisted together to the tubes, but not twisted from the PT...as of right now, when I power the la-2a on...it does not hum At ALL...its totally silent...my problem is that, it will not pass audio, and the needle is pegged at 20

I decided to take out some of metal film resistors in the circuit( and replace with CC of the same value), and then re measured VDC at R29..it went up to 356 from 353 after removing to MFR and replacing with CC resis in place, with same value...

 

[Biasrocks]

Ok..Im gonna re-do the wiring over the next few days for sure

I want be clear though Mark...I am not having any HUM in the unit...none at all...My heater wires are twisted together to the tubes, but not twisted from the PT...as of right now, when I power the la-2a on...it does not hum At ALL...its totally silent...my problem is that, it will not pass audio, and the needle is pegged at 20

Good on you for redoing the wiring.

I understand that it's not passing audio. My suggestion is a general one related to building tube equipment w/AC heaters.

Mark

 

[college101]

I just see everybody building 2a's with the UTC input and output trannys...where those trannys have built in grounds that attach to grounds on the chassis... where my Sowter tranny's do not have built in grounds..and do not hook up the same way...

For example...The A-10 Input tranny has #10..which is the bottom wire of the secondary TIED to ground and then the ground is tied to a chassis ground...Seperated by another ground connected to the Neon..

On mine...There is no ground to hook #10 up to..So I have to just tie it directly to seprate chassis ground..which then connects to the neon...

SAME with the Output tranny...#10 on the secondary is supposed to be tied to a ground in on the tranny, on my Sowter 1010, there is just the shield to the core...which i have Wired to the ground of tb#2..but #10 on my set up...goes to a split turret point that one side connects to t4b ground, and the other links to c4 insulated terminal which has the R20 1k connected to Pin3 of the V2...

To me, these subtle difference may be what is causing it to not work, inless you guys think otherwise?

I am still going to re-wire, and hope for the best... I definitly dont want to buy all new input and output trannys

 

[college101]

Wait a second!!  My green wires...the heaters are center taped!( i have the wire CT linked to the ground on the back plate!!) I dont need to have the 100 ohm resistors correct?  Yes /- No?

 

[simonsez]

No, u don't need them... ;D

 

[college101]

I took off the two 100k cc resis

Re-checked VDC at C7B..its at 320.8 VDC...Is this good guys?

I also started checking for AC on the R5,R6,R7 side, and nothing...my meter just drops to .003 everytime...why would that be happening?

 

[college101]

Well...SIGH...DC voltage has come down...still nothing has changed..still wont pass audio...Going to buy some brand new tubes..and then re-check voltages...I am going to re-wire over the next few days...If that doesn't work...Ill be taking it to a electronics guru, because I will be giving up...

 

[Sredna]

When you measure the AC on R5,R6,R7, do you have a signal source (oscillator, CD, guitar, whatever) connected to the input?

320V at C7B is still ~50V too high.

Shield or core connections to ground of in/out transformers doesn't have anything to do with the voltages, niether do carbon comp/metal film resistors. It must be a more fundamental error so a
rewiring could help you to get a better owerview. When you do it, measure every resistor before you solder them in, check and double check. Same for capacitors and correct direction of the electrolyts.

When trouble shooting, try to concentrate on the makup gain circuit, thats the upper part of the schematic from the gain pot right to the output transformer.

When buying tubes, its always a good idea to order some backups as well.

 

[college101]

Thanks Sredna..I got AC readings finally..but only if I did not have the black common connector from my DMM hooked up...IF I had it connected to ground, it would just drop to .003, if I tool it off..then I got readings aroung 3 volts AC in the R5,R6,R7 area

re wire is needed....

 

[simonsez]

re-wiring? then make it look like this...
http://www.groupdiy.com/index.php?topic=22899.msg271707#msg271707

 

[Sredna]

;D Yeah, thought about these pictures too! Überneat!

@college101: It would help us to help you if you could answer the questions that are asked!
Did you have a signal source connected when you measured AC at R5,R6,R7?
You have to connect the common of the DMM somewhere (ie. ground) otherwise you´ll measure random noise.

 

[college101]

NO..I did not have a microphone connected to the input of the 2A when I measured the AC...

When I did connect the common, the ground off of TB#1, and set my Meter to measure AC, and then checked R5,...it would just drop to .003 on my meter...but I did not have a microphone hooked up to it...

I have a tube Mic I built from a few months back, that works, and I have recorded with...to check for signal...

If I need to have that plugged in and ON...I will re-check tonight!

 

[Sredna]

Ok, that explains why you dont have AC at the junction R5/R6.

Connect a CD player or something to the primary on the input transformer and measure again.
Try different output levels from the CD player.

When you reported that there was no output signal, did you have anything connected at the input?

A microphone might not give enough signal on the input depending on the input transformer.
What are the ratio of the Sowters?