All things LA2a related

[Toobme]

After going through everything, it was a matter of cleaning up a couple of cold solder points on the 12BH7.
I have fairly decent sound going through, testing speech with an omni.
It’s a tad on the warm side if gained over 15 or 20, as others pointed out, but very tasty and warm.

Now, my switch is wired wrong for sure, and I tried doing it again differently, but I’m still just getting GR metering (and that works great), but no +4 or +10 metering.

Here’s my voltage chart now:

V1 12AX7
Pin 1: 114.9 V
Pin 3: 0.987 V
Pin 6: 246 V

V2 12BH7
Pin 1: 128.2 V
Pin 2: 45 mV
Pin 3: 5.2 V
Pin 6: 249 V
Pin 8: 133.3 V

V3 12AX7
Pin 1: 98.3 V
Pin 3: 1.01 V
Pin 6: 98.5 V
Pin 7: 32 mV
Pin 8: 1.011 V

V4 6AQ5a
Pin 2: 4.32 V
Pin 5: 211.6 V
Pin 6: 83.3 V

Neon Bulb: 52 V

After 1n4007: 375 V

Heaters: 3.6 V

That is looking closer now, and my negative value is now positive in the BH7.
Now I just have to keep improving the ground, as there’s a tiny bit of hum when I get a test lead close to the heater wire, while the machine is powered off, but plugged in and I’m monitoring my recorder’s input.

1) DOes the above look correct? Time to button it up?

2) Since I’m not (really) using the input terminal strip, can I simply connect the sowter input wires directly to the xlr pins? I don’t see why not, but wanna check.

3) Speaking of the Sowter trafos, is it normal to need grid stoppers on pin 2 and 7 of V3?

Thanks very much! It’s so close!!!

 

[beatnik]

V1 pin6 looks wrong it should be somewhere around 100V-120V, and around 1V on the cathode (pin8)

V4 pin5 also looks a bit hot, again it should stay around 100V-120V

For the heaters you should have 6.3V AC , either 3.6 is a typo or you have something wrong with the heater wiring

 

[Toobme]

Heater is 6.3, was a typo.
I must have a wrong value of R somewhere, must recheck.

UPDATE:

WTF - Heater is 3.567 V! All the way back to the PT. I hope my transformer isn’t messed already!
Not sure how I did that - looking at 3.567 and rounding up to 3.6, then reversed it in my brain.

Powering from my isolated Variac, coming in at 119.9 VAC.
I note the schematic states 117V, but 3 volts is not going to explain my highs on V1 and V4, nor my 50% heater power.

More time extending the project - more troubleshooting!

 

[fallout]

Now, my switch is wired wrong for sure, and I tried doing it again differently, but I’m still just getting GR metering (and that works great), but no +4 or +10 metering.

There's really not a whole lot of circuirty in the metering section. I'd follow the schematic and check every that connection is correct, cold solder joints, etc. Check the connections coming off of the output transformer and R23. If that's all correct check and make sure the switch itself isn't broken and that it's wired correctly.

 

[Toobme]

I have since ordered a new switch. Mine that’s not working (probably ME) has 12 points of contact and 3 positions of rotation.
I found a Mallory 2 Pole 3 Position switch which looks like the schematic above 8 lugs instead of 12. I’m guessing I could use my current switch, but due to having multiple mistakes in my build, I’m going to use the new one to keep from fathoming anything incorrectly.

Folks, I have and continue to really appreciate your work and comments on this thread! Thanks a million!

 

[fallout]

You can probably use the switch you already have, as long as it's wired correctly. You should be able to figure out how your switch works with your DMM.

 

[beatnik]

What power transformer are you using ?

If it has a center tap on the heater winding, that should be grounded.

Maybe you have grounded the wrong end of the heater winding

With such a low filament voltage the tubes are not warming up and conducting properly, that could explain the high voltages

 

[Toobme]

It’s the Hammond 369JX.
I have the Red/Yellow-stripe and the Green/Yellow-stripe (CTs) grounded to my main chassis ground point close by.
Purple, Blue/Yellow-stripe, White/Black-stripe, Brown/Yellow-stripe, and Black/Red-stripe are all shrink-tubed and tucked under that little turret.
It’s messy word-wise, but here goes for a pic:

 

[beatnik]

Is the heater voltage low even without the tubes inserted ? If without tubes it goes back to 6.3V it could be a damaged tube

Otherwise double check the heater wiring, make sure there isn't a short somewhere

You could also disconnect the transformer heater wires just to make sure the transformer is ok, but to be honest I doubt the transformer is the problem.

 

[JMan]

This may seem like a silly thing to ask, but in the interest of eliminating variables, how are you measuring your heater voltage? (As in, where are you placing the probes). If you’re saying the low voltage goes all the way back to the transformer itself, then it seems to me that there are only two possibilities—the transformer is bad, or you are measuring incorrectly. You aren’t by any chance probing between a heater winding and chassis ground, are you?

 

[Toobme]

Is the heater voltage low even without the tubes inserted ?

Yep, same same.

Going from Cayacosta’s schematic, he’s got green, red, and white all going from secondary PT to the chassis ground point. I’ve got just the green and red with yellow stripes (the CTs of the High Voltage and Filaments) grounded. I’m lost looking for this missing white wire to ground.

 

[Toobme]

You aren’t by any chance probing between a heater winding and chassis ground, are you?

Well, yes - I am indeed measuring that way. I’m guessing that’s incorrect, so thanks for pointing it out.
So I’m measuring HALF of the AC heater circuit that way, right?
I will measure against the two heater pins on one valve then to get the total.

UPDATE: Yes, I’m getting 6.4 on the heaters all around. With and without tubes inserted.
I’m ripping out my wiring for all switches and knobs. Redoing and paying more attention.
Will report back next week.

Patience right?
And I’m thankful for your help, all!!!!!!

 

[JMan]

Well, yes - I am indeed measuring that way. I’m guessing that’s incorrect, so thanks for pointing it out.
So I’m measuring HALF of the AC heater circuit that way, right?

I mean, all I can say is that I made that guess from a place of personal experience. Glad we found the answer to that particular issue!

 

[hayes0444]

It’s the Hammond 369JX.
I have the Red/Yellow-stripe and the Green/Yellow-stripe (CTs) grounded to my main chassis ground point close by.
Purple, Blue/Yellow-stripe, White/Black-stripe, Brown/Yellow-stripe, and Black/Red-stripe are all shrink-tubed and tucked under that little turret.
It’s messy word-wise, but here goes for a pic:
View attachment 104466

Hey just chiming in but do you have a .001uf cap in the C13 spot?
The one from the top 1M pot (stereo adjust) to pin 6 on the 6AQ5 (V4).
I can't really see what you have in the C12 spot on the side chain 1M pot underneath (R37).
Did you maybe put the 510 pf cap there? its smaller, orange and could be obscured by the also orange .02uf cap.
If so maybe you swapped C12 and C13.
To clarify, the .001uf blood colored chiclet cap goes on the lower 1M pot (behind the circle ceramic cap), and the orange tictac 510pf cap goes on the upper 1M pot to the V4 pin 6 (where you have the .001uf cap in the picture).

I don't know if that will solve your problem but double capacitance somewhere and half capacitance somewhere else will prob cause some voltage issues.

 

[Toobme]

Holy crap! I’m at work right now, but will check when home.
I have a feeling you’re right.
You caught yet ANOTHER mistake in my build (or cobble-together, build is too generous now!), so thank you .

As someone stated above, it’s highly unlikely to be the traffo, so you may just have super-sleuthed it!
Love your town by the way!

Update: here’s what I have



- yes it’s as you say, with the 510pf behind my .02M ceramic. And I’ve got the 1000pf connected to V4’s screen.
OOPS - will swap those out and have a test.
THanks so much again, for pointing that out.

Cheers everybody!

 

[Toobme]

Howdy, folks.
I am sorry to post again, but another thing is stumping me (big surprise!).

I’m confused about the Sowter 1449e input tranny.
On the data sheet, the green and grey are listed as secondaries.

I have my green going between R5 and R6, and the grey going to chassis ground.
Is this backwards? Or would it even matter?

Here’s a pic of that area:
Thanks!

 

[fallout]

Sounds like you have the input transformer wired correctly. It'd only affect phase if it were reverse.

 

[Toobme]

@fallout thanks - that’s put that to rest then.
I’m just now on the hunt for less fuzz.
Hum is very minimal, but the detail/clarity is gone if I speak or sing with anything above a whisper.
It’s like internally, between valves, there’s an over-gain somehow.
Again, gain reduction is working great - the reduction curve seems very close to the graph of a ‘68.
It’s just the fuzzines of the sound if I get over a whisper.
Any ideas super-sleuth’s?

 

[JMan]

You may have covered this earlier, I can't recall, but have you tried swapping out the tubes, particularly either of the two in the audio path?

 

[Toobme]

have you tried swapping out the tubes,

I tried the 6aq5a, but not the 12bh7a or 12ax7a.
It’s a good idea, and having a spare set won’t hurt, so I just ordered another set and we’ll see.

You got me thinking, though - the 6aq5a never sits right (square in its socket) and is a little Leaning Tower.
I might replace that socket as well.
Also, my i/o transformers are touching the case and that means the cans are grounded. I thought I’d want that, but could that add noise -mshould they be isolated with a strip of insulation between them and the case?

Really straw grasping here, folks.
But i know it will work one day, with your help (and patience - thanks!) snd my perseverance.