D-LA2A Support Thread

[dmp]

Actually what i realise by watching the calibration video is that i have no peak reduction showing

If there is no gain reduction at all, doublecheck everything through the sidechain.
You can also post a good pic of your board and may get more help.

 

[tedsorvino]

If there is no gain reduction at all, doublecheck everything through the sidechain.
You can also post a good pic of your board and may get more help.

Sorted. Actually had placed the wrong way the T4bs sockets. Instead of taking them out ( a nightmarish scenario), just filed them in order to make a channel for the T4b bottom and all done correctly.
As i was expecting it more than 25db of gain reduction shown on the meter and the zero calibration was especially easy.
Now it’s just a matter of fine tuning quality. Thanks for the help Dmp.

 

[justinheronmusic]

Considering swapping out my Carnhill output transformer for an AMI UT24. Will the difference warrant the cost?

 

[dmp]

Considering swapping out my Carnhill output transformer for an AMI UT24. Will the difference warrant the cost?

I used a $20 Edcor WSM for the output transformer, and my LA2A sounds amazing.

 

[justinheronmusic]

I used a $20 Edcor WSM for the output transformer, and my LA2A.

I have experience with the Edcor WSM series and have used them on other builds. I'm glad you are happy with your build.

Perhaps I am not speaking coherently enough. Allow me reframe my question; is the output transformer contributing to the overall tonal aesthetic to the LA2A enough to warrant being picky about the brand/model of transformer used?

 

[flyweight]

Yes, shielded neat cabling is important in a LA2A. The shield of the cable should be connected at chassis ground on one end.
Also when running cable to the front panel switches be cautious of using twisted pair cable as it adds a capacitance across the pot or switch. Better to use single wire shielded cable.
If you used AC heaters make sure the heater wiring is neat (I don't remember how the heaters are done for this build off hand)

Thanks a lot, will check it.
1st the AC heaters and then will redo the cabling with shielded one.
Had really long twisted cables.

Anyhow the left was working fine and I checked the cabling etc.
They are build the same so the AC heater is the 1st one to check.

Unfortunately I cut off most wires from the right so have to redo it anyhow.

Cheers

T

 

[snowow]

Finished my D-LA2A a couple of months ago. Fun build and very nice sounding unit.

 

[dmp]

Finished my D-LA2A a couple of months ago. Fun build and very nice sounding unit.

Nice looking build.
I found that using twisted pair cable to a pot adds a capacitance in parallel with the pot - a treble boost. Cable specs list a pF/foot. May not be noticable in your case, but something to keep in mind.
And if you've connected the cable shields to audio ground and you find the noise/buzz performance is poor, changing all the shield to a chassis ground, with a single connection from chassis to audio ground may help.

 

[snowow]

Thanks for taking the time to inspect my work and give me feedback! I appreciate that a lot!

I haven't noticed any noise and the frequency response seems pretty flat so I probably won't deal with any of your remarks right now. But I surely will take it with my for my next build!

 

[snowow]

By the way, I use shielded cables for all the pots, the twisted cables by the front panel is from the VU-meters & bypass switches.

 

[dmp]

I meant shielded cable with a pair of wires, typically they are twisted together. Hard to tell from the pictures, but I thought that is what the white/grey cable was.
If you used a single conductor shielded cable to the pot and another from the wiper coming back you won't have the issue.

 

[FunkyTek]

yes, it can be hard to find 2cond shielded cable where the two conductors are not also twisted. I found some Belden ultra thin mic cable that is really handy for chassis wiring. Next time I am at the parts trailer, I will look up the number.
Really nice assembly, Snowow. Nice balance of wire length vs tidy routing.

 

[FarisElek]

How important is it to very close to the original capacitance of C113/C213 (C13 in original schematic)? I figure most caps back then are often within 20% of their marked value. I noticed on this board it's 500pf, but on the teletronix it's 510. I accidentally only ordered 1 500pf mica. So to keep both channels even I used two 470pf/630 2.5% siemens polystyrenes I had. I figured they're still within 10%. Think this will he okay in this position?

 

[FarisElek]

But it is possible to steal some voltage from the heaters secondary to do this right? running the relays at 5v ? or i can use a second toroid (have lots of 10va anteks lying around).

Thanks !

I'm using a little 5v antek transformer along with the main transformer for b+ and heaters. I don't forsee it being a problem as I've done the same thing for phantom power on a tube preamp build.

 

[[silent:arts]]

C113/C213: I used 500pf since it was the nearest value available. A few hundreds of D-LA2A builders did too. No complains up so far. I don't think 470pf will be a problem.

 

[flyweight]

Yes, shielded neat cabling is important in a LA2A. The shield of the cable should be connected at chassis ground on one end.
Also when running cable to the front panel switches be cautious of using twisted pair cable as it adds a capacitance across the pot or switch. Better to use single wire shielded cable.
If you used AC heaters make sure the heater wiring is neat (I don't remember how the heaters are done for this build off hand)

Hey thanks a lot, the problem was with the heather wire ring.
At some spot a solder joint was sqeezed true the wire.

I put everything back as before and no hum!!!

Now I can tight things up and later calibrate, add resistors for the response and move to another tube in the pre stage,

Thanks a lot for your hints

 

[snowow]

If I want to use a 12AT7 in V1 do I have to change plate resistors and/or any other components to not stress the tube?

 

[mrclunk]

If I want to use a 12AT7 in V1 do I have to change plate resistors and/or any other components to not stress the tube?

100K plate res with a 12AY7, not tried a 12AT7.

 

[FarisElek]

I did the math and it appears this transformer I'm using for the 5v rails is a little too close for comfort:
AN-0105 - 10VA 5V Transformer[/URL]
so i'm upgrading to 2.5amp version which I believe should be good for the relays and VU lamps, correct me if I'm wrong.

Also, I'm getting distortion when I turn the left channel gain up past 3 o'clock which sends the meter flying even with no signal passing through, I think that it might be a little too close to the VU wire, so I think I just need to zip tie the wires in a more ideal way.
Otherwise, I believe I'm done. It sounds amazing and was mostly perfect at first start-up, with voltages all being within 10-15% of their ideal voltages.
Thank you for the great project!

Ryan

 

[FarisElek]

The more powerful transformer smoked a 1n4007 and is buzzing really loudly in circuit. Can anyone enlighten me on this?