D-LA2A Support Thread

[herrmann]

Carnhill VTB2291 : ungapped, 9600:600

Ok, I guess it will work for the input too ? (reversed)

 

[Randyman...]

I've got 4 of the original LA2A Arco 465 Trimmer Caps on the way for C104/C204 and C114/C214.  Will these fit directly into the PCB - or will I need to get creative to mount them properly?  I also planned on using a bournes 50K trimmer resistor for R25 (or whatever the resistor is that needs to be set for the T4B - most people use a 33K IIRC) to get this thing as tweaked out as possible.

Is there a calibration procedure somewhere?

I've got the main PCB stuffed (heaters wired, too) - so just waiting on the transformers and chassis before I power her up for the first time!  Any suggestions before I slam HV into it for the first time?

Thanks!  8)

 

[audiophreak]

Hi,
    were did you get the Arco 465 Trimmer Caps ?  I've been looking for any trimmer cap, but cant seem to find any in the pf range needed - ( 1-120pf to 330-500pf )  I.ve found a couple part numbers from Data sheets, but when I call or send email, they don't have them in stock and 3-5 MONTHS to get them .

 

[Randyman...]

Azone's BOM has them listed in the "Substitution/Upgrade" part from surplussales.com :

http://www.surplussales.com/Variables/Compression/CompCap1.html  (scroll down)

It's the Arco 465 #CTM-465 according to Azone and a few others...

Any tips on a calibration procedure?

8)

 

[alexc]

Carnhill 9600:600 reversed would 'work' for the input - sure
But it is not really meant for the role of an input transformer, being unshielded, quite large
and of a power rating more suited to an output. There's probably other reasons too 

I think it would be preferable to find a shielded input transformer of 600:10K or 600:15K
or even 600:50K as I have seen. My pref would be 600:15K.

But it would work. As do the Edcor open frame unshielded transformers.

 

[audiophreak]

Thanks Randyman,
                              I did check those last week, but they listed them as " Out of Stock "  -  as for calibration ..  check pages 12 , 13 of this thread, I'm sure there's more but that was all I could find right now. hopeing to get mine together over the next month or so ...  good luck with yours, keep us updated 

 

[helterbelter]

I'm pretty much interested in buying the carnhill for the output too.

Which inputtransformer do you use ?



And a shameless bump for the question I asked earlier today ! I have checked my build again today, but didn't find the cause.

Hi all,

I "finished" my La2a about 6, 7, maybe more months ago. (yes, a D-LA2a board, but just one channel populated).

Back then I've tested it and it works, but it doesn't pass low frequencies. At that time I didn't have much time to continue searching for the mistake, and I kinda thought the Edcors (WSM 15k/600 and 600/15k) were to blame for the lowcut, but by now I think this isn't the reason.

I've read the whole thread again, looking for similar problems, but I didn't find any. Does anybody here have a clue what might be wrong ? I'm 99-100% sure there's no short somewhere, and all voltages measure fine. (I had another problem to solve earlier, so I already checked this stuff).

 

[Randyman...]

Thanks Randyman,
                             I did check those last week, but they listed them as " Out of Stock "   -  as for calibration ..   check pages 12 , 13 of this thread, I'm sure there's more but that was all I could find right now. hopeing to get mine together over the next month or so ...   good luck with yours, keep us updated  

I'm not sure if they got more in (I think these are old stock) - but I know I have (4) on the way to me (in transit).  I believe they still have some left...

Re-Reading the calibration stuff on Page 12-13 - but still don't see many specifics.  Some links are dead (like the UA link!).  I'll keep searching...

8)

 

[[silent:arts]]

I've got 4 of the original LA2A Arco 465 Trimmer Caps on the way for C104/C204 and C114/C214.  Will these fit directly into the PCB - or will I need to get creative to mount them properly?

You need to go the creative way
I used fixed values, since those Arco trimmers were impossible to source.

I also planned on using a bournes 50K trimmer resistor for R25 (or whatever the resistor is that needs to be set for the T4B - most people use a 33K IIRC) to get this thing as tweaked out as possible.
Is there a calibration procedure somewhere?

SSLtech wrote an excellent LA2A calibration procedure somewhere in another thread.

I've got the main PCB stuffed (heaters wired, too) - so just waiting on the transformers and chassis before I power her up for the first time!  Any suggestions before I slam HV into it for the first time?

if you are worried leave the T4Bs out the first time.

 

[[silent:arts]]

...
Back then I've tested it and it works, but it doesn't pass low frequencies. At that time I didn't have much time to continue searching for the mistake, and I kinda thought the Edcors (15k/600 and 600/15k) were to blame for the lowcut, but by now I think this isn't the reason.

http://www.groupdiy.com/index.php?topic=26119.msg429195#msg429195

 

[alexc]

I used Jensen 1:4 inputs  37.5K:2.3K wired backwards if I remember correctly.
They were a deal from a forum member. Very transparent and very quiet.

My other one uses a Sowter 4383 600:10K 1:4. It's also very clean and quiet.

If I was doing another, I'd probably use the Sowter again.
Or try and find an Altec or UTC maybe.

I like Edcor but not for inputs  - they can pickup a little too much RFI for my liking.
You can move them away from the psu and all that, but even so - haven't had to do this with
shielded input traffos.

I started the DLA2A and also DOAC with Edcors but changed them to the Jensens for that reason.
Depends on how much money you want to part with

 

[alexc]

To track down why and where you are losing low-freq, I would try and trace the signal
with a cro and a signal gen set to a low frequency. If it is noticable by ear, it should be
visible on the cro.

The LA2A is not all that complex. Excluding the sidechain
    input traffo -> attenuator  -> gain pot -> 12AX7 gain stage x2 -> white cathode follower stage -> OP traffo

You could also rig up a cheap audio probe going to cheap computer powered speakers and follow the chain,
making sure you probe the low DC voltage points ie. *after* DC blocking caps.

Listen for where the low freqs disappear. Do some searching for a suitable audio probe.

*** As always - be very careful probing High Voltage circuits. If in doubt, don't do it. ***

My guess is the usual suspects : wiring errors - check the wiring of the input and output transformers,
component values - check caps and resistors. Also try tube swapping. Faulty tubes do occur, particularly
in the first 12AX7.

Another thing to do - use a soundcard with a spectrum analyser that can do a frequency sweep

Good luck!

 

[helterbelter]

http://www.groupdiy.com/index.php?topic=26119.msg429195#msg429195

Yes, that's why I suspected the trannies.  However, mine are not the XSM, but are WSM, as recommended by SSLTech. I will follow Alexc's lead first, after that, it'll be transformer swapping time.

AlexC :

I'll swap the first 12AX7 to begin with, if that doesn't work,  probing it will be the next step. Not really looking forward to it with these high voltages, hence the question where the cause might be. So, thanks for your suggestions,

Thanks for the replies !

 

[herrmann]

The VTB9071 10k:10k may be more adequate ? Then why not adding a 3PDT to lower the input impedance at 2k5 ? Maybe it's a stupid idea, don't know...

 

[alexc]

10K:10K may work - I haven't tried it.

I think the step up in voltage provided by a 600:10K  or 600:15K is neccessary in order to ensure sufficient signal level to drive the T4B into compression.

I wouldn't recommend it, but I seem to recall reading about it somewhere.

 

[alexc]

I used the XSM  Edcors in this build and didn;t notice any excessive low end roll off.

it does help to do a sweep to determine how rolled off and at what freq.

-3dB at 20Hz is a standard measure of the low freq cutoff.

I would regard anything more than -3db at 40Hz to be an issue, usually meaning the
transformer is not well matched to the application.

I don't think you would hear it until it was > -3dB rolloff  at 80Hz or so.

Some points regarding probing tube circuits - never hurts to reiterate!

With respect to probing, take your time, know in advance which point your are going to examine,
then  power up, do the measure and power down.

Always keep one hand in pocket to avoid the possibility of an across-the-chest shock.

Aa always, make sure the main PSU capacitors are drained EVERY time you power down.
It's a discipline - a power down 'make safe' routine.

It is useful to keep a multimeter attached to the psu caps always on indicating the voltage.
I use a second one as a 'double check' every time I'm probing.

You may need a grounding probe with resistor for the purpose of draining the psu caps.

That aside, probing should show you very quickly where the loss of bass occurs.
There's only a couple of points to check :

- farside of C1/grid of 12AX7 #1 (V1 pin7)
- farside of C2/grid of 12BH7 (V2 pin 7)
- farside of C5/primary of Output traffo

If it's not at any of the points above, it's in the input side of the tubes - the input traffo or T4B.


Let us know if you have questions and good luck

 

[herrmann]

10K:10K may work - I haven't tried it.
I think the step up in voltage provided by a 600:10K  or 600:15K is neccessary in order to ensure sufficient signal level to drive the T4B into compression.

This is actually the "jensen mod", but I can't find any comparaison review. Also on ebay there's some cinemag 15k:15k that are selling as LA2a input transformer.

 

[drgrateful]

Hi all,

I would like to wire two 12v VU lamps and one 6.3v on/off bulb. If I got it right, I see that it is OK to wire the VU lamps to the fifth secondary of the Rondo transformer (9vAC), as long as the in/bypass relays keep working, since this secondary is not in the audio path: is this correct?

Now, what is the best way to wire the 6.3v on/off bulb? I don't think it should be wired directly to the third or fourth secondary (both 6.3vAC) of the Rondo transformer, correct?

Thanks, Matteo

 

[Soeren_DK]

Ready for take off

Anything I should be aware of before power up it up?
Should i leave the T4 out at first? and what about tubes?

Anybody have some voltages around the board the I can compare to?

Cheers
Soren

 

[alexc]

I usually check the basic voltages of heater, B+ at the tube sockets and t4b socket before adding tubes and t4b.

Then power up briefly - look for smoke release. Check those basic voltages.

I like to wear some eye and ear protection when I first power up a tube unit. And don't lean over it until
you think it's all clear

If all seems (smells)  OK, add tubes and t4b and repeat. Check for voltage drops on those places as you go.

When you power on without smoke for a couple of minutes, should be all good.

Then do detail checks of voltages as per the schematic.

There's only a couple of places where it may smoke if you made errors.

Good luck!