D-LA2A Support Thread

yes ,i found it on another CJ's post :
"I subbed in an 12AY7 for the AX7a first stage amplifier. Less gain which helped the oscillation, plus you are  likely to find a 12AY7 that sounds better than your typical AX7a. More of a vintage sounding tube."
or :
"Drop a 6072 in for V1 and then tell us how you like it."
and agin  :
"Do you have a 12AY7 laying around? This tube makes a much better input stage than the 12AX7 for two reasons:
1) On the average it will sound nicer than the harsh 12AX7
2) It will reduce your gain so you can turn up the Gain pot above 1 with a CD type level input."

seems worth a try and maybe he's suggestion about R11 is with a 12AY7,
i also found this post from another member about changing 2 resistors :
"I also went ahead and replaced R9 and R13 with 100k resistors
(had a 12ay7 in for V1 for a while, just hadn't changed the resistors)."
and another :
"If you want to tame it even more, replace R9 and R13 with 100k.  This also has the effect of making clipping more symmetrical by biasing the tube closer to the mid-point.  I haven't tried these yet, but maybe one day..."
did you tried this tube change kingston ?
the R9 R13 100K and 12AY7 seems to be the perfect companion
for a R11 50k in series with a 470k pot mod, ;-)
can't wait to get my D-LA2A back and try this.
thank you so much for your explanation !!
regards,
francois

Kingston said:

Yes, R37 is the high-pass filter. Has rather extreme effect on sound so like I said, put it on the front panel.

R11 is the global feedback resistor. LA2A make up gain amp is absolutely swimming in negative feedback with that 68k. It's a very linear amp, complete devoid of any recognizable tube distortion. You won't know it's tubes until someone tells you because the excess feedback removes all distortion.

Click to expand...

Really, no distortion??

My LA2 is set up with original UTC transformers. I can get a ton of distortion out of my LA2A. In fact when I first hooked it up I thought it was broken because the distortion was so high when I cranked the gain knob. I don't think I can get the gain knob past 70 without distortion in most cases.

I have used my LA2 as a distortion device from time to time, in fact did it on a vocal track just the other day. It can get very dirty and overdriven.  Why you'd want to increase the distortion is beyond me unless your using different ratios for transformers which may in fact help to clean up the distortion on the audio path vs the stock transformers A10/HA100X, A24.

Agreed with the SC HP on the front panel, very useful.

Regarding ARCO's, I haven't put them into my PTP build yet which is currently using Silver Micas in those positions. So no comment on how they compare soundwise. It doesn't seem like a big deal to mount them, no idea how you go about that on a PCB. Perhaps mount them temporarily to determine the correct value and them solder in a silver mica and call it done.

Mark

Biasrocks said:

Really, no distortion??

Click to expand...

Not in the normal use case with even blast out mastering limited levels (10'oclock in the LA2A like they say). THD is below maybe 0.1%. That's in the linear region before you run out of headroom, no tubes to be heard, only transformers. With the amp swimming in negative feedback the roof of the headroom is very sharp, and since 12AX7 even without NFB sounds pretty bad, it's really the door to nasty-land. A great effect sometimes, that's why I have a switchable 15dB pad on the output transformer to really abuse it.

But I mostly like amps that budge out softer.

With less NFB and something like 12AY7, maybe even 12AU7/6SN7 we have more THD at lower levels but the distribution of harmonics is nicer. There might be 1% THD and everybody just says it's "more 3D", you know what I mean. And the roof of headroom is more forgiving. Easier sound shaping.

pacemaker said:

"If you want to tame it even more, replace R9 and R13 with 100k.  This also has the effect of making clipping more symmetrical by biasing the tube closer to the mid-point.  I haven't tried these yet, but maybe one day..."
did you tried this tube change kingston ?

Click to expand...

Yes, but with several russian alternative tubes. I wanted to "break this horse" so to speak. I have another big tube experiment going but after that I might even go back and mod some more. The line amp of LA2A is ripe for it.

Changing those 220k plate resistors to something lower is indeed a good idea. The 220k are only good for the really high mu 12AX7's. You could even go and check the tube datasheet if you really want to optimise them (and the cathode bias resistors) to a certain tube. Making the distortion more symmetrical is not automatically better of course. Ears will tell, and since you can mod two channels at a time the D-LA2A is a good testbed for it.

  Kingston and All, thanks so much for this recent conversation, I am in the early stages of a D-LA2A and am acting like a sponge and soaking it all in    ;D

mmm output pad ?
very nice idea kingston,
do you have a rough schematic ?
Is it a T-pad, H-pad or other ?
for example 2 x 419R and a 220R resisotrs ?
thanks for the allreadygiven help
regards,
Francois

Make it a 600in 600out ohm H-pad, http://www.nu9n.com/tpad-calculator.html

attenuation of 10-20dB works. Don't bother with accurate values from that calculator, just pick the closest resistor you have. 1dB here 2dB there, we're not fussy.

nice explanations Kingston!

however, like with every project around here I would suggest:
build it stock first, mod it later if it doesn't fit your needs / expectation.

if you are planing any mods use sockets etc for the parts in question.
but build it stock first.

any possible trouble shooting will be easier if you started with a working stock unit.

yes you"re right volker ,
but as i've built 2 D-LA2A,
i wanted to add mods to the second one,
in order to explore all the aspect of this wonderful piece of gear ,
have two different units, listen to them
and if possible , learn a few things about electronics
;-)
Thanks to Volker for this amazing boards,
and thanks to all the brain around, CJ ,Kingston,SSLtech
who helps beginner like me ,
regards,
francois

Indeed - Thanks for the continued input!  It's a goldmine of info.

I think I will include the Arco caps in my build and possibly consider swapping out to a fixed value once I hear exactly what the differences are with the rolloff and my Edcor transformers.  I guess I can just find some type of plastic standoff and use a dab of silicone to mount the trimmer caps for now?  They look like they have a strange PCB mounting scheme - but they obviously won't fit on the D-LA2A PCB - and they don't have any mounting provisions aside from the leads that I can tell - but I haven't scrutinized them to the ends of the earth just yet.

I'll heed Volker's advice and start stock with the 12AX7 (already have them) - and ponder a 12AY7 and resistor mods after I finish the session I'm hoping to use the D-LA2A on!  The switchable front-panel Sidechain High-Pass will likely make it into the initial build.

Now, If I could just get my frigging case already and get to wiring! 

Thanks again for the wealth of info!  8)

did some search for you and updated the first post with some additional support links 8)
hope this helps to answer some FAQs.


and for sure I have to add the last two builds to our family  ;D
nice & unique work, gentlemen ;D ;D ;D

Can someone explain for my benefit what the 100 ohm resistors to ground do on the 6vac inputs to the heaters?

Is there any links or companies other than Par Metal, Hammond, Bud... for custom chassis/cases ???

Also, where could I get Vu meters that are angled 90 degree's opposed from each other. ><

@ volker thank you 

thestudio said:

Is there any links or companies other than Par Metal, Hammond, Bud... for custom chassis/cases ???

Also, where could I get Vu meters that are angled 90 degree's opposed from each other. ><

Click to expand...

i got my case from (german) ebay. there are some black 2 HEs around 27 euro... quite cheap but ok for custom things.
also cheap but very nice VUS can you get at hairball audio

Can someone explain for my benefit what the 100 ohm resistors to ground do on the 6vac inputs to the heaters?

Click to expand...

For transformers without a center tap on the heater winding a pair of well matched 100 ohm resistors to ground creates an artificial center tap. Without this you'll get a lot of hum.

-pH

Hi
I'm about to finish mine. Just a quick question : is it ok to have about 90V DC on the output of C5 ? (i'm using sprague polarized).

C5 should block all DC - if you have 90V in front of the transformer there is something wrong.

That's what I thought... In fact, yesterday I had Solen caps, and one channel was working, but on the other, I had 50v dc on the out of the cap (wich has burnt one of my carnhill  :-\). That's why I change both caps.

Well, I really don't know what happens. All my voltages are good, I have 90V into that cap from the 12BH7, but they simply won't block this DC. Both.

did you connect all grounds (correctly)?
could be just a measuring error - you are measuring in relation to where?

All grounds are connected together to the case (3 pins on pcb + jumper). I'm measuring from this star ground. I maybe retry with the good Solen, as this is the only thing I've change since yesterday.