rainton's P2P LA2A chassis "original style" discussion thread

[beatnik]

Just get rid of the carbon comps, as you can see they have poor tolerance, high temperature coefficient and they are also very noisy. Use some nice metal films like Vishay/Dale RN65
Don't believe the BS you read around saying the carbon comps have some sort of magic sound, it's all rubbish

 

[Zim]

Just finished my Analogdevices La2a and, somewhat surprisingly, all fired up and seemingly working as intended right from the off. My first build of this sort (previously just guitar pedals and a couple of 500 series kits) and, gotta say really delighted with this thing. It's super quiet - no hums and buzzes whatsoever, which is probably more luck than judgement but... all in all a really satisfying and rewarding project!

A couple of things I'd still like to get sorted if anyone has can help...

• Tube shield for the 6AQ5... the one that came with the base in the BOM is too short, found a longer 60mm one on eBay that just about squeezes on but the 'nipple' on the top of the tube sticks out of the top. Found another 70mm one for 7-pin socket again on eBay, but that's for a larger diameter base. I've googled, obviously, but can't find anything. Unless my 6AQ5 is longer than normal (and datasheets I've found say 58mm, so doesn't seem to be..) I don't get it! Anyone know where I can get the right part - can't believe this is the thing that's got me stumped!!

• I installed the NE2 bulb in parallel with the diode, just for the look really and had read that this was fine to do etc. The bulb doesn't light though. I had a bit of a poke with a chopstick which made no difference until I slipped and brushed a finger. Got zapped, but it got the bulb going until I turned the unit off, then back to nothing. Haven't tried that again, and not really important as functionally it makes no difference, but if anyone's tried something similar, I'm curious...

• Last thing - not sure it matters as it all seems to be working as expected, but voltage on pin 5 of the 6AQ5 seems a little high. I'm getting 153V which is within 15% of the 135V on the schematic, but in the 'all things La2a' thread I waded through that's way higher than most were seeing. Don't really want to do the resistor mod that's often referred to in that thread if I don't need to, especially as it's seems to be working great as it is. Just wondering if this is way off what everyone here that's gone with Rainton's build is seeing?

Appreciate anything anyone an offer re the above, and lastly, a few pics - just because...

 

[Zim]

So one thing I’ve just noticed... the La2a flips the phase. I’ve doubled checked my wiring with the build diagrams and pins 2 &3 of the XLRs are wired same as the diagrams, but that results in the phase being flipped.

I notice that in the wiring diagram the pin 2 of xlr is connected to pin 1 of the input transformer and pin 3 to pin 6 of the transformer, while on the outputs that is reversed with pin 6 going to pin 2 of the xlr and pin 1 going to pin 3.

is it supposed to be like this?

 

[j.colt]

Just finished my Analogdevices La2a and, somewhat surprisingly, all fired up and seemingly working as intended right from the off. My first build of this sort (previously just guitar pedals and a couple of 500 series kits) and, gotta say really delighted with this thing. It's super quiet - no hums and buzzes whatsoever, which is probably more luck than judgement but... all in all a really satisfying and rewarding project!

A couple of things I'd still like to get sorted if anyone has can help...

• Tube shield for the 6AQ5... the one that came with the base in the BOM is too short, found a longer 60mm one on eBay that just about squeezes on but the 'nipple' on the top of the tube sticks out of the top. Found another 70mm one for 7-pin socket again on eBay, but that's for a larger diameter base. I've googled, obviously, but can't find anything. Unless my 6AQ5 is longer than normal (and datasheets I've found say 58mm, so doesn't seem to be..) I don't get it! Anyone know where I can get the right part - can't believe this is the thing that's got me stumped!!

• I installed the NE2 bulb in parallel with the diode, just for the look really and had read that this was fine to do etc. The bulb doesn't light though. I had a bit of a poke with a chopstick which made no difference until I slipped and brushed a finger. Got zapped, but it got the bulb going until I turned the unit off, then back to nothing. Haven't tried that again, and not really important as functionally it makes no difference, but if anyone's tried something similar, I'm curious...

• Last thing - not sure it matters as it all seems to be working as expected, but voltage on pin 5 of the 6AQ5 seems a little high. I'm getting 153V which is within 15% of the 135V on the schematic, but in the 'all things La2a' thread I waded through that's way higher than most were seeing. Don't really want to do the resistor mod that's often referred to in that thread if I don't need to, especially as it's seems to be working great as it is. Just wondering if this is way off what everyone here that's gone with Rainton's build is seeing?

Appreciate anything anyone an offer re the above, and lastly, a few pics - just because...

View attachment 85053View attachment 85054View attachment 85055View attachment 85056

Very nice build! Regarding the 6AQ5 shield, I found some labelled "EBY Cinch 7 Pin TS10203 Tube Black Shields Branded Philco 842944 3 6AQ5 Size" on eBay that fit well. You may find some that are similar. Regarding the neon lamp/diode... it is really either/or. I chose to use the neon lamp for "authenticity" but everyone advise to use the diode for reliability! Lastly 15% from spec seems ok got me but I am no expert.. if it sounds good it's probably ok!

 

[Zim]

Thanks J.colt. Really handy info! Had a look for those shields but can only see some which are about £180 for 34 at the moment, so will just have to keep an eye out. I got another 6aq5 to try on the advice of Bill at Kenetek and, sure enough that second tube draws quite a bit less power, so the high voltage is clearly down to the 5ube rather than my build so happy with that.

still haven’t figured out the reverse phase issue though - is that something you see with your La2a?

Thanks

zim

 

[j.colt]

Thanks J.colt. Really handy info! Had a look for those shields but can only see some which are about £180 for 34 at the moment, so will just have to keep an eye out. I got another 6aq5 to try on the advice of Bill at Kenetek and, sure enough that second tube draws quite a bit less power, so the high voltage is clearly down to the 5ube rather than my build so happy with that.

still haven’t figured out the reverse phase issue though - is that something you see with your La2a?

Thanks

zim

No reverse phase issue in my build. You could just flip the hot and cold on the output? I have tested transformer that had a flipped phase/incorrect wiring. Maybe check the phase of you A-24?

 

[Zim]

Thanks. Had thought of just flipping hot and cold, but didn’t know if whatever was causing it could be having any other detrimental effects so thought I’d better find the culprit. Had looked at the wiring diagram for the AMI transformers to see, but honestly not clear to me what should be doing what and therefore how to check if one of them’s flipping the phase (still trying to learn this stuff)

guess it’s an opportunity to try and get a better understanding of schematics etc!

thanks for the feedback

 

[j.colt]

Testing the transformer phase is easy if you have a 2 channel scope: Connect a signal generator on the primary with the hot on the transformer primary "black dot" (the black dot indicated on the AMI 24 wiring diagram). Place your first scope prob on your signal generator to monitor the input signal. Then place your second scope probe on the transformer secondaries "black dot" and the ground of the probe on the other side of the secondary. Your scope will show the input and output signal. If they are mirror images, you know the issue is with the transformer. This is not a 1:1 transformer so you may need to adjust the scale on your scope. I was looking at a UTC A-20 clone the other day that had its secondary reversed.

 

[Zim]

Testing the transformer phase is easy if you have a 2 channel scope: Connect a signal generator on the primary with the hot on the transformer primary "black dot" (the black dot indicated on the AMI 24 wiring diagram). Place your first scope prob on your signal generator to monitor the input signal. Then place your second scope probe on the transformer secondaries "black dot" and the ground of the probe on the other side of the secondary. Your scope will show the input and output signal. If they are mirror images, you know the issue is with the transformer. This is not a 1:1 transformer so you may need to adjust the scale on your scope. I was looking at a UTC A-20 clone the other day that had its secondary reversed.

Thanks for very much for this - apologies for the hand-holding!

Don’t have a scope yet, but it’s something I’d been looking into. Probably time to accelerate that purchase as there had been a couple of things previously that it would have been useful for and is probably one of the last tools for this hobby that was looking to add to the bench.

That info will certainly prove invaluable once I’ve got it
appreciate your time - thank you

Zim

 

[j.colt]

No problem! This is fairly new to me as well!