All things LA2a related

[andYz00m]

I just went to the studio to do some more testing.

First off, it sounds good! Its wild how hard you can hit this thing without it sounding bad. In fact, I couldnt make it sound bad, I tried.

I tested four different tubes in V1.

Plate voltages:
Cheap 12AX7:100/104
Old Fender branded 12AX7: 98/101
Random Fender 12AT7: 78/84
Mullard 12AT7: 50/57

Sound wise, the higher the plate voltage the higher the output (I left the settings the same), which makes sense. But once I level matched them, the were as follows:

Cheap 12AX7: didnt do a listen test
Old Fender branded 12AX7: little muddy in the lows and low mids, bright
Random Fender 12AT7: slightly muddy with clean high end
Mullard 12AT7: clean low and mid with a smooth top end

So I dont know what the deal is with the lower voltages on the 12AT7s and the Mullard in particular. I dont know if this is a problem. I could always lower the plate resistors (R9, R13) to bring the plate voltage up, but I dont know if thats the right thing to do, or even necessary.

 

[Toobme]

Regarding wire, I did search this out a fair bit, but have yet to find the answer(s) [some results read it’s nothing special to worry about].

I’m planning to use:

• 20 awg solid for jumpers on the board
• Mix of 18 awg solid/stranded cloth wire (rated 600V) for everything else.
• Mogami 2549 (mic cable) for the i/o

Does this list make sense or not?
I’m mainly worried about the turret board jumpers, but any other hints are appreciated.
Thanks all!

 

[dmp]

I don't know what PCB you're using as this is a general thread.
But in general, the cut off leads from resistors work great for jumpers.
stranded 18amg will be fine for power connections.
But you want to use shielded cable for gain pot / switches / etc in a LA2A, to prevent oscillations. If you use a twisted pair cable to a pot/switch, keep in mind the capacitance of the cable is in parallel with the pot/switch. In some cases, separate shielded wires is good

 

[Toobme]

Thanks for your advice, @dmp - for this one, I'm doing a turret board build, no pcb.
That has meant that my leads' cutoffs are not long enough to span two turrets as a jumper.
I'm thinking of buying a roll of 18 awg buss wire to use as jumpers, or should jumpers remain insulated?

I appreciate your response!
Cheers

 

[dmp]

I have a small roll of solid uninsulated wire that can be cut to length that I would use in that case. It is your own preference I suppose if you want the jumpers to be insulated.

 

[JMan]

I'm thinking of buying a roll of 18 awg buss wire to use as jumpers, or should jumpers remain insulated?

Insulation is just insulation, so you only “need” it if you think there’s a chance of the exposed wire making physical contact with something else and shorting - hopefully not an issue since that would put your turrets, which are much bigger targets, in a similar position. Insulation won’t provide shielding, so no need to worry about anything special here. You could just use the cut leads of some components from elsewhere in the build and call it a day. I’m not sure exactly what your layout looks like, but I know that on my turret boards 22awg jumpers were fine, no need to fuss with thick wire (unless you you have jumpers in the power supply section, where it’s probably not a bad idea).

 

[dmp]

I'm thinking of buying a roll of 18 awg buss wire to use as jumpers, or should jumpers remain insulated?

You may also want to use heavier gauge wire for the ground if you've designed the turret board for a ground bus. I've used 12 awg pulled from a scrap of romex.

 

[Toobme]

Romex stripped to bare copper is what I'm using for grounds.
Thanks @JMan and @dmp for your help.
It seems I'm over-thinking it a bit. Just wanna do it as right as I can!
Cheers, all!

 

[dbonin]

All the P2P la2a 's I built I used insulated, multi colored 22 awg hook up wire. After tracing wire all over the units it's just easier on the eyes and brain. Good luck!

 

[Toobme]

Thanks for the replies, folks!

On another note, I’m doing one with no cutouts for the i/o terminal strips. Instead the XLRs are going to be used exclusively.
There’s more going on in the i/o strips than just the 3 pins of the XLRs, so I’m wondering if I can just trace the routes of all the connections that would go to the terminal strips and make those nodes directly on parts instead?
I just don’t want to mangle this case’s rear plate by drilling a bunch of holes out to fit a pair of terminal strips and thought I could just “bypass” them altogether.
Any harm in doing this, assuming I make all the connections?
Thank very much!!

— Cancel that — I’ve just gone ahead and mounted the strips on the inside of the case (where they would normally go), and decided to live with it being a modern take. I didn’t want to experiment too much on my first La2a build, so this keeps the typical circuit layout the same, just no feed to the external world aside from XLR.

 

[paulcheeba]

I wanted to make my ADL 1500 sound more like a vintage LA2A without spending a fortune. Where are the best places to spend the cash? I have plenty of valves. So BLA Opto attenuators or transformer’s and I don’t want to import UTC vintage ones from the U.S.?

 

[Toobme]

Put in some Mundorf or other expensive caps, maybe? I’m going to try that on my second La2a.

First things first - I’ve settled on a layout - anyone see trouble with my heater wiring?
I’ve read it may be better to travel across the valve sockets, as opposed to circumnavigating them, as i did.
Is that something you’ve noticed, anyone?

 

[wutty]

Put in some Mundorf or other expensive caps, maybe? I’m going to try that on my second La2a.

First things first - I’ve settled on a layout - anyone see trouble with my heater wiring?
I’ve read it may be better to travel across the valve sockets, as opposed to circumnavigating them, as i did.
Is that something you’ve noticed, anyone?

How did you get the shell? Is it made by myself? It's not convenient to buy a ready-made shell where I am, so I plan to make one myself

 

[Toobme]

Diy-racked in Slovenia. Good guys!
This case comes with two cutouts for the T4B, one for normal, one for fast or superfast, etc.

 

[GussyLoveridge]

Merlin Blencowe's book has the heater wires going directly across the sockets avoiding I believe what he called in the book hum loops. It might be worth a re-wire before you move along with the rest of the wiring. Looks beautiful! Nice work.

 

[Toobme]

Ahh, I thought I’d heard/read that somewhere. Thanks for the feedback - I’ll certainly do this before attaching the rest.
Going to try to source that book as well. Thanks again!

— Update — I ended up getting the Designing High Fidelity Tube Preamps and also his book on tube PSUs. Amazing, but a bit beyond me at this point. I’m thankful for you pointing out the author, @GussyLoveridge - I’m having to re-re-read each chapter, but things are slowly starting to cement for me. The more I read, and get schooled, the more I realize this is an ocean and I’ve managed to muster a drop or two.
I need to get the Art of Electronics, as well, but these are expensive copies these days.

 

[aaquilato]

I wanted to make my ADL 1500 sound more like a vintage LA2A without spending a fortune. Where are the best places to spend the cash? I have plenty of valves. So BLA Opto attenuators or transformer’s and I don’t want to import UTC vintage ones from the U.S.?

I had my ADL 1000 modded some years ago. Adding the UTC i/o transformers was by far the best sonic upgrade. Obviously the HA input transformer is ridiculously overpriced BUT for exactly this reason. It is the sound of the LA2 imho.
Second was the grounding path and power transformer.
Then cc resistors and pio caps.
I had to sell that ADL a few years ago to a local producer who said he wouldn’t think sell it back to me. Not even for twice what he paid for it.
The sound is in the iron.

 

[GussyLoveridge]

Ahh, I thought I’d heard/read that somewhere. Thanks for the feedback - I’ll certainly do this before attaching the rest.
Going to try to source that book as well. Thanks again!

https://www.valvewizard.co.uk/
You can find the books there. New edition of the valve power supply book out just this year or last. All three books are wonderful. A gold mind of great information with built out tests. I am not a math genius and I can understand a good deal of it. Each time I re-read these books I get a little bit more. Good luck!

 

[Toobme]

Thanks, @GussyLoveridge - much appreciated!
Books are where it’s at - I only have the Audio Cyclopedia (an amazing artifact of a book!) and am certainly craving some tube knowledge. Thanks for providing the link!
And thanks again for confirming the quieter way to lay out the filaments’ lines. I just desoldered them off the build and am about to go at it again, crossing - not circling the sockets.
Just you confirming this sent me on a bit of a quest and I’ve since figured out a better route for the leads, in addition to the “not circling the socket” wisdom.
I’ll post the new layout when it’s done.
Gotta love this forum - thanks all!!!!

 

[Toobme]

Phew, done the rewire of the heaters now.
Yep, tube-noob here, so thanks again for the direction!
Doing it properly is so much trickier handwork with the wires across vs around the socket.

Not much room with the awg20 solid I’m using, making me a bit worried about the insulation touching the other pins’ edges.
I threw on some heatshrink on the parts that sit between pins, but still.
What gauge do most use?
I WANTED to use my nice cloth 18 solid but there’s no way I could keep the twists consistent across the sockets. Just too fat, like me!
Anyway, here’s the “across” layout. Since pins 4 and 5 are connected, would it matter if the wire went to pin 4 OR 5?