All things LA2a related

[andYz00m]

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?

Looks clean!

It shouldn’t matter if the pins are jumpers together which one you go to with the actual wire. I usually just strip a bit longer lead of the heater wire and jumper it with the lead itself.

You do want to make sure your heater lines are consistent from tube to tube though (don’t switch back and forth between which run of the pair you are connecting to the particular pin)

Hope this answers your question.

 

[Toobme]

Thanks, @andYz00m - that was what I thought (and was hoping to be true).
I did keep it same same along the chain as to which wire was feeding from which pin.
Now just to finish up the PSU and meter!

The hard thing, for me so far, has been containing excitement (first tube build) - not to rush through to completion is hard!
But whenever I felt like hurrying or pushing through, I knew it was time for a break - and to check this forum!

 

[Toobme]

Anyone have an idea how to mount the lamp holder for the meter, without drilling into the faceplate?
I was considering super glue, but if someone developed a better technique, and is willing to share . . .

 

[JMan]

If you have the Teletronix meter or similar, there are two unused mounting bolts on the rear of it. You could probably rig something up with one of those pretty easily.

FWIW, I built two using @rainton’s chassis, which does utilize a countersunk bolt through the faceplate, and it looks fine (that’s an understatement, his chassis is a work of art). That said, I totally understand being averse to drilling new holes on the front if you don’t have to.

 

[Toobme]

Thanks @JMan - I’m not using the nice meter! Next time!
For now I just epoxy’d it and that should be fine (for now).

I’m only getting 250V between chassis ground and C7B (or the second PSU cap), with no tubes or T4B.
Is it supposed to be that low? I’d have thought around 300V or more needed.

Time to find a voltage sucker somewhere. I did some serious scraping down to bare metal on every chassis point.

 

[Toobme]

Time to put tubes in and test ‘er out.

Question: any worth in putting in a thermister for protecting tubes from high initial inrush current?
Does using a variac make sense for first fire-up with tubes and T4b in there?

Thanks for your words of wisdom!

 

[beatnik]

The Teletronix meter from don-audio is basically a Sifam AL-39WF and for that meter it's available an illumination kit that gets screwed on the two posts on the rear of the meter.

I strongly recommend anybody who wants to purchase this meter to get it from Sifam directly, it costs peanuts compared to the don-audio prices which are a complete ripoff. The Sifam team is very helpful and offers top quality customer service compared to don-audio who in my experience ignores emails most of the time

 

[TwentyTrees]

Agreed, the Sifam team are great - I buy collet knobs from them direct, and they're always helpful.

 

[beatnik]

Question: any worth in putting in a thermister for protecting tubes from high initial inrush current?
Does using a variac make sense for first fire-up with tubes and T4b in there?

I think the components that can get damaged by inrush current are the filter capacitors rather than the tubes. I would probably use a 500V capacitor in the first position and not worry about thermistors.

I definitely always use a variac to power up gear that is in unknown state (weather it's something I just built it or it's a purchase that is sold as untested or for repair)
Combined with a current meter to visualize if there is any short or some problem that draws excessive current

 

[Toobme]

Thanks for the feedback, @beatnik - I just happen to have a bunch of 500v orange polys.
Cheers

 

[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.?

I have a similar dilemma but bought BLA opto’s and thought they would be plug and play but the old ones are soldered in a line on the circuit board. Anyone know how to install them as they are supposed to be compatible, anyone done this? Thanks!

 

[TDJ]

hey all! just finished a new la2a build based on the layout/schems that have been floating around forever. all is well except for my voltage at c4/r21 is 20-22 - the schematic says it should be 27. my question is - would it be unwise to lower the value of r21 to something like 68k to get the voltage where it needs to be? or would that have a negative impact on the circuit? thanks in advance!

 

[Toobme]

Good question @TDJ !
Not that I know anything, but for my reference and curiosity, were all your components selected by you after measuring on the dmm - to get the closest values? I’m wondering about responses to your question in general practice - how much leeway does one have in this/these case(s)?

I felt like measuring and recording all values - just so I would know for later troubleshooting. But I’ve wondered about tweakabiliy and varied values.
Share a pic, too, if you wouldn’t mind!

 

[sr1200]

Anyone have a source for decent LA2A cases. There used to be some selling in the white market, but I'm no longer seeing them.

 

[JMan]

I highly recommend @rainton ’s chassis, available at AnalogVibes.com — it’s on the pricier side, but his work is incredible.

 

[dmp]

all is well except for my voltage at c4/r21 is 20-22 - the schematic says it should be 27. my question is - would it be unwise to lower the value of r21 to something like 68k to get the voltage where it needs to be? or would that have a negative impact on the circuit?

I don't think you want to do that. The 12BH7 is setup as a white cathode follower. The voltage at the plate of the lower half (100v on the schematic) is what is feeding the DC voltage at R21/C4. You see that R21 and R22 form a voltage divider from the plate voltage - I'm guessing your lower plate voltage is low, maybe ~81v. So the DC voltage at R21/C4 doesn't matter at all - that is a feedback path to the cathode of the gain stage before it. The amount of feedback is controlled by the ratio of R21 and R22, R11 and R10. Changing R21 will change the amount of feedback, which you don't want to do.
So why is the DC voltage different? Assuming the resistors around the WCF are all correct (R17, R20 etc) then the tube is just biasing a little different based on it's characteristics. I would not tweak the circuit, I'd try different 12BH7. Compare the top plate voltage, bottom plate voltage, and bottom cathode voltage and you'll see different tubes bias at different points.
And there is explanation for a WCF online you can look into if you want to know more about the bias point.

 

[Toobme]

I just put this one together - case from DIY-Racked.
It’s not a standard hinge-style case, but is 3U and has a nice front panel.

Now to troubleshoot the buzzing I’m getting!

 

[EarthTone]

Anyone have a source for decent LA2A cases. There used to be some selling in the white market, but I'm no longer seeing them.

Dan at Collective Cases has cool stuff! I think he has an La2a case? Try it! I have made like 6 AML 1073’s with his cases and they look awesome. Here’s 4 of them. Love those red LED’s popping out of the front! I did have to drill out the holes a bit to get the grayhills and pots and led to slide through. The paint powdercoat got put on pretty thick.

www.collectivecases.com

 

[EarthTone]

I just put this one together - case from DIY-Racked.
It’s not a standard hinge-style case, but is 3U and has a nice front panel.

Now to troubleshoot the buzzing I’m getting!
View attachment 102311

That looks awesome! Point to point or PCB build?

 

[Toobme]

Thanks, @EarthTone - This one’s a Turret board. Halfway between p2p and pcb, maybe.
First time for tube gear for me, so it’s a bit scary to be honest!
Collective Cases looks awesome, thanks for the link!