All things LA2a related

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FarisElek

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Oct 26, 2019
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scott2000 said:
arrgh... I had that happen once....Nice pair of smoked glass 6sn7s the seller taped together  and in my excitement I separated them with too much force  and one shot off to the tile floor...

Hopefully your close.... I've read the tolerances can be as much as 20% in this thing so maybe...

That's the worst! This unfortunately was a very nice NOS RCA so I feel you. I'm replacing it with an EH. This circuit doesn't really need super nice tubes and I like EH tubes in general. They're fine.
 

justinheronmusic

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FarisElek said:
That's the worst! This unfortunately was a very nice NOS RCA so I feel you. I'm replacing it with an EH. This circuit doesn't really need super nice tubes and I like EH tubes in general. They're fine.

I've got all RCA tubes throughout, but I saw the EH 12BH7 and it sparked my interest - for some reason I am drawn to the reliability of new production tubes - but perhaps it is just an illusion I am buying into and they aren't any more reliable than the used/vintage on ebay.

I tested an EH 6922 in a Rode NTK once upon a time. The top end was extended, or open rather, and the low end was deep and slightly bloated in that circuit. Would have been awesome for recording bass. I settled on a new production Genelex Gold Lion 6922 after testing a rather noisy Amperex 6DJ8 and the Gold Lion was so close in dimension and freq response (only without the noise) it just made sense.

Is there anywhere I can find subjective notes on different tubes in the LA2A? Part of me wan't to do a video on different 12BH7 and 12AX7 brands in the LA2A.
 

FarisElek

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382
justinheronmusic said:
I've got all RCA tubes throughout, but I saw the EH 12BH7 and it sparked my interest - for some reason I am drawn to the reliability of new production tubes - but perhaps it is just an illusion I am buying into and they aren't any more reliable than the used/vintage on ebay.

I tested an EH 6922 in a Rode NTK once upon a time. The top end was extended, or open rather, and the low end was deep and slightly bloated in that circuit. Would have been awesome for recording bass. I settled on a new production Genelex Gold Lion 6922 after testing a rather noisy Amperex 6DJ8 and the Gold Lion was so close in dimension and freq response (only without the noise) it just made sense.

Is there anywhere I can find subjective notes on different tubes in the LA2A? Part of me wan't to do a video on different 12BH7 and 12AX7 brands in the LA2A.

Depends on the circuit and the tubes used in it. U67 is really sensitive to the type of EF86 it has in it, but you can use pretty much anything for the gain 12ax7 in the LA2A and it will sound pretty much the same. I believe it has to do with negative feedback. I've messed with the LA2A circuit before and the biggest difference I heard was playing with the output capacitor and using a good quality t4b. I'm sure you could run some sweeps and see for yourself. It's difficult to do that with microphones but pretty easy to do with line level gear.

 

FarisElek

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FarisElek said:
Depends on the circuit and the tubes used in it. U67 is really sensitive to the type of EF86 it has in it, but you can use pretty much anything for the gain 12ax7 in the LA2A and it will sound pretty much the same. I believe it has to do with negative feedback. I've messed with the LA2A circuit before and the biggest difference I heard was playing with the output capacitor and using a good quality t4b. I'm sure you could run some sweeps and see for yourself, or maybe prove me wrong. It's difficult to do that with microphones but pretty easy to do with line level gear.
 

justinheronmusic

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FarisElek said:
I've messed with the LA2A circuit before and the biggest difference I heard was playing with the output capacitor and using a good quality t4b.

Wow, not even the input transformer?
 

FarisElek

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justinheronmusic said:
Wow, not even the input transformer?

Transformers probably do too. By how much I couldn't say. I'd imagine if they're good transformers they'll sound good though. I know there is a common Jensen mod for the LA2A which uses different impedence at the input I believe. That probably changes things more than swapping between several different good transformers of the same ratio. I'm not an expert though. Who knows?
 

FarisElek

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Oct 26, 2019
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382
Okay, finished my point to point build of the LA2a. All voltages appear to be within 20% or less except pin 8 of V1, which has nothing.

Gain is working just fine EXCEPT, for the loud buzz which is louder than the gain. Also, no low end. Using edcor transformers. I didn't load these any particular way, I just followed the recpro layout.

The buzz is probably the heater wiring. I'm going to make sure it's really tighly sound and not touching a bunch of other wires later. I'll follow the path of pin 8 and see if u can fix whatever is preventing it from being the right voltage. If anyone can enlighten me on what might cause no low end and how in supposed to terminate the war transformers I don't know much about termination in general.

Ryan
 

Lee_M

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Nov 23, 2013
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Dorset, UK.
Just curious, how are people physically mounting C14 (the variable cap near the output tx) on their units?

I've got some of the Arco 465 caps, but I can't find any pics showing how they're actually mounted on the rear panel. I was gonna go with insulated standoffs, with the cap terminals and other components (C6, R30) screwed onto the standoffs using ground lug washers, but I figured I'd check here to see what other people are doing before I do anything.
 

dbonin

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Jan 20, 2012
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Just curious, how are people physically mounting C14 (the variable cap near the output tx) on their units?

I've got some of the Arco 465 caps, but I can't find any pics showing how they're actually mounted on the rear panel. I was gonna go with insulated standoffs, with the cap terminals and other components (C6, R30) screwed onto the standoffs using ground lug washers, but I figured I'd check here to see what other people are doing before I do anything.
 

Lee_M

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Nov 23, 2013
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Dorset, UK.
So the consensus is just forget the Arco caps for that position and use a fixed 150pf instead?

That actually makes things less straightforward tbh, as it involves ordering parts I don't have (150pf caps) and using them in place of parts that I already own.
 
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