All things LA2a related

Scenaria

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Joined
Jun 3, 2004
Messages
948
Location
Florida
Hey guys,

if anyone has questions in regards to the infamous LA2a, feel free to post your questions here... theres bits and pieces in thread fragments in this forum and I felt it would be better to try and keep the collection in one place :)

this is for ALL la2a questions....

peace!

:guinness:
 

fum

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Joined
Jun 3, 2004
Messages
840
Location
Seattle, USA
Ok,

For my LA2A's, I'm using Allied part #6K88VG for the power transformer. Let me run down the wiring to make sure I've got it clear ( and everyone else has it too =) ):

Primary:

2 Black wires. Hooked up to the feed from fused, switched mains

Secondaries:

250V
2 Red wires: hooked up to the 1N4007 diodes ( one each)
Red/Yellow ( center tap of two reds): Goes to ground?

6.3V
2 Green wires: Heater supply and to VU Lamp
Green/yellow (center tap of two greens): Goes to ground?

Grey: shield, goes to ground

Regards

ju
 

Cainester

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Joined
Jun 12, 2004
Messages
219
Location
Gulf Coast, Alabama
[quote author="fum"]Ok,

For my LA2A's, I'm using Allied part #6K88VG for the power transformer. Let me run down the wiring to make sure I've got it clear ( and everyone else has it too =) ):

Primary:

2 Black wires. Hooked up to the feed from fused, switched mains

Secondaries:

250V
2 Red wires: hooked up to the 1N4007 diodes ( one each)
Red/Yellow ( center tap of two reds): Goes to ground?

6.3V
2 Green wires: Heater supply and to VU Lamp
Green/yellow (center tap of two greens): Goes to ground?

Grey: shield, goes to ground

Regards

ju[/quote]

My question too, and in addition to that:
I have looked for links that tell exactly how to hook up the heaters and I have found nothing. I did find one breakdown of the elements in a tube and noticed that a 12ax7 has 3 connections to the heaters. Pins 4 and 5 seem to connect to either side of the element, and pin 9 connects to the middle. So, which pins do I connect the 6.3v to?

The 6aq5's heater is on pins 3,4, so this is a simple 1 to 1 connection, right?

Thanks,
Caine
 

PRR

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Joined
Jan 30, 2010
Messages
11,144
Location
Maine USA
> how to hook up the heaters and I have found nothing.

http://www.lynx.bc.ca/~jc/hum.html

At first glance, heaters are just heaters, you just have to give them power.

The 12A?7 family gives you two heaters that can be fed 12V series or 6V parallel.

BUT: the cheapest source of heater power is raw AC from the transformer. And that is equivalent to a 6V 60Hz tone leaking all over the audio. So heater wiring layout is critical. The link above gives good tips and a photo of a bad job.

Most guitar amps put the audio components either very low on the socket or on a tag-strip, and then the best layout is to bring the twisted heater wires straight-up from the socket. Most pro audio tube layout favors setting the heater wiring in first, pressed hard against the chassis, and wiring the audio circuits high. For a row of tubes, you bring the twisted heater wiring out perpendicular to the row, then bend and run parallel to the row, then bend into the next socket, to keep the heater wiring away from the audio.

Good well-twisted well laid-out AC heaters don't hum.... decades of very clean gear was built this way and very little of it resorted to DC heater power. Bad AC heat will hum bad.
 

cayocosta

Well-known member
Joined
Jun 3, 2004
Messages
262
Location
SW Florida USA
Here's a layout for the La2a heaters:

http://www.vacuumbrain.com/The_Lab/TA/LA2/la2a_heater_layout.jpg

Thanks to CJ for hosting.
 

Scenaria

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Joined
Jun 3, 2004
Messages
948
Location
Florida
NE2 is the neon lamp (used for regulation)

check one of the bags... look for the piece made of glass.. bag is labeled "neon lamp"
 

Cainester

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Joined
Jun 12, 2004
Messages
219
Location
Gulf Coast, Alabama
I thougt the neon lamp was to light the meter.
Can it be moved over there to perform that function, or should the meter be lit with a separate light?
 

Scenaria

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Joined
Jun 3, 2004
Messages
948
Location
Florida
nice thought but I dont think its going to be bright enough....

I simply strap a bayonet style leaded socket to the meter bracket and tie it to the power switch..

on the UA version theres a small hole in the front upper right hand side above the meter to hold a lamp bracket... I ommitted this hole as I thought that screw looked tacky...
 

CJ

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Jun 3, 2004
Messages
14,147
Location
California
The bulb is used as a regulator. If you remove it, you have to add some zeners.
 

gyraf

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Jun 4, 2004
Messages
10,129
Location
Aarhus, Denmark
Or do like the re-issues - keep the bulb there for authenthic looks, but use some zeners hidden in heatshrink to do the real job.. :grin:

Jakob E.
 

gyraf

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Messages
10,129
Location
Aarhus, Denmark
the neon is just there for stabilizing the reference for the meter in GR-mode. Not chritical, to say the least..
 

Ethan

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Admin
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Jun 3, 2004
Messages
1,604
Location
DC
I've had 2 zeners burn up and die there, replaced them with neons and no problems since. Also "upgrading" the 2W resistors in the PS to 4W seems to keep them cool and doing so will probably help them live a longer life.
 

Ad0lescnts

Member
Joined
Nov 21, 2004
Messages
6
Location
California
Hi guys,
I'm new here, I usually am on the Homerecording.com bbs and recently found this one, there's a lot of good information here.

Ok so for my question,
I know it's kind of open ended but:
I just got finished with my DIY LA2a using Dave's book, but I have A LOT of buzzing. Whenever I turn the gain knob I get a bunch of different buzzing noises, and once i pass the 6 o'clock point it sends my meter to the top. I tried grounding everything all over the place and to the centertap of the 250V wires/AC plug ground. This just gave me some different buzzing sounds.

Could my problem be that my Hammond transformer doesn't have a centertap for the heaters? Where should I start on fixing the problem?

Thank you very much,
T
 

NewYorkDave

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Joined
Jun 4, 2004
Messages
4,378
Location
New York (Hudson Valley)
Your heaters are just floating with no reference to ground? If that's the case, that could certainly be part of the problem! Try two 100-ohm 1-watt resistors, one from each "leg" of the heater supply to circuit ground.
 

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