All things LA2a related

soundsactive · Nov 19, 2014

so i'm in trouble shooting mode for my p2p build based on Cayocosta layout. first problem seem to be that none of my tubes will fire except the 6aq5 (v4). i'm getting 3.49v on all of my filaments for all the tubes. i have the hammond 369jx power transformer with the sowter i/o transformers. any ideas? thanks so much!

sr1200 · Nov 21, 2014

You're measuring AC on those heaters right? The most common problem with the P2P build that ive run into (ive only done 3) is a ground missing somewhere. Every build I've done, theres been one ground somewhere that was off, gummed up the whole thing.

Bonsaimaster · Jan 10, 2015

I have a problem with my P2P LA2a. All tubes work and the voltages below. I am having oscillation issues with the unit. I do not have any grid stoppers installed and using a Sowter in place of T1 (HA100X). In +10 mode the meter just jumps. Gain reduction position and the meter does not go to zero. Cannot get sound to pass thru unit. Also the gain reduction does not function on the meter.

C7C - 191V
C7D - 115V
C7B -262V
R29 - 350V
R34 - 262V
Across NE - 52V

12ax7#1
Pin 1 - 98V
3- 1.0V
6 - 100V
8 - 1.03V

12BH7A
Pin 1 - 109V
3 - 3.5V
6 - 209V
7 - 31V (*) not normal according to what I read. Before R15 voltage is 109
8 - 113

12ax7 #2
Pin 1 - 100V
3 - 0.7V
6 - 100V
8 - 0.8V

6AQ5A
Pin 2 6.8V
5 - 118V
6 - 107V

Any thoughts?

Janalex · Jan 20, 2015

Hi

Which tube socket, base, shield combination will work with the 6aq5a and where to source it? It apears to be a tall tube and everything I found seems to be for shorter tubes.

dbonin · Jan 21, 2015

Put a test tone into the unit and carefully test if the signal makes it to the secondaries of your input transformer (ie start at the input end of the circuit) then continue breaking the circuit into logical chunks and start tracing, using the schematic and a highlighter to keep track of where you have traced.

Bonsaimaster said:
I have a problem with my P2P LA2a. All tubes work and the voltages below. I am having oscillation issues with the unit. I do not have any grid stoppers installed and using a Sowter in place of T1 (HA100X). In +10 mode the meter just jumps. Gain reduction position and the meter does not go to zero. Cannot get sound to pass thru unit. Also the gain reduction does not function on the meter.

C7C - 191V
C7D - 115V
C7B -262V
R29 - 350V
R34 - 262V
Across NE - 52V

12ax7#1
Pin 1 - 98V
3- 1.0V
6 - 100V
8 - 1.03V

12BH7A
Pin 1 - 109V
3 - 3.5V
6 - 209V
7 - 31V (*) not normal according to what I read. Before R15 voltage is 109
8 - 113

12ax7 #2
Pin 1 - 100V
3 - 0.7V
6 - 100V
8 - 0.8V

6AQ5A
Pin 2 6.8V
5 - 118V
6 - 107V

Any thoughts?

sr1200 · Feb 11, 2015

Check your grounds. All the units ive built, 100% of the off voltages i've run into were caused by a ground not connected properly somewhere.

Deepdark · Feb 11, 2015

Check out for missing missing wire wire, not just ground. I've had trouble with some off voltage because of that

bancho · Mar 4, 2015

I have some basic questions …
Anyone tried UTC LS-50 as output? There is also the black version of this tx… it looks like the case is being handmade at blacksmith's…

Would these (the black LS versions) be anyhow superior to "ordinary" grey version (like A series) or it doesn't matter?
(by superior I mean more vintage sound)

beatnik · May 13, 2015

I have a problem with my LA-2A ptp build

For the second time V1 has broken and the compressor started to sound distorted

I replaced V1 with a fresh one and the compressor sounds fine, but voltages on V1 and V3 are not good compared to the recommended values

V1

1- 168V
3- 0.8V
6- 202V
8- 0.3V

V2

1- 97V
3- 2.9V
6- 222V
7- 75V
8- 102V

V3

1- 168V
3- 0.4V
6- 168V
8- 0.4V

V4

2- 6.4V
5- 128V
6- 117V

I am wondering what is causing V1 to break ? I have triple checked all the wiring and components and everything is fine.

Can someone help me pinpoint the problem ?

Deepdark · May 13, 2015

I would check again along the cayocosta layout AND the schematic. A lot of time, it's a missing ground point/wire. Something is sucking out too much current. Check your résistors values. I had trouble with voltages once and it was a missing ground wire. I found best to take a highliner and trace over the layout as you check all components. That way, if their is something missing, you'll know. When I cross a resistor, I validate it's values before going further.

beatnik · May 13, 2015

hi deepdark,

i have checked a lot of times and i am sure all the components values and connections are fine

i am afraid i have a damaged resistor or capacitor along the line, i can't figure out what is wrong

the compressor works and sounds fine, but with that high voltage on v1 and v3 these tubes are going to break again i guess

is there any way i can isolate the sections and narrow the problem ?

Deepdark · May 13, 2015

first confirm that R9, R13 and R33 are 220K, and not 220R, or 22K, or what ever. R12is 470K, R16 is 68K and not 6K8,

beatnik · May 13, 2015

yes those are 100% correct

Harpo · May 13, 2015

...and double check R17 and R27 (maybe R34 as well) are 10K for real, not 120R resistors read backwards.

Deepdark · May 13, 2015

about R34, be sure it's rated 2W. I even place a 1/2W instead of a 2W and my voltages were higher than they add to.

Oh and also, check R10, R12, R14 and R31 and R22.

beatnik · May 13, 2015

all the resistors checked again and their values are exact. they measure ok with multimeter so i think none of them are burnt

i did the full check with schematic and layout on hand and again everything appears to be fine

i can't really figure out what's happening here !

in the meantime another 12ax7 has been damaged

Deepdark · May 13, 2015

something is wrong for sur. Check to be sure to have not a resistor that could touch another pin, ground turret, etc. it's seems to have too much current passing by the 12ax7. If possible, strap your DMM in series before the 4k7, between the 2 power caps. A LA2A should take about 15ma.

Deepdark · May 13, 2015

oh and take a few pictures please, as close as possible of both 12ax7

beatnik · May 14, 2015

i have checked many times for shorts, missing connections, wrong component values. everything is right

maybe 12bh7 tube is bad ? although voltages are fine... this is the only tube i didn't try to replace, because i don't have another one.

i am thinking if there is any way to isolate the sections and try to make them work step by step

the problem is very hard to spot, i've lost so many hours trying to find it

i am totally stuck with this, it's been the most painful build i've ever done

Deepdark · May 14, 2015

Can you take some pics please? Also, thé 12bh7 is the last stahe of thé amp section. There is a feedback circuit connected between it and thé first 12ax (the first stage of the amp section.). You can look around thé feedback section but i doubt thé culprit is there. I dont think thé 12bh is bad.

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