La2a replica - need help

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barthman.de

Well-known member
Joined
May 26, 2007
Messages
87
Location
Erfurt / Germany
I have been struggling with a La2a replica for several weeks now, which I built using P2P wiring. Unfortunately, the device does not work properly yet and despite extensive attempts and tests, I am not able to achieve my goal. One thing first: the tubes are not the problem, I've already replaced all tubes once.

The problem is:
When I look at the plug-in emulations of the La2a, the signal level at the input should also be at the output if the gain control is in the “30” range (without compression). The La2a should have a lot of gain to compensate the peak reduction. Unfortunately, this is not the case at my unit: When I feed a signal (sine 1 kHz; + 4dbU = 1.228V = 0VU) into the input, a corresponding voltage is only applied to the output in the "80" position and has a THD of approx. 20%. So something can't be right.
I've looked through the circuit more than once and can't find any fault with the wiring.

So in order:
I used this circuit:

With the following components:
UTC A10 input transformer (XLR hot/cold wired to pin 1 an 6 – no C.T. used)
UTC A24 output transformer (XLR hot/cold wired to pin 1 an 6 – no C.T. used)
R34 10K
R29 4k7
R38 and C13 installed
I only write this because there are different La2a revisions.

If I understand the circuit correctly, the tube amplifier part of the circuit can operate independently of the compressor and VU meter part. So I'm currently focusing exclusively on the amplifier when troubleshooting.
In order to test the voltages of the amplifier in the first step, I proceed as follows:
R3 fully CW
R16 fully CW
Peak Reduction fully CCW
Gain fully CCW

The following voltage is applied to B +: 276V
NE2 (lamp) 64.5V
I measure the following DC voltages at the pins of the 12AX7A tube:
1 - 122.8V
3 - 1.03V
6 - 124V
8 - 1.16V
I measure the following DC voltages at the PINs of the 12BH7:
1 - 99.7V
3 - 3.97V
6 - 227V
7 - 74.1V
8 - 104.8V

The voltages look pretty good, only the voltage between R11 and R22 is significantly lower than noted on the circuit diagram and is around 18V instead of 27V. R11 and R22 have the correct resistor values.

I also measured the signal voltage (AC):
I go with a sinus voltage of 1 KHz; + 4dbU = 1.228V (0 VU) from my Focusrite 4i4 into the La2a.

Then I measure:
A10 Pin1 and Pin 6 (input): 0.668V / -1.30dBm
A10 at pin 7 to ground (output): 5.51V / 17.04 dBm
12AX7A / pin 2 or middle pin of R1
Gain at 30: 79mV / -19.78 dBm
Gain at around 80: 1.228V / + 4dbm
A24 Pin7 to ground (input)
Gain at 30: 483.9mV / -4.1dbm
Gain at 80: 6.85V / + 18.9dBm
A24 Pin 1 and Pin 6 (output)
Gain at 30: 88mV / -18.9dBm
Gain at 80: 1.228V / + 4 dBm

Here is the frequency chart (Gain at 80):

THD.jpg

So I’m stumped, any ideas?
 

moamps

Well-known member
Joined
Sep 24, 2005
Messages
1,861
Location
Croatia, HR
The makeup gain of the LA2A is about 40dB IIRC, so you need to look for the problem somewhere around V1 12AX7. You can remove V3 and V4 and T4B during testing.
 

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
4,557
Location
Sussex, UK
Be aware that any voltage readings you take with V3 & V4 removed will be higher than normal since the PSU has less of a load. Personally I would leave them in for that reason. There is really no benefit in removing them since if the gain reduction control is fully down they are not affecting the levels in the audio part of the circuit.
 

barthman.de

Well-known member
Joined
May 26, 2007
Messages
87
Location
Erfurt / Germany
Yes, it's better not to remove V3 and V4.

BTW I found my mistake :) A classic - R12 has to be 470k - I had installed 470R😬 Now I have enough gain and can optimize the cable routing a little.It appears that the connection between R17 and 12BH7A is particularly sensitive to interference.Thank you for helping me isolate the problem.
 

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