All things LA2a related

[Creamman]

Hi Rock Soderstrom,

you wanted to know which power supply I am using: Hammond 370EX.

I use the 5AR4 tube in the power supply which is mounted outside the housing, just like the Hammond.

Looking for grounding problems I noticed that the B + somehow collapses as soon as the power supply is loaded, which ultimately results in a loss of 50V, so that B + is only 233V?

While looking for my mistake I noticed that I installed LM317T.
LM317HV would be correct according to the circuit diagram.

I ordered some and hope they fix my 100 Hz hum.

Thank you for your help, I get back as soon as the new voltage regulator is soldered.

 

[Creamman]

So..... I installed the new voltage regulator. Checked all groundings again, all soldering points and all components.

Unfortunately none of this helped !!!!
I noticed that the hum only appears after the heating process is completed. I also noticed that this humming can be heard without a loudspeaker, directly from the electronics ...
But so far I could not hear from which component it comes from. ?? But it is most noticeable near the power supply board.

 

[scott2000]

So..... I installed the new voltage regulator. Checked all groundings again, all soldering points and all components.

Unfortunately none of this helped !!!!
I noticed that the hum only appears after the heating process is completed. I also noticed that this humming can be heard without a loudspeaker, directly from the electronics ...
But so far I could not hear from which component it comes from. ?? But it is most noticeable near the power supply board.

Can't recall the frequency of the noise addressed but this thread went in to some rabbit holes. Maybe you can get some more ideas from there??. His final solution is mentioned in the last post.

https://groupdiy.com/threads/rca-ba-2c-buzzing-issue.78167/page-4

 

[rock soderstrom]

I also noticed that this humming can be heard without a loudspeaker, directly from the electronics ...
But so far I could not hear from which component it comes from. ?? But it is most noticeable near the power supply board.

If you can hear the buzz without speakers, it is probably mechanical hum. Most of the time it is the mains transformer that starts humming. However, other components can also cause it.

Let's start with the power transformer. Can you feel the vibrations of the transformer with your hand? If yes, the hum can have several reasons. Which transformer do you use?(Its a Hammond 370EX). Is it sufficiently dimensioned? Some transformers tend to hum at high load or overload. Some just do it for no obvious reason...What happens when you mechanically disconnect the transformer from the case? Do you have another transformer to test?

As scott mentioned above, here is a user with a similar problem and his solution.

https://groupdiy.com/threads/rca-ba-2c-buzzing-issue.78167/post-1002287
If it's not the transformer, try to locate the source of the vibration. Are you using a choke? Check if it vibrates. Have you changed the rectifier tube? Are all capacitors polarized correctly? Note the errata in the documentation.
The Drip PSU board gives the possibility to use diodes instead of tube rectifiers. You have to choose one, either tube or diodes, not both at the same time.

 

[rock soderstrom]

I took a look at the data sheet of the Hammond 370EX. It can deliver 4 amps at 6.3 volts AC. Your LA2A needs about 3,3A of heating current (the 5V/ 3A in AC for the rectifier tube will be added), in DC(!) for the Drip Opto 7. From my point of view the power transformer is overloaded.

The Valvewizard writes the following about it:

"Another important thing to be aware of is that rectifying AC to DC in this way introduces a power-factor loss of about 0.5, which loads down the transformer more. In other words, to supply a heater with 300mA of DC the transformer actually has to deliver about 600mA of AC, So be careful not to overload the transformer."

Can you still touch the power transformer after a while?
What happens if you unplug V3 and V4 on both channels to relieve the heating winding. Less buzz?

http://www.valvewizard.co.uk/heater.html

 

[Creamman]

The humming comes from the power supply. It's not getting warm at all.
I swapped the rectifier tube for the diodes. No success.
I disconnected the entire secondary side of the 370 Ex. Transformer still hums.
I reconnected everything, disconnected the Travo from the case, removed V3 and V4 from the motherboard and switched it on. Now the transformer literally began to dance, with a very loud mechanical hum. I don't use a choke, just a resistor.

I will now start to search for a new Transformer.

 

[rock soderstrom]

I disconnected the entire secondary side of the 370 Ex. Transformer still hums.

If the transformer is humming without any load, there are only two possibilities:

1. the primary wiring is wrong. How is the transformer connected at the moment?

2. the transformer is defective.

 

[Biasrocks]

Troubleshooting my P2P LA2A build. I'm getting very little output from the sidechain amp.

I've got signal into the the 6AQ5 off the Stereo Link pot ~37vac with the Stereo Link and HF adj maxed. 1khz @-18dbv tone into the input transformer.

I'm getting very little AC voltage into C11 feeding the T4B ~8.5vac with the gain reduction control maxed.

I have no voltage drop across 10K/2watt resitor R34 ~346vdc on the supply side and ~340vdc on the side connected to C11. The resistance across R34 is 9.94k.

All tubes in the sidechain amp have been swapped.

I replaced C11 because the dielectric had escaped and wasn't working and had an issue with a bad tube socket on the sidechain 12AX7 which I've sorted.

Could it be a bad 6AQ5 tube?

This was a working build for a number of years. But stopped working recently.

Any pushes in the right direction.

Mark

 

[mrclunk]

I Could it be a bad 6AQ5 tube?

Yes, i’ve had 2 of 4 6aq5 die in mine.
I’m using the Russian mil equivalent in my latest build (not pin compaterble)

 

[beatnik]

what are your dc voltages on the sidechain tubes ?

 

[Biasrocks]

what are your dc voltages on the sidechain tubes ?

6AQ5
Pin 1 0.57vdc
Pin 2 0.37
Pin 5 340vdc
Pin 6 2.6
Pin 7 -0.6

12ax7
Pin 1 115.5 vdc
Pin 2 0
Pin 3 1.04
Pin 6 115.5
Pin 7 0
Pin 8 1.1

 

[beatnik]

Something is definitely wrong around the 6AQ5. There should be about 130V on the plate and 100V on the screen.

The tube itself has got to be the prime suspect, but also check the components around it. R36, C10, R38, C7-D, C13, R35

And make sure the tube socket doesn't have a loose contact or oxidation

 

[Biasrocks]

I have a few NOS 6AQ5's coming which I'll swap in. I will also check the components around it to make sure they are in spec


Any suggestions for a good sounding 12ax7, 12bh7 combination for the output amp. I currently have an NOS GE 12bh7a in there and a JJ (ick) 12ax7

Update: I put a NOS RCA 12AX7 grey plate in that position. Sounds great. Still waiting on my 6AQ5's to see if that fixes the sidechain issue.

 

[andYz00m]

Question:

Do I need to mess around with the 250Ohm input and the center tap to the terminal strip of the original schematic? If I know I'm only going to be using it for XLR modern usage, is there any point in wiring the CT and 250 leads? My logic says no.

Also, does the stereo adjust do anything to the sound of the circuit? Seems like you could remove the stereo adjust pot, and go straight from plates of V2 to limit response pot to Grid of the 6AQ5, and then to the C6>C14/R30>Ground.

What am I missing?

 

[beatnik]

The stereo adjust really is only useful for pairing up dual units, when two T4B don't compress the same. You can replace the pot with just a 1M resistor to ground and that will result in maximum signal going to the T4B
Regarding input impedance, the lower taps perhaps are useful when connecting certain microphones straight into the LA-2A for different tonalities

 

[andYz00m]

Wanting to use this switch for the GR/Output selector switch, but on the spec sheet there is no pinout information. Im feeling dumb.

Any help with understanding how to wire this switch up?

https://www.tubesandmore.com/products/switch-rotary-3-poles-3-position

 

[beatnik]

Use the dmm in resistance mode or on continuity check and you'll figure it out.

I think for that switch might be better to have non shorting (break before make)

 

[andYz00m]

Use the dmm in resistance mode or on continuity check and you'll figure it out.

I think for that switch might be better to have non shorting (break before make)

Yeah, I figured it out. This switch is break before make so I think im good there.

 

[andYz00m]

Alright build is complete but having some issues.

The gain reduction is not working properly. Im feeding it a 1k sine at +4 and can get the amp to output properly, but when cranking up the GR knob, it doesnt show any gain reduction on the meter.

Triple checked all of my wiring and connections.

Also Im using a 12AT7 in V1 and the plate voltage is down to ~57DCV for both plates (I believe this should be more like 120V) but without tubes installed all the B+ rails are showing about 360.

Any suggestions of what to start looking at to troubleshoot?

 

[andYz00m]

Alright, I fixed the GR issue. Super stupid but the stereo adjust pot was maxed out (fully CCW) which was causing the GR to not function... derp.

Still having the low plate voltage issue on V1, but the unit is working properly. On the spec for the 12AT7 is says the plate voltage should be 100-250 but mine is showing 50/57 respectively. This seems like a problem.

I tried throwing a modern regular 12AX7 in there and it popped the plate voltage up to ~115 which is where its supposed to be.

The only other thing is the junction at C4 is stated as being 27V on the schematic but on my unit its 16V. Otherwise all the readings are within reason.

Thanks in advanced for any feedback.