LA2A build

[mihi_fuchs]

Hi Michael and thanks for your answer
I m not sure I understand it all but the hum I have I had before making the mod with the rectifier diodes. The mod gave the 6.3v at heater but the hum is still the same.
I don’t understand what you mean with: if you have the ground rails of rail 1 and 2 connected by the jumper. I don’t understand “connected by the jumper”
Ground of rail 1 and 2 goes from a 2 screw connector with green twisted wires to chassis where it meets with ground cables from iec and power tx.

Nick

from what i see you have a half wave rectifying system in at least one line. have wave rectifiers only use one amplitude (e.g. the positive curve, the negative part is neglected (for information see here: https://en.wikipedia.org/wiki/Diode_bridge and here https://byjus.com/physics/half-wave-rectifier/ .
if you check the last post you can find the disadvantages of these setups which includes more ripple noise (caused by longer breaks inbetween the rectified voltage).

connected by the jumper means that you connect the two ground rails electronically so you basically jumper those together by the wires.

if it is your heater that causes the problem you can try a ground lift (remove wire to star ground) if the voltages on that rail are low enough (i do not know the transformer voltage you come into that heater rail, make sure that you take all safety precautions necessary).

if the problem consists maybe go for a full bridge rectification on the other rail(s) too? that should minimize ripple noise.

however, if others got that psu to work, maybe it is only caused by the alignment of the components as rock suggested. however, those two things i would give a try if i had that psu in my hands …

take care with the high voltages when handling and modding the thing.

br
michael

 

[thelivingroom]

Hi Rock, thanks for answering
As I think I have all grounds connected as should be.
B+ and heater ground goes from the 2-connector on the red pcb with green wires to the chassis where it meets with ground from IEC and power TX
The power tx is https://www.don-audio.com/g-pultec-mono-power-trafo-sec-220v-9v-5V_1
Audio transformers is LL 1922 and LL5402
I have rotated, tilted and even moved the power tx to be laying flat on the floor with no diffrence at all.
I have replaced the LM 338 sitting at the chassis with no difference. Maybe replace the other 2 regulators in the psu?
The hum is pretty massive and constant at around -35dbfs
The picture is the best I found in my phone but I can of course take more photos.

This is a shot in the dark, but is that a black plate RCA in V1, and have you tried a different one? When I built my D-LA2A using them in V1 it had a loud hum in channel 2 (I can’t remember if it was 50 or 100Hz though), and I knew they are notorious for that, so before I started chasing my tail I swapped the tubes in V1 and that’s what it was.

 

[moamps]

Hi Rock, thanks for answering
As I think I have all grounds connected as should be.
B+ and heater ground goes from the 2-connector on the red pcb with green wires to the chassis where it meets with ground from IEC and power TX
The power tx is https://www.don-audio.com/g-pultec-mono-power-trafo-sec-220v-9v-5V_1
Audio transformers is LL 1922 and LL5402
I have rotated, tilted and even moved the power tx to be laying flat on the floor with no diffrence at all.
I have replaced the LM 338 sitting at the chassis with no difference. Maybe replace the other 2 regulators in the psu?
The hum is pretty massive and constant at around -35dbfs
The picture is the best I found in my phone but I can of course take more photos.

First you need to ask the seller of the board to help you with the power supply board because it is obvious that the schematic is wrong in the rectifiers part. Maybe the board is designed correctly (full wave rectifier).
Second, the total heater consumption of all vacuum tubes at 6.3V is more than 1.5A and if your transformer can only supply 1A on that secondary, it will be overloaded and overheat. You should also check that.
If you have a DMM, measure the voltages for heating and +B in the DC and AC positions on the output pins of the rectifier board, and send it here.

 

[Nick6691]

from what i see you have a half wave rectifying system in at least one line. have wave rectifiers only use one amplitude (e.g. the positive curve, the negative part is neglected (for information see here: https://en.wikipedia.org/wiki/Diode_bridge and here https://byjus.com/physics/half-wave-rectifier/ .
if you check the last post you can find the disadvantages of these setups which includes more ripple noise (caused by longer breaks inbetween the rectified voltage).

connected by the jumper means that you connect the two ground rails electronically so you basically jumper those together by the wires.

if it is your heater that causes the problem you can try a ground lift (remove wire to star ground) if the voltages on that rail are low enough (i do not know the transformer voltage you come into that heater rail, make sure that you take all safety precautions necessary).

if the problem consists maybe go for a full bridge rectification on the other rail(s) too? that should minimize ripple noise.

however, if others got that psu to work, maybe it is only caused by the alignment of the components as rock suggested. however, those two things i would give a try if i had that psu in my hands …

take care with the high voltages when handling and modding the thing.

br

Michael, as I understand your point of view It could be bad thing to have a full wave rectifier on one rail an a half wave rectifier on another rail and connect both to ground. The mod on the heater was necessary to get 6,3 V (had only 5,3v before the mod) Doing the mod on the high voltage rail will probaly result in changing a some components due to the higher voltage. I give groundlift a try.
Thanks

 

[Nick6691]

This is a shot in the dark, but is that a black plate RCA in V1, and have you tried a different one? When I built my D-LA2A using them in V1 it had a loud hum in channel 2 (I can’t remember if it was 50 or 100Hz though), and I knew they are notorious for that, so before I started chasing my tail I swapped the tubes in V1 and that’s what it was.

Hi thelivingroom, It is Blackplate RCA and I have 2 of them and tried both with no difference.
Maybe I should try with something else during the hum search?
Thanks
Nick

 

[Nick6691]

First you need to ask the seller of the board to help you with the power supply board because it is obvious that the schematic is wrong in the rectifiers part. Maybe the board is designed correctly (full wave rectifier).
Second, the total heater consumption of all vacuum tubes at 6.3V is more than 1.5A and if your transformer can only supply 1A on that secondary, it will be overloaded and overheat. You should also check that.
If you have a DMM, measure the voltages for heating and +B in the DC and AC positions on the output pins of the rectifier board, and send it here.

Hi Moamps and thank you for answer.
Intresting with the 1A on secondaries. Maybe have the wrong transformer? I`ll try to get som voltage reading up as soon as possible.
Nick

 

[Nick6691]

First you need to ask the seller of the board to help you with the power supply board because it is obvious that the schematic is wrong in the rectifiers part. Maybe the board is designed correctly (full wave rectifier).
Second, the total heater consumption of all vacuum tubes at 6.3V is more than 1.5A and if your transformer can only supply 1A on that secondary, it will be overloaded and overheat. You should also check that.
If you have a DMM, measure the voltages for heating and +B in the DC and AC positions on the output pins of the rectifier board, and send it here.

Hi again Moamps
Sorry maybe I attached a link to the wrong power tx, I have this:

Tube Power Transformer - Pri.: 2x115v- Sec.: 250v, 24v and 9v​

Primary: 2 x 115v (230v)
Secondarys:
250V 100mA
24V 0,5A
9V 2,5A
High-Quality Audio Transformer with integrated Shielding-Sheet

• Integrated Don-Audio Trafo Shield
• Optimized windings for ´Tube Projects
• Epoxy filled
• CE, RoHS Certification
• Special soft construction
• Isolated rubber base
• High-End Audio Transformer, made in Germany

It says 2,5A on the one used for heater

The voltage readings is like this
First rail (heater) AC 9,62V DC 6,28V
Second rail B+ AC 280V DC 245V
Third rail AC 27,7V DC 11,5V
at the moment i dont use the third rail but im going to use it for lights to vu meter.
Thanks
Nick

 

[moamps]

It's good that you still have a suitable transformer. Now set the DMM to measure AC and measure the AC voltage at the output connectors for DC heater T2 and B+ T7. The purpose of this measurement is to determine whether there is a superimposed interfering AC signal at the outputs of the power supply.

Also check that pin 2 of T2 is connected to main board CN6 pin 2 and that pin 2 of T7 is connected to pin 2 of CN1. Also make short pins 1 and 2 on connector T3 (the unmarked connector in the middle of the power supply schematic).

The next step will be to make a full wave rectifier mod on the B+ input as well.

The seller has a supporting thread here
https://groupdiy.com/threads/build-honeybadger-audio-la2a-official-build-help-thread.88237/
so contact him there to help you.

 

[Nick6691]

It's good that you still have a suitable transformer. Now set the DMM to measure AC and measure the AC voltage at the output connectors for DC heater T2 and B+ T7. The purpose of this measurement is to determine whether there is a superimposed interfering AC signal at the outputs of the power supply.

Also check that pin 2 of T2 is connected to main board CN6 pin 2 and that pin 2 of T7 is connected to pin 2 of CN1. Also make short pins 1 and 2 on connector T3 (the unmarked connector in the middle of the power supply schematic).

The next step will be to make a full wave rectifier mod on the B+ input as well.

The seller has a supporting thread here
https://groupdiy.com/threads/build-honeybadger-audio-la2a-official-build-help-thread.88237/
so contact him there to help you.

Thanks Moamps
I didn´t understand the AC measuring at output. I will do that when home this evening.
"Also make short pins 1 and 2 on connector T3" I dont understand this! Do you mean connect those 2 with a jumper? Hope you mean the GND plint.
Green connector witn 2 green wires.
Thanks
Nick

 

[Nick6691]

It's good that you still have a suitable transformer. Now set the DMM to measure AC and measure the AC voltage at the output connectors for DC heater T2 and B+ T7. The purpose of this measurement is to determine whether there is a superimposed interfering AC signal at the outputs of the power supply.

Also check that pin 2 of T2 is connected to main board CN6 pin 2 and that pin 2 of T7 is connected to pin 2 of CN1. Also make short pins 1 and 2 on connector T3 (the unmarked connector in the middle of the power supply schematic).

The next step will be to make a full wave rectifier mod on the B+ input as well.

The seller has a supporting thread here
https://groupdiy.com/threads/build-honeybadger-audio-la2a-official-build-help-thread.88237/
so contact him there to help you.

Hi Moamps
Ac readings at output connectors: Heater, first 10mV drops fast to 1mV then stabilized at 0,5mV
B+ 496mV
Connections between psu and main board checked and ok.

Nick

 

[HoneyBadger Audio]

Hello All

Looks like there are some questions about the board, specifically to do with full and half bridge rectification. I dont think the hum is caused by the rectification. It is most likely a bad capacitor or more likely a bad regulator. I have done various tests with full and half bridge rectification specifically on the LA2A project and it matters verry little which is applied so long as the output voltage is adequate for the system to operate. I'm sure some might point out that there is a longer period between spikes but there is adequate filtring to achieve a relatively ripple free signal (about 1mV or so).

The original 2A build uses a half bridge on the high voltage and AC voltage on the heater, mixing them does not matter much as long as done properly.

When I designed the board I was using some Hammond transformers with much higher secondary outputs. I now recommend the Don-Audio units as they are about half the price and more redily available. This switch needed a full wave bridge mod to be done on the heater in some instances that the heater was not able to achieve the required 6,3vdc. Basically switching the polarity and output of two diodes D2 and D3.
Take a look at the files atached from another builder @timtom , thanks for the pics

 

[analogstudio7]

Well, I was going to buy this set of UTC -10 & UTC A-24 transformers off eBay for my (hopefully) soon to be started La2a build. BUT alas... I just don't have the funds for these right now since I retired, now on Social Security and having already spent my "build allowance" on the other parts right now.

Anyway, I thought I'd pass this eBay listing along to you guys who could probably use these transformers right now. It's gonna take me a while to fill up my piggy bank again before I'm able to buy transformers!

Here's the ebay link 'Set of UTC A-10 & A-24 Transformers' https://www.ebay.com/itm/387401872739

Greg

 

[ubxf]

you can build it with cheaper transformers and swap them later. If you are not in a hurry you'll find them eventually for a more reasonable price

 

[VanGenz]

from what i see you have a half wave rectifying system in at least one line. have wave rectifiers only use one amplitude (e.g. the positive curve, the negative part is neglected (for information see here: https://en.wikipedia.org/wiki/Diode_bridge and here https://byjus.com/physics/half-wave-rectifier/ .
if you check the last post you can find the disadvantages of these setups which includes more ripple noise (caused by longer breaks inbetween the rectified voltage).

connected by the jumper means that you connect the two ground rails electronically so you basically jumper those together by the wires.

if it is your heater that causes the problem you can try a ground lift (remove wire to star ground) if the voltages on that rail are low enough (i do not know the transformer voltage you come into that heater rail, make sure that you take all safety precautions necessary).

if the problem consists maybe go for a full bridge rectification on the other rail(s) too? that should minimize ripple noise.

however, if others got that psu to work, maybe it is only caused by the alignment of the components as rock suggested. however, those two things i would give a try if i had that psu in my hands …

take care with the high voltages when handling and modding the thing.

br
michael

I builded a stereo LA2A 10-12 years ago, love it and use it "all the time". When i first tested it the was quite a lot of hum in the audio. I suspected the AC heather, put in a bridge rectifier and a "big" capasitor and voila! the hum was gone.