Agree!!!
But I have seen the reading with Fluke tester; I think with this very low voltage just some cm of dsitance you read different value. Also for the ground routing and how long it is
Walter.
I replaced the coupling caps with the stock one...pls see the pictures
I replaced the 300B and used the Western for this measurement
No change at all...
The measured TP10 starts at 60mV and slowly constantly increases to more than 120mV as you can see on the pictures.
AC is 234V now here in my workshop (10V AC higher than two hours ago)
I only measured the right channel and unplugged the 300B from the left channel.
2: 2.8V (REF: 2.8V)
4: 57.6V (REF: 70V)
6: 65.4V (REF: 75V)
8: 215V (REF: 205V)
10: 0.0632 => 3min => 0.1068 => 3min => 0.1205
12: 64.5V (REF: 60V)
14: 69.5V (REF: 12 + 5V)
16: 388V (REF: 390V)
18: 396V (REF: 405V)
20: 312V (REF: 330V)
22: 415V (REF: 420V)
Due to the higher supply voltage of 234V AC, the supply related values are much closer to the reference values now.
However, the grid offset is still present and seems to increase overt time.
I replaced the 300B and used the Western for this measurement
No change at all...
The measured TP10 starts at 60mV and slowly constantly increases to more than 120mV as you can see on the pictures.
AC is 234V now here in my workshop (10V AC higher than two hours ago)
I only measured the right channel and unplugged the 300B from the left channel.
2: 2.8V (REF: 2.8V)
4: 57.6V (REF: 70V)
6: 65.4V (REF: 75V)
8: 215V (REF: 205V)
10: 0.0632 => 3min => 0.1068 => 3min => 0.1205
12: 64.5V (REF: 60V)
14: 69.5V (REF: 12 + 5V)
16: 388V (REF: 390V)
18: 396V (REF: 405V)
20: 312V (REF: 330V)
22: 415V (REF: 420V)
Due to the higher supply voltage of 234V AC, the supply related values are much closer to the reference values now.
However, the grid offset is still present and seems to increase overt time.
You're right. I have not noticed this.
However, here is a test certificate I've downloaded from a Western dealer.
It shows a comparable chart.
I decided to rip up the whole device for a complete resolder and inspection job.
I'll keep you updated....
I'll keep you updated....
Hi guys,
I resoldered all pads on the PCBs today, cleaned the board and used a Milli-Ohm-Meter to measure each trace on the board. Then I measured all parts for their proper values according to the schematic. Everything okay....
Then I reassembled the device and started measuring again.
All testpoints are okay, except TP9 and TP10.
Still in the range of 200mV, depending on the tube.
I made the following tests:
1) Swap the tubes from one position to the other. Then the offset also changes either. So, this is definetly tube related.
2) Removed the DC coupling caps completely
3) Replaced the coupling caps with the stock ones
4) Changed the 12BH7 to another pair, which I have here. No 12AU7 available unfortunately.....
5) Changed the Chinese 300B to the WE300B and retestet 1), 2), 3) and 4)
During all tests, the cathode voltage and therefore the Bias was okay and in the range of -62V
All other TPs were okay either.
BUT, and I think this is important.
Right after startup and if the 300Bs were cold, I measured a "proper" value on T9/10 in the range of 30mV. It was a little bit noisy and changed rapidly between 20....45mV, but in average it measured 30mV.
But approx. after 5 minutes, when the tubes warmed up, the voltage on T9/10 started to increased slowly.
All other TPs still tested fine and especially the Bias didn't change.
After 30 minutes of warmup the offset reached the previously measured 210mV and 250mV on the WE300B
Important:
I measured the voltage drop over the 220k Grid Leakage resistors R113/R213.
The positive side of the voltage was on TP9/10, so on the grid.
This means the current, which generates the offset flows from the grid via R113/R213 to ground.
IMHO, this makes sense and is in harmony with the data given in the test certificate of my WE300B (posted above)
It states:
Tube 312511: Grid current: 0.86uA
Tube 312513: Grid current: 1.00uA
=> Voltage on TP9 according to test certificate with grid leak resistor of 250k: 0.86uA * 250k = 215mV
=> Voltage on TP10 according to test certificate with grid leak resistor of 250k: 1.00uA * 250k = 250mV
And these are the values I measured after a warmup time of approx. 30 minutes!
Around 210mV on one and 255mV on the other WE300B.
@vkung
Have you measured T9/10 on cold tubes during the first 5 to 10 minutes after power up?
After all the measurements I did today, I agree to the statement from Rod Coleman, who wrote yesterday in the first post:
However, and this is most important:
The Amp works pretty well, even with this "high" offset according to the Elekit manual.
I wired the AMP to 8 Ohm dummy loads and connected it to my Audio Precision Analyzer.
Input: 1kHz sine with 1V RMS from AP Analyzer
WITH FEEDBACK after 30 minutes warmup
Output power in W THD in % Left and Right unweighted <10Hz .... >300kHz
Max output power at 10% THD
Residual noise IEC_A 8Ohm
This matches exactly the value given in the manual
WITHOUT FEEDBACK after 30 minutes warmup
Output power in W THD in % Left and Right unweighted <10Hz .... >300kHz
Max output power at 10% THD
Residual noise IEC_A 8Ohm
Non-Feedback might sound better, but from measurement it's pretty worse compared to the feedback variant.
I've got no complaints, when looking at these results 🙂
The amp works and sounds pretty well. No noise, hum or anything else at the speakers, even when the volume is turned up to max.
@vkung
Could you please tell me the value of C107/207, which you were using in your builds?
In my schematic these caps are specified with 8.2uF 400V and there were 4 pieces of 8.2uF in the kit
But I saw pictures of boards in the web, where 7.5uF 400V caps were mounted.
7.5uF is a pretty strange value for a cap. So, I wonder why Elekit has choosen such a value?
I hope this is not a possible reason for my measurement issues on TP9/10? 🙂
I resoldered all pads on the PCBs today, cleaned the board and used a Milli-Ohm-Meter to measure each trace on the board. Then I measured all parts for their proper values according to the schematic. Everything okay....
Then I reassembled the device and started measuring again.
All testpoints are okay, except TP9 and TP10.
Still in the range of 200mV, depending on the tube.
I made the following tests:
1) Swap the tubes from one position to the other. Then the offset also changes either. So, this is definetly tube related.
2) Removed the DC coupling caps completely
3) Replaced the coupling caps with the stock ones
4) Changed the 12BH7 to another pair, which I have here. No 12AU7 available unfortunately.....
5) Changed the Chinese 300B to the WE300B and retestet 1), 2), 3) and 4)
During all tests, the cathode voltage and therefore the Bias was okay and in the range of -62V
All other TPs were okay either.
BUT, and I think this is important.
Right after startup and if the 300Bs were cold, I measured a "proper" value on T9/10 in the range of 30mV. It was a little bit noisy and changed rapidly between 20....45mV, but in average it measured 30mV.
But approx. after 5 minutes, when the tubes warmed up, the voltage on T9/10 started to increased slowly.
All other TPs still tested fine and especially the Bias didn't change.
After 30 minutes of warmup the offset reached the previously measured 210mV and 250mV on the WE300B
Important:
I measured the voltage drop over the 220k Grid Leakage resistors R113/R213.
The positive side of the voltage was on TP9/10, so on the grid.
This means the current, which generates the offset flows from the grid via R113/R213 to ground.
IMHO, this makes sense and is in harmony with the data given in the test certificate of my WE300B (posted above)
It states:
Tube 312511: Grid current: 0.86uA
Tube 312513: Grid current: 1.00uA
=> Voltage on TP9 according to test certificate with grid leak resistor of 250k: 0.86uA * 250k = 215mV
=> Voltage on TP10 according to test certificate with grid leak resistor of 250k: 1.00uA * 250k = 250mV
And these are the values I measured after a warmup time of approx. 30 minutes!
Around 210mV on one and 255mV on the other WE300B.
@vkung
Have you measured T9/10 on cold tubes during the first 5 to 10 minutes after power up?
After all the measurements I did today, I agree to the statement from Rod Coleman, who wrote yesterday in the first post:
I think Elekit should specify the test condition for the voltage measurements: Cold after startup or fully warmed up
However, and this is most important:
The Amp works pretty well, even with this "high" offset according to the Elekit manual.
I wired the AMP to 8 Ohm dummy loads and connected it to my Audio Precision Analyzer.
Input: 1kHz sine with 1V RMS from AP Analyzer
WITH FEEDBACK after 30 minutes warmup
Output power in W THD in % Left and Right unweighted <10Hz .... >300kHz
| 0.01W | 0.065% | 0.049% |
| 0.1W | 0.086% | 0.065% |
| 1W | 0.27% | 0.19% |
| 2W | 0.39% | 0.28% |
| 4W | 0.66% | 0.69% |
| 6W | 3.1% | 3.3% |
| 7.6W | 7.4W |
| Bias | -62.2V | -62.9V |
| T9/10 | 250mV | 210mV |
Residual noise IEC_A 8Ohm
| Volume 0 | 15uV | 14uV |
| Volume 100 | 37uV | 33uV |
WITHOUT FEEDBACK after 30 minutes warmup
Output power in W THD in % Left and Right unweighted <10Hz .... >300kHz
| 0.01W | 0.11% | 0.16% |
| 0.1W | 0.30% | 0.46% |
| 1W | 1.00% | 1.35% |
| 2W | 1.42% | 1.78% |
| 4W | 2.11% | 2.62% |
| 6W | 3.1% | 5.94% |
| 7.1W | 6.9W |
Residual noise IEC_A 8Ohm
| Volume 0 | 50uV | 41uV |
| Volume 100 | 140uV | 110uV |
Non-Feedback might sound better, but from measurement it's pretty worse compared to the feedback variant.
I've got no complaints, when looking at these results 🙂
The amp works and sounds pretty well. No noise, hum or anything else at the speakers, even when the volume is turned up to max.
@vkung
Could you please tell me the value of C107/207, which you were using in your builds?
In my schematic these caps are specified with 8.2uF 400V and there were 4 pieces of 8.2uF in the kit
But I saw pictures of boards in the web, where 7.5uF 400V caps were mounted.
7.5uF is a pretty strange value for a cap. So, I wonder why Elekit has choosen such a value?
I hope this is not a possible reason for my measurement issues on TP9/10? 🙂
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