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Tubelab Simple SE Help

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Hi Everybody,

I need some help with troubleshooting my Tubelab Simple SE project. I originally bought the PCB and parts around 9 years ago and I am finally trying to get it to work! Unfortunately, on initial testing in Triode wiring I have almost inaudible sound (ear pressed against the speaker) and a fair bit of hum. Everything lights up and looks great but no sound.

These are the parts I have gathered:
PT - Edcor XPWR 059 (240V in Australia)
Output – Edcor GXSE 15-8-5k
Tubes: 6L6GC STR, 5U4GB rectifier, 12AT7
I do not have a choke (although might later)

I have gone through the Ty_Bower checkout (which is great) and got these results:
1. ~100 kOhm - Ground to rectifier pin 8
2. 222kOhm - Gr. to 12AT7 pin 2
3. 221kOhm – pin 7
4. 221kOhm – pin 3
5. 220 kOhm – pin 8
6. 222 kOhm – Gr. to V11 pin 5
7. 221 kOhm – Gr. to V21 pin 5
8. 550 Ohm – Gr. to V21 pin 1
9. 549 Ohm – pin 8
10. 549 Ohm – Gr. to V11 pin 1
11. 550 Ohm – pin 8
12. 99 Ohm – V11 pin 3 to 4
13. 99 Ohm – V21 pin 3 to 4
14. 7.4 VAC - 12AT7 heater voltage pin 4
15. 7.42 VAC – pin 5
16. 7.41 – V11 pin 2
17. 7.43 VAC – V21 pin 2
18. 6.12 VAC – Rectifier filament voltage pin 2
19. 406 V – Rectifier voltage pin 4
20. 405 VAC – pin 6

It seems as though the power tube triode strapping numbers are the only major problem. Is this because of my R1 150 Ohm resistor use instead of the choke?
I have tried fiddling with the output polarity and removed the volume control to see if this has changed anything. It didn’t. I am using my phone as an input and (not ideally) largish speakers. Any help would be much appreciated.
 
I don't have a board or schematic handy, but on the 12AT7, the grid resistors should be 220K, the cathode resistors should be 220 ohms.

Check to make sure you have the right value cathode resistors.

The hum is likely a combination of the wrong cathode resistors, and no choke.

Win W5JAG
 
I don't have a board or schematic handy, but on the 12AT7, the grid resistors should be 220K, the cathode resistors should be 220 ohms.

Check to make sure you have the right value cathode resistors.

The hum is likely a combination of the wrong cathode resistors, and no choke.

Win W5JAG

Thanks for the reply. I've typed these values incorrectly. 4 & 5 in my list should be 220 Ohm. I've also checked the board and retested with my multi-meter.
 
Thanks for the help so far. I've been checking all the resistors on sight and also individually tested their resistance. They're all as they should be. Maybe a poor solder somewhere in the mix?

With the input wiring i've checked the continuity of my solders, leads and input to R11&12 and R21&22. I think this is OK. Unless I'm missing a ground somewhere.

As a test I added a preamp to boost the input and sound from the speakers. It appears very distorted and still very low. Should I be considering a fault at 12AT7 or maybe the ICs?

Thanks again.
 
Are both channels bad? A decent picture of the board with as much of the associated wiring might help us find a mistake.

It would also help if you could provide DC voltage readings for the 12AT7 and output tubes. Put the black voltmeter lead on ground and touch the red lead to pins 1, 2, 3, 6, 7 and 8 of the 12AT7 and write down the numbers, then do pins 1, 3, 4 and 5 of each output tube. This will tell us if each stage is at least operating normally.
 
Are both channels bad? A decent picture of the board with as much of the associated wiring might help us find a mistake.

It would also help if you could provide DC voltage readings for the 12AT7 and output tubes. Put the black voltmeter lead on ground and touch the red lead to pins 1, 2, 3, 6, 7 and 8 of the 12AT7 and write down the numbers, then do pins 1, 3, 4 and 5 of each output tube. This will tell us if each stage is at least operating normally.

Yes both channels are the same. I'll add a few photos to this message. I'm getting some bizarre DC voltage readings

Ground to 12AT7
- Pin 1: -42.0mV
- Pin 2: 0mV
- Pin 3: 0mV
- Pin 6: -4.1mV
- Pin 7: 0mV
- Pin 8: 0mV

Ground to V11
-Pin 1: 0mV
-Pin 3: 0.3mV
-Pin 4: 0.3mV
-Pin 5: 0.3mV

Ground to V21
-Pin 1: 0mV
-Pin 3: 0.3mV
-Pin 4: 0.3mV
-Pin 5: 0.3mV
 
Some pics
 

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According to those readings you have no HT/B+, either that or no ground. For a 12AT7 or any 12**7 valve you should have 100v - 200v ish on pins 1 and 6 (anode), 0v on pins 2 and 7 (g1) and around 1- 8v for pins 3 and 8 ( cathode). These are just ballpark numbers.

Have you checked that you have heater/filament voltage on pins 4/5 and 9? There should be 6.3v AC.

It might be an idea to start back at your power supply, check you have HT/B+ at your smoothing/reservoir cap/s, if not do you have high voltage AC out from the transformer seconary's? With power off, mains lead unplugged and any caps discharged check you have continuity from the power supply/reservoir cap to your board.

Andy.
 
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My previous readings were with an unseated rectifier tube! I had changed it out for another temporarily. I have re-measured the DC voltages as follows:

Ground to 12AT7 pins:
1. -0.98V
2. -0.84V
3. 0.7mV
6. -1.09V
7. -0.91V
8. 0.80mV

Ground to V11 pins:
1. 34.7V
3. 435V
4. 346V
5. 67mV

Ground to V21
1. 36V
3. 435V
4. 435V
5. 113.5mV

Sorry for the confusion.
 
According to those readings you have no HT/B+, either that or no ground. For a 12AT7 or any 12**7 valve you should have 100v - 200v ish on pins 1 and 6 (anode), 0v on pins 2 and 7 (g1) and around 1- 8v for pins 3 and 8 ( cathode). These are just ballpark numbers.

Have you checked that you have heater/filament voltage on pins 4/5 and 9? There should be 6.3v AC.

It might be an idea to start back at your power supply, check you have HT/B+ at your smoothing/reservoir cap/s, if not do you have high voltage AC out from the transformer seconary's? With power off, mains lead unplugged and any caps discharged check you have continuity from the power supply/reservoir cap to your board.

Andy.

Thanks for your reply. I have re-measured the DC voltage, but they are obviously still different from the reference numbers you mention.

The heater/filament voltages are: 6.89VAC at 12AT7 pins 4 and 5, but only 0.8VAC at pin 9.

I can confirm that I have continuity from the PT to the rectifier tube. I also appear to have continuity throughout the rest of the power supply. However, maybe I'm missing something obvious. I also have ground at all points noted in the schematic as well, via my grounded inputs.
 
The pin 1 voltages on your output tubes confirm that they are operating properly, as is the rectifier / power supply circuitry. The 12AT7 circuit is not working. The usual suspect here is the 10M45 chips either installed upside down, or bad / counterfeit. Some builders in Europe have received counterfeit chips on Ebay.
 
The negative lead of C2 looks like a cold joint.

The trace for the B+ voltage for each 12AT7 plate comes off the positive lead of C2 - follow that trace and check both sides of each component on the trace with a voltmeter to see where the B+ goes away.

Would not hurt to touch up the positive lead of C2, even though it is apparently a good enough connection to send power to the output tubes.

Win W5JAG
 
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