Voltage Issue Troubleshooting [resolved]

[Toobuzz]

Hi everyone,

I had a blast building this kit over the last week!  I’ve learned a ton and this was certainly the right kit for me as it’s just a bit outside my comfort zone.

Issues:
1. Kreg voltage low (5.2V) on the A side of the Regulator board.
2. LEDs not illuminating on B side of C4S board that is mounted over the A tube socket.

Current voltages (targets in parentheses):
275vDC on the Power Supply Board (275V) = 283V
+6.3vDC on the Power Supply Board (55-65V) = 58V
-6.3vDC on the Power Supply Board (50-60V) = 52V
IA on the A side C4S Board (275V) = 283V
IA on the B side C4S Board (275V) = 283V
Breg Regulator Board A (220V) = 218V
Breg Regulator Board B (220V)= 218V
-reg Regulator Board A (0V) = 0V
-reg Regulator Board B (0V)= 0V
Kreg Regulator Board A (8-12V) = 5.2V
Kreg Regulator Board B (8-12V) = 10.5V

History:

I originally had an additional issue where my +275V on the PS board was measuring 235V (and no B side LEDs lit on either C4S board), but cruising this forum pointed me toward the jumper wires on the C4S boards.  I had a mistake where I ran the long red wire jumper on OB to IA instead of OA to IB on the board above the C tube socket. Once I resoldered this jumper my +275V is now reading 283 and B side LEDs are now illuminating on the C4S board above the C tube socket.

Any help would be greatly appreciated!

[Paul Birkeland]

How is the solder joint at pins 4 and 5 on the 12AU7 socket?

You have an amp that looks like it's working nicely, except one half of the 12AU7 is not quite happy.  This could also be a loose joint on that socket as well, or perhaps one of the resistors going to pins 2/7 isn't quite all the way connected. 

[Toobuzz]

Thanks!  I’m getting close. The joints on pin 4 and 5 look ok, same with the resistors. I reflowed these just now and tested again. Unfortunately, still getting the same readings, and no glow from the 12AU7 (tried several known good tubes).

Here are some photos of the 12AU7 socket and overall photos of both sides of the boards.

[Paul Birkeland]

The 12AU7 has to have at least one side that's glowing in order for you to get the voltages you're getting.

[Toobuzz]

Apologies. I dimmed the lights and now see the glow.

[Paul Birkeland]

OK, do both halves of the 12AU7 appear to be glowing?

[Toobuzz]

Yes, both sides are glowing.

[Paul Birkeland]

You could certainly try a different 12AU7 if you'd like.  Technically the amplifier is working.  If you find the same imbalance with a different tube, then I would look more carefully at the resistors connecting to pins 2 and 7 on the 12AU7 socket, as well as the solder joint at pins 4/5.

[Toobuzz]

I can’t get the bias lower than 215V on either side. And I have no LEDs kit on the B side of the A tube. I think I have trimmers bottomed out. This is ok?

*edited to add....

Voltages still the same with various tubes. I did a listening test (leaving it at that ). I’ll dig into redoing/testing those areas of the 12au7 socket. I’m fine with this being a journey and am grateful for your help!

[Toobuzz]

I de-soldered (wick) and re-soldered 2, 4, 5, and the center of the 12au7 socket.  While not particularly pretty, this is what I have, The issue is now worse.  I'm getting 3.3V (Kreg B) and 4.4V (Kreg A).  Perhaps I fried these little stoppers?

[Paul Birkeland]

I can’t get the bias lower than 215V on either side.

Oops, that I did not notice! That could definitely throw off the Kreg voltages on its own.

When you put the blue trim pots on the board, did you fold the pair of leads over and solder them together as instructed in the manual?  (It looks like you did, but I couldn't quite see them)

What are the OA/OB voltages on the center PC board?  Do they change when you twist the screws on the blue trim pots?

As far as the dark LEDs, that could easily be a flaky solder joint on one of the PC boards.  I would reflow all the joints on the suspect board other than the LEDs themselves.

[Toobuzz]

I definitely remember folding over those leads on the trim pot, but anything is possible at this point.

OA = 3.27V
OB = 3.34V

After 10 clockwise rotations each...

OA = 2.97
OB = 2.85

Interestingly I am also now getting

Kreg A = 8.52
Kreg B = 6.45

Bias voltages still over 205V

I’ll reflow that board without the leds but this other issue seems to be the one to focus on.

[Toobuzz]

Whoa, I’m getting somewhere!

After 15-20 more clockwise rotations of the trim pots I’m able to all the voltages within range!

[Paul Birkeland]

They will rotate infinitely.  When you are at the end of the range, they will click with every rotation to let you know that you're at the end.  I do not see any wiring errors in the section that would allow for the cathode voltage of the 6C45PI tubes to be established, so you should be able to set them down closer to 1.5V to get the plate voltages down to what's mentioned in the manual, then you can get a clearer picture about what's going on with the amp.

[Toobuzz]

I mean it sounds fantastic straight away. No weak or loose bass. Dead quiet. I must say I built a fine amp!