Kreg Voltages both low

[Randy Gage]

Hey everybody,

My first bottle head build, a Mainline Amp. I have been through the amp twice now and do not see an obvious reason my Kreg voltages are 3.04 A side and 1.91 B side. Otherwise everything seems fine tube are lit and other voltages are within ranges specified on page 88. I must be missing the obvious? Any help is appreciated.

Randy

 

[Paul Birkeland]

I would check that all of the wires going to that center PC board are in the correct spots.  I have seen issues like that when the Kreg wires were swapped by accident (or alternatively the +reg wires).  On a Mainline, this is kind of hard to do.

What I would recommend is swapping the high current C4S boards between channels.  If you have an A side C4S that is passing substantial extra current, then you can end up with a low Kreg voltage.  If this is the case, that Kreg voltage should move to the other side of the center PC board after swapping the high current C4S boards.

 

[Randy Gage]

Hey Paul,

Thanks for the quick response. I did read the post about the uneven Kreg voltages and swapping locations of C4S boards. Since mine are both very low I decided to slow down little. All resistance and Voltages are otherwise "Normal", So I am reading everyones notes first.

In taking my time here since I suspect it is probably "Operator/Technician Error". I just noticed that although the LED's are oriented correctly on both C4S boards none of the B side LED's are emitting light. A side LED's are fine, I am in a dark room so seems like I may have a more basic problem.

 

[Paul Birkeland]

Dark B side C4S LEDs and very low Kreg voltages means the 6C45P tubes are not drawing any current and it's probably all (or mostly) going through the regulator.  Are you able to perform the biasing procedure?  What are the IA, OA, IB, and OB voltages on each high current C4S board?  (Are the blue trim pot leads soldered together on the bottom side of the center board as indicated in the manual?)

 

[Randy Gage]

Hi Paul,

Busy week so when I can I swivel over to my project here in my office so I am slow to reply.

So IA A side is 269vdc  IA B side is 269 vdc
then OA is 220 and OB is 230

Seems initially like I couldn't get a good probe connection OB side went from 260 down to 217 acting like it was discharging through my meter?

Anyway I'll have some time this weekend to look deeper into my issue. Yes the POTS are correctly aligned and wired but first time I inserted them and soldered I had them backwards, caught that afterI finished the board when doing a final visual long before any power was applied BUT I may have damaged a lead when I reconfigured them. I will desolder this weekend and inspect them both, then resolver and re-trace everything before I move on.

I was unable to adjust bias A side stays at 236VDC Pin 20 on the Capacitor lead,  while C side is at 216Vdc Pin 30 capacitor lead will not adjust

Thanks again for the help here. Do you happen to know where the Kreg points are in the schematic? I don't see a full representation of the test points on the board on the schematic?

 

[Paul Birkeland]

Kreg pads are on the center PC board.

The lack of adjustment of the 6C45P plate voltages is what I would suggest going after.  This kind of thing can happen from missing jumpers on the center PC board, possibly from the pots being damaged from being uninstalled (you can put them in backwards, no issue there!).  There's an OA and OB on the center board that provide the bias voltage to the 6C45P and allow for fine tuning of the plate voltage.  If you turn each of the plue pots ten turns CCW, then measure OA/OB, what do you see?  Now turn both pots CW ten turns and remeasure those test points.  I would suspect those voltages won't change, and that will help to focus your efforts a bit.