Voltage issue with High Current C4S

[Toobuzz]

Alright, I replaced the 2N2222A and the TIP50 on the problem high current board. All measurements are now spot on except the Kreg on the side with the bad HC board (Now the B side of the LC board).

On the LC board I get
KregA 5.5V
KregB 2.5V (side with suspect HC board)

By the way, this is pretty much what I originally had when it was working (except KregA was low). I may have had 2.8V on the Kreg, but I figured it was close enough to 3V. Im no longer going to be that cavalier. Now, since the issue has moved to the other side, following the board, that gives me confidence that the underlying issue is not wiring or sockets. It’s also not the LC board as it performs fine when connected to the other HC board.

Any suggestions for getting that Kreg up?

I did put in some equivalent parts (until stock parts arrive).

I’m using an 2N2483 (NTE says it’s a 2N2222A equivalent) and a TIP50 D1.  These are on high current side and wouldn’t seem to affect the Kreg, but disclosing for sake of completeness.

[Paul Birkeland]

2.8V is barely enough.  Does this follow one of the 6AQ5s if you swap them side to side?

[Toobuzz]

Yes, it does! Sort of. Simply swapping the 6AQ5s brought the KregA up to 3.75 and the KregB down to 4.35. Technically all voltages are in range now!  I don’t have that warm and fuzzy felling yet, however.  Is the next logical thing to try a fresh set of 6AQ5s?

Whoa! That TwoQuiet went from “real quiet” to pretty darn amazing sounding! 

[Paul Birkeland]

6AQ5s are one of those tubes that really need to be awoken to work all the way.   When I test them before sending them out, it can take a while for them to come up to full emission. I would leave the amp on for three or four hours, then check the voltages.  I would bet that you'll find that both of your Kreg voltages climb after some run in time.

[Toobuzz]

Thanks again, PB!  I really enjoyed working through this with you and I am super grateful. Even though I may have come across more as frustrated (I was).  This forum is 1000x better than a tutorial or YouTube video in terms of learning about and hence, appreciating the amp. I consider this experience excellent value for money!

[Paul Birkeland]

We are always here to help, and the C2A is certainly one of the more complicated circuits to debug.  Hopefully your fixes are all that's required for years of reliable operation.

[Toobuzz]

So all has been well for the last few days, all voltages well within range. However, I lost the right channel after a 12au7 tube swap this morning.  She was fine for 20 minutes then I powered down to swap the 12au7.  Im hoping you can help me zero in on what would cause OB on HC board to jump up to 145V?  OA on the LC board is also reading 145V.

Is this the part of the circuit that involves the plate of the 12au7 and the MJE350 and PN2907?

These are the only 2 measurements (resistance and voltage) that are affected right now.

Thanks!

[Paul Birkeland]

Can you put the other 12AU7 back in?

[Toobuzz]

I did try that. Still seeing that 145V.

The Kregs are both rock solid at 4.4V now, btw.

[Paul Birkeland]

What's OA and OB on the low current center C4S board?

[Toobuzz]

OA = 145V
OB = 91V

[Paul Birkeland]

OK, likely the only issue with your amp is that high OA voltage on the center PC board.  That can happen from any of the following and this is the order I would use to debug them:

1.  The solder joint at pins 4/5 on the 9 pin socket isn't solid and one half of the tube isn't heating up.  This will also manifest as only half of the tube glowing when you look at it. 
2.  The solder joint on the center leg of the MJE350 feeding OA isn't 100% soldered and working properly.  Just reheat it until you see the solder wick into the board to be sure.
3.  Possibly an LED isn't working properly, but they don't break on their own.  A voltage of 145V on OA and a DC voltage across an unlit LED of about 12V DC would indicate a failed LED.  It's really important that when you see an LED not illuminating that you don't fixate on it and resolder it to death.
4.  Some other wire is broken or loose. 

[Toobuzz]

I found the issue and I’ll spare you a pic.  12au7 rolling made the pins move and the lead attached to the center pin of the right channel snapped.  I’ll put a little slack in my replacement.

[Paul Birkeland]

You can also install a wire between pins 3 and 8 for the time being to get the amp back up and running. 

[Toobuzz]

Thanks for the tip!  Do you think it would be wise to order some new resistors and a little more wire (in addition to the LED) and clean up that 12au7 socket. It’s definitely the sloppiest part of what I’ve done here. I was thinking the heater wire is fine but maybe redo everything else attaching to the 12au7 lugs?