Voltage issue with High Current C4S

[Toobuzz]

Thanks for your help PB!  I’ll work on getting a replacement.

[Toobuzz]

Ok, I replaced the regulator on the A side of the low current board and unfortunately I’m still getting 60V on IA. 

Just to double check, is this a compatible part?

https://www.mouser.com/ProductDetail/on-semiconductor/lm431aiz/?qs=fAPocZeZcjgLlO1ZPti8PA%3D%3D&countrycode=US&currencycode=USD

[Paul Birkeland]

Yes, that's acceptable.   With all the tubes in and the amp running, can you give me IA/OA on the high current C4S board on the bad side, as well as Kreg and the OA/OB voltage on the middle board on the bad side?

This is somewhat likely to just end up as a repair on my bench, so keep that option in mind also, though you'll need to replace the caps on the low current C4S board with what we provided first. 

[Toobuzz]

High Current Board on bad side
IA 195V
OA 60V

Low current board
IA 60V
OA 40V
Kreg A 9.5V
IB 150V
OB 70V
Kreg B 5.5V

Please let me know if there is anything else that I can try!

Is there anything to the fact that the KregA voltage was always low (1V or even less) and after the regulator change it’s now high (9.5V)?

Sam

[Paul Birkeland]

Your initial 1.1V on Kreg A is resolved, so that's good.  Now pull the 6AQ5 and verify that OA on the high current C4S board pops up.  Once you have verified that, put the 6AQ5 back in and pull the 6080, then perform the same test to see what OA on the high current C4S board settles to.

[Toobuzz]

Without the 6AQ5 I’m still only getting 60V on OA of the HC board.  Could there be an issue with the 7pin socket itself?

[Paul Birkeland]

Without the 6AQ5 I’m still only getting 60V on OA of the HC board.  Could there be an issue with the 7pin socket itself?

Pull the 6080 and the 12AU7 and the other 6AQ5, then recheck to see if the OA voltage there pops back up.

[Toobuzz]

Yes. With no tubes at all the OA is back up to 200+V on the HC board. It’s the 6080 alone that makes it drop. 

Does This make Q1 and Q2 on the B side of the HC suspicious?

[Paul Birkeland]

Well hold on a sec, try with just 6AQ5s, then the OA voltage on the HC board should cruise down to about 150V, then try with the 6AQ5s and the 12AU7 and everything will be OK other than the OB voltages on the big C4S boards.  You would also expect those Kreg voltages to be quite low, but I think the regulators will still work without the 6080 sucking down current.

[Toobuzz]

You are correct. It’s not perfect. With only the 6AQ5s the OA on the HC board did come down to 161V.

Adding the 12AU7 (still no 6080) I get:

HC board
IA 195
OA 162V
bA 0V
OB 0.1V
bB 162V

LC board
IA 162V
OA 71.5V
KregA 2V
bRegA 162V
IB 188V
OB 69V
KregB 2V
bRegB 188V

At least the LM431A seems to be working well. Is that right, since the Kreg voltages match on both sides?

[Paul Birkeland]

Yeah, the Kreg voltages are too low for the regulator to work effectively. 

So to continue debugging, the two paths would be:

1.  Undo the cap mods to see if things go back to normal.  If there's something amiss with the output coupling caps that could present itself as an issue with the 6080 in the socket but not be an issue at all with no 6080.

2.  Try a different 6080.  Since the amp was working before the mods, my money would be on the caps being somehow problematic.

It's worth mentioning that Audyn and Dayton 100uF film caps will fit the space in the amp pretty well.  Those Mundorfs don't look like they fit all that well and the extra 100V of voltage rating makes them bigger than would otherwise be necessary.

[Toobuzz]

I tried a different tube and the stock 100uf caps. Unfortunately, no change.  I’m confident in my wiring and soldering (famous last words), and I’d really like to tackle this myself. I’ll buy a whole new set of parts, whatever is necessary. If you think it’s my wiring then I’ll order some and redo a whatever sections are needed.

Based on my last set of measurements, and knowing that the caps were good, doesn’t that point to Q1 and Q2 on the B side of the HC board (the B Side of the HC board is the only place where the LEDs won’t light)?

[Paul Birkeland]

If you had just built this amp and it had never worked, then I would be more suspicious of the current sink load for the 6080.  At this point you can swap the high current boards to see what happens.  If the problem switches channels, replacement of at minimum the TIP50 and 2N2222 transistors would be in order, but there is generally a reason for a failure like this to happen, and ascertaining that may present a bit of a challenge.

[Toobuzz]

The issue followed the board to the other side.  I see, you are doubtful that a bad component is the root cause. If I replace the bad components the new ones might go too. Is it worth a shot to try replacing them and see what happens after that?  I did see and replace some sketchy wiring on terminal 6. Perhaps that contributed.

Thanks again for all your attention to this!

[Paul Birkeland]

Yes, I would replace the transistors on that half of the board, but keep an eye on the amp for a while to see if more problems develop.  Sketchy wiring at 6U could cause a fault that took out your 431 chip, but that shouldn't really cause problems with the 6080 and its current sink load.