Voltage issue with High Current C4S
- Thread starter Toobuzz
- Start date
Toobuzz said:
Pull the 6080 and the 12AU7 and the other 6AQ5, then recheck to see if the OA voltage there pops back up.
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.
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?
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?
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.
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.
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)?
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)?
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.
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!
Thanks again for all your attention to this!
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.
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.
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.
2.8V is barely enough. Does this follow one of the 6AQ5s if you swap them side to side?
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!
Whoa! That TwoQuiet went from “real quiet†to pretty darn amazing sounding!
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.
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!
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.
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!
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!
Can you put the other 12AU7 back in?
What's OA and OB on the low current center C4S board?
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