Bottlehead Forum
Bottlehead Kits => Mainline => Topic started by: Loquah on May 06, 2020, 10:21:08 PM
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I had a drop-out in the left channel today and discovered a broken wire caused by a recent episode of tinkering. Having fixed the broken wire and re-tested all voltages, I decided to check the biasing while the amp was upside down and powered.
The B side bias was no issue at all, but the A side bias only has a voltage range of up to about 6.5V
Any ideas what might be going on?
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I had a drop-out in the left channel today and discovered a broken wire caused by a recent episode of tinkering. Having fixed the broken wire and re-tested all voltages, I decided to check the biasing while the amp was upside down and powered.
The B side bias was no issue at all, but the A side bias only has a voltage range of up to about 6.5V
Any ideas what might be going on?
Had a thought to swap the tubes left/right and the issue switched too. Guess that's a spontaneous dead tube, yes?
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The story continues...
I installed some different tubes I had here and the B-side LEDs on the C4S for tube socket C stopped lighting up. Voltage checks show that the kreg on the B-side is only at 4.5V or so.
I'm going to back away from the amp for now and wait for some advice because I feel like each fix I find reveals another issue...
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I'm really very confused by your information. You are giving a voltage of 4.5-6.5V, which voltage are you reporting?
The manual says to measure terminals 7 and 20 to set bias. If those are measuring around 5V DC, you have a serious problem, but I don't know if that's what you mean?
If you have 4-6V at Kreg on one side, that can just mean that one 6C45PI isn't conducting.
Which wire in the amp did you find was broken? It could be that this caused a failure in the amp that we now need to diagnose.
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Hi Paul,
Sorry for any confusion. I'll step it out carefully so I can make some sense.
1) wire connecting 19L to +reg is what broke
2) after connecting new wire, the bias voltage (measuring terminals 7 & 30) was only around 5-6V
3) I swapped tubes and the C board LEDs didn't light up now (B-side of C board) so I ran voltage tests again
4) During voltage tests, kreg on B side only 6V
Have I gotten "lucky" with a dead tube in my spare set? Could the original 19L-to-+reg wire break kill the tube that was installed?
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Does the low voltage move with tube swapping?
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I'll check when I get home.
Do you think the original wire break could have killed the tube I had installed at the time? (Different to the tubes I am now testing with)
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It's not impossible.
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Does the low voltage move with tube swapping?
No. After swapping tubes, the B-side LEDs above socket C do not light (or very dimly light) and the Kreg voltage on the B side of the regulator board is still low (around 5V)
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Just realised that it's possible the 19L to +reg cable broke when I was investigating the original issue of no sound from right channel. That would make sense based on the C-socket board not working right, wouldn't it?
Either way, the current problem is the Kreg voltage and non-lighting LEDs so I'll await your next instruction or diagnosis.
As always, thanks so much for the help!!
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Either way, the current problem is the Kreg voltage and non-lighting LEDs so I'll await your next instruction or diagnosis.
Isn't the problem that your bias voltage can't be set to 150V?
What are IA/OA voltages on the big board on the side that isn't working?
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It looks like we may be having some confusion due to terminology. Lachlan, can you give us all terminal voltages that are out of spec?
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+275 vDC = 271-272V
+6.3vDC = 6.29V
IA side A = 270V
IA side B = 269.2V
Breg = 220.5 (B side) and 219.2 (A side)
-reg = 0.5mV both sides
Kreg = 5.16V (B side) and 11.36V (A side)
Sorry for the confusion. The issue is that the original fault / out-of-spec measurement shifted while I was trouble shooting. I'm assuming the tube caused the original biasing issue and now we're dealing with something different.
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The B side C4S on the side with your low bias isn't supplying current to the 6C45, or there's something weird going on with that socket.
The easiest way to debug this is to swap the two big C4S boards to see if the bad voltage follows a board. If it does, then you can set your meter to beep for continuity and test each pair of transistor legs for each transistor on the B side of the offending C4S board. If any transistor has a short, just replace them both and consider replacing the LEDs as well.
If the low voltage does not follow a board, then I would be looking at the 6C45 socket wiring and double checking that the middle section of the middle PC board doesn't have any broken wires (around the area with the blue trim pots and 2N2222 transistors).
-PB
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Brilliant. Thanks Paul. My money is on a connection around the socket failing during the tube switch, but I'll work through the steps as described.
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Swapped C4S boards and the error remained at Socket C. Going to check socket and centre board next
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I've checked everything now to the best of my ability and cants solve the issue. What I've done:
- removed and reconnected the power regulator board
- reflowed any suspect joints on reg board
- tested for continuity across the regulator board transistor pins (no continuity between any of three pins on both)
- swapped C4S boards (i.e. they are currently opposite from their original arrangement)
- tested and reflowed joints on C socket
After full reassembly I've got the same issue of the two LEDs that light up later on the C socket C4S failing to light.
I'm guessing I've got a faulty component somewhere, but don't know how to test for it. Any tips would be hugely welcome.
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The center board can be rotated 180 degrees, that is how I would check the center board.
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Oh, I tried that, but didn't include it in my list because I wasn't 100% sure that was meant to work. The result was the LED issue switched sides. The second pair of LEDs above Socket C also didn't light, but I realised after that the tube was still out of the socket.
I'll retest this with the tube in to be certain, but working on the assumption that the problem changed side (and the C socket LEDs just needed a tube installed), what's my next step for diagnosis.
Sorry this is a challenging one!
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If rotating the center board also rotated the problem, then there's a trimmer pot, a 2N2222 transistor, and a resistor that are being swapped in that process, as well as the high voltage regulator components. Before swapping the center board again, what voltages do you see at OA and OB on that center board?
I have to be absolutely insistent that you not say anything about lit or dim LEDs moving forward. It's absolutely 100% necessary to have voltage measurements. Dark LEDs don't really tell me anything, while voltage measurements tell me a lot.
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No problems, Paul. Only using the LEDs as an indicator. Happy to focus on voltages now you've told me what to measure. I'll test as soon as I can and report back
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Original orientation
OA: 3.32V
OB: 2.28V
Centre board turned 180 degrees
OB: 2.22V
OA: 3.2V
Other measurements with centre board rotated 180 degrees:
+275: 286.6
+6.3: 6.29
IA: 289 (A) and 289 (C)
Breg: 220.3 (B side, now closest to A socket) and 157 (A side, now closest to C socket)
-reg: 0.4mV both sides
Kreg: 4.45 (B side, close to A socket) and 1.56 (A side, close to socket C)
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Breg: 157 (A side, now closest to C socket)
Kreg: 1.56 (A side, close to socket C)
These have changed. Can you pull the 12AU7 and recheck?
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You might be onto something...
Kreg B 1.16 and A 1.04
Breg B 286 and A 286
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With no 12AU7, are you still seeing that super low voltage at OB on the offending channel?
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With no 12AU7, are you still seeing that super low voltage at OB on the offending channel?
189V at OB on socket C
For reference, it's 262V for socket A
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OB on the two outboard C4S boards shouldn't be that high. You need to run the amp with no 12AU7 but with the 6C45PI tubes.
OB on the outboard C4S boards is of interest under these conditions.
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That was with 6C45Pi installed on both sides.
Confirming measurements WITH 6C45s and WITHOUT 12AU7. All references to sides are based on the A and C sockets on the amp to avoid confusion from the rotated regulator board. Apologies for any redundant info. Just trying to provide a complete picture
+275 = 288
+6.3 = 6.3
IA = 287 (A) 288 (C)
Breg = 285 (A side) 285 (C side)
-reg = 0.3mV (both)
Kreg = 1.16 (A side) 1.02 (B side)
OB: 259 (A) 187 (C)
IB: 285 (both)
IA: 286 (A) 287 (C)
OA: 284 (both)
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That was with 6C45Pi installed on both sides.
Confirming measurements WITH 6C45s and WITHOUT 12AU7. All references to sides are based on the A and C sockets on the amp to avoid confusion from the rotated regulator board. Apologies for any redundant info. Just trying to provide a complete picture
+275 = 288
+6.3 = 6.3
IA = 287 (A) 288 (C)
Breg = 285 (A side) 285 (C side)
-reg = 0.3mV (both)
Kreg = 1.16 (A side) 1.02 (B side)
OB: 259 (A) 187 (C)
IB: 285 (both)
IA: 286 (A) 287 (C)
OA: 284 (both)
Just rechecked all my wiring because of your concerns about OB voltages. OB on C4S boards remained as per quoted post here.
OB on regulator board reads 2.24V (connected to C socket) and OA is 3.37V (connected to A socket)
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OK, so the OB voltage that really wanted to spend the rest of its life at 6V has popped back up without the regulator in the way.
I believe your issue will be resolved by replacing the 431 regulator on the offending channel, with the slightest possibility that you'll also need a 12AU7.
-PB
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OK, so the OB voltage that really wanted to spend the rest of its life at 6V has popped back up without the regulator in the way.
I believe your issue will be resolved by replacing the 431 regulator on the offending channel, with the slightest possibility that you'll also need a 12AU7.
-PB
Thanks Paul - I appreciate your help and patience. I've ordered the required parts and will advise once installed
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I've replaced the TL431 and swapped in a fresh 12AU7. All voltages are correct EXCEPT Kreg (B side) is still reading only 6V.
All tubes are new compared to when original issue arose.
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Are you able to adjust the bias per the instructions in the manual?
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Yes! I think the A-side trim pot was out of range if there's such a thing. A few extra turns and I'm back in range.
Thanks so much, Paul. Can I ask one more question?
Should I be wary of trying any of the previous tubes back in the amp? If the tubes have failed could they cause problems if I reinsert them?
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Good question, looking forward to Paul's reply. I would suggest that you download data sheets and use your meter to check for shorts.
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The amp should tolerated shorted 6C45PI tubes pretty well. Things will heat up if you leave them in there running for a long time though.
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The amp should tolerated shorted 6C45PI tubes pretty well. Things will heat up if you leave them in there running for a long time though.
Thanks, Paul. I'll fire it up quickly, check for sound (with sacrificial test headphones) and respond accordingly