Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: snm on February 02, 2023, 07:27:25 AM
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Hello, first timer here. I'm at the stage where I'm measuring resistance and while most of my measurements are in the expected values, there are a few terminals where the resistance checks are a little off. I was hoping somebody might be able to tell me what I should look for to troubleshoot the differing values.
| Terminal | Expected | Measured |
| 1 | * | 22.39K Ohm |
| 2 | * | 269 Ohm |
| 4 | * | 269 Ohm |
| 5 | * | 22.38K Ohm |
| 13 | * | 269 Ohm |
| 20 | 0 Ohms | -0L |
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I would recommend posting some build photos.
The lack of a solid 0 ohm reading on 20 tells me that you have a black wire that's poorly soldered somewhere. Often the junction of the two black wires on the headphone jack is not well soldered, but I suspect it would help to redo all of the joints where black wires land.
Are you sure the DC resistance reading on 2 and 4 is not 269K?
Expect at least a blown fuse on power-up, if not smoke, so let's resolve these before powering up.
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If the reading is actually 269 ohms that value tends to indicate a possible miswire, as that reading seems to imply that the big white 270 ohm resistor is grounded at one end when it should not be. You might want to post pics of the wiring of the terminal strips on either side of the power transformer and chack closely where all the black wires have been attached.
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Appreciate the help. I went through all my wiring and noticed I did indeed miswire the black wire on 12U to 15U by mistake :/ Corrected it and rewired it to 14U as instructed.
My resistances seem to match what’s expected but I think i need some clarity around the terminals with an asterisk. I know the docs state to watch out for 0 readings at those terminals as they would indicate something is wrong, which luckily none of mine are showing as 0. However, I do see these readings around 1.5M to 1.6M while steadily dropping. Is this normal?
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That that's millions of ohms, which is sufficiently far away from 0 to not be concerned.
If you saw something like 5 ohms, then that would be concerning.
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Awesome, thanks! Tested it today after checking voltages and what a great treat it was!
I've moved to the speedball and when measuring my voltages, 0A measures out to 44V pretty consistently, while 0B measures out to 60V.
IA and IB are both about 177V and B-A/B is 0V.
I redid 0A just to be sure and it's still measuring out to 44V. Since this is below the 60-90V range I'm hesitant to move forward. Any thoughts?
I should note that my tubes still glow just fine and the LEDs are lighting up without apparent issue.
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How were the voltages without the Speedball? We absolutely recommend using the amp for a week or two in its stock configuration before moving onto the Speedball.
Your OA and OB are very low. We have seen that from a backwards Q1 transistor and from soldering issues.
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Voltages looked in like with what the Crack manual stated. Uploaded some photos to google drive of the small board
https://drive.google.com/file/d/1LhXRqyxwbuZHoOKZh2RwfkTb8tT3hjm3/view?usp=drivesdk
https://drive.google.com/file/d/10oa-NoPOEticlKJL2ZpV9byZ47RdljXl/view?usp=drivesdk
https://drive.google.com/file/d/1LhXRqyxwbuZHoOKZh2RwfkTb8tT3hjm3/view?usp=drivesdk
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Can you tell me about the solder and soldering iron you're using? That looks like it could potentially be an iron that isn't hot enough at work here, or oddball solder.
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Sure, I’m using a Hakko FX-951 and some Cardas Soldering Wire Quad Eutectic Silver Solder with rosin flux
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I would turn the iron up to the maximum temperature if you haven't already.
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So, I reflowed the components on the small board at max temperature and still get around the same readings. I also checked voltages at B1 and A6 and they both come out to around ~45V, though I’m obviously no electrician and not sure if that measurement even matters/makes sense :)
To rule things out I’ve rewired A6 to 1U redid the small white wire from 1U to OA but the measurements are still the same.
Also, for what it's worth, my 8 pin tube doesn't sit all the way flush with the socket, seems its a bit tight but I don't know if that matters, figured I'd mention it anyways in case it was applicable in troubleshooting.
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Do those voltages change if you pull the 6080 and run the amp with just the 12AU7?
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No, they appear consistent. IA and IB go to 223V, up from 177V before.
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Can you remove that PC board and temporarily reinstall the 22.1K resistors and recheck the voltages at terminals 1 and 5?
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I have to order some replacements as I cut the leads and originally installed them pretty close to the terminal strip. I'll report back as soon as I get some replacements put in :)
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Ok, with resistors reinstalled:
Terminal 1: ~67-68V
Terminal 2: 229V
Terminal 3: 0V
Terminal 4: 229V
Terminal 5: 83V
This is without the tube installed
With the tube reinstalled, my readings are:
Terminal 1: 57.6V
Terminal 2: 179.2V
Terminal 3: 0V
Terminal 4: 179.1V
Terminal 5: 71.1V
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Can you carefully measure the voltage at A8?
The 67V seems pretty low, but you could have a 12AU7 with one half that's very potent, or there could be another issue at work.
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A8 looks to measure out to 1.595V (with both tubes plugged in)
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How about A3?
Is the jumper installed on the volume pot between the lugs where black wires land?
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I’m reading 1.588V at A3.
Can you clarify what you mean by the jumper?
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There should be a bare wire between the upper and lower lugs on the volume pot where the black wires connect.
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Oh, my apologies. That's installed. Here's a picture for reference:
https://drive.google.com/file/d/1DkNyuo6HC6RzHT0YkimH5LeOLhx0bzWh/view?usp=sharing
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Does the amp otherwise sound good and work in what would be considered a normal way under these conditions? Can you post some photos of the 12AU7 you have as well?
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Here's some photos:
https://drive.google.com/file/d/1WavPdI3cm4lhIitNHnJvoSA0dp9ozpGN/view?usp=share_link
https://drive.google.com/file/d/1ttI2jqRog3ECBBqV5n1_lzI9kbhbcln0/view?usp=share_link
https://drive.google.com/file/d/1PjVoFfVaEBoRWcgbwQAZPz7ra2_nMxBc/view?usp=share_link
https://drive.google.com/file/d/1HTEElxpBPS05NpP2oTBvS3E-fhj1uH-4/view?usp=share_link
The amp sounds fine to me, atleast I haven't noticed anything abnormal, though this is my first tube amp.
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I would pop the PC board back in and give the amp a listen. It may just be that your 12AU7 is unusually potent.
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Am I fine to listen with the small board only? I haven’t proceeded past testing voltages yet, but wanted to confirm first. Appreciate your help with this!
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Yes, the amp will work with just the small PC board installed provided the 3K/10W resistors are still present.
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I've been running the stock Crack for a few days now, without the small board just to see if any issues arise. Only thing I have a question about is the TRS connector. After listening for about ~30 minutes today, I unplugged my headphones and noticed the TRS connector was slightly warm. I assume this is normal?
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Sure, the whole chassis will be warm after playing for a while. The Brits call vacuum tubes thermionic valves. Emphasis on the 'therm' syllable here. Tubes only work if their heaters are hot, and of course that heat energy transfers into the rest of the amp. The power transformer also gets quite warm.