Bottlehead Kits > Crack-a-two-a

A buzz in both channels

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Skipperrik:
Out of the blue my Crackatwoa has developed a buzzing sound in both channels. I looked at this post, https://forum.bottlehead.com/index.php?topic=14712.0 and saw where his problem was a dirty 12AU7 pin. I have changed all of the tubes multiple times with no resolution so it must be something else.

The voltage tests results are:

Low Voltage C4S
IA  193
OA  72.9
KregA  8.29
bRegA  163.7
IB  150.2
OB  72.8
KregB  5.42
bRegB  150.2

High Voltage C4S (A/B)
IA  192.9 / 190.5
OA  163.2 / 150.2
bA  0.3 / 0.6
IB  0.3 / 0.6
OB  100.2 / 93.1
bB  163.9 / 150.2

I also tested it with only the 12AU7 tube in place:
IA  216
OA  64.3
KregA  9.0
bRegA  217
IB  217
OB  64
KregB  1.4
bRegB  217

These values seem high with the exception of OA and OB which are normal and KregB which is low.

Do any of these values give a clue as to where the problem could be?

Paul Birkeland:

--- Quote from: Skipperrik on January 11, 2025, 06:33:50 AM ---Low Voltage C4S
IB  150.2
bRegB  150.2
High Voltage C4S (A/B)
OA 150.2

--- End quote ---
This is one channel's worth of regulated high voltage connections.  They are all wired together and they all have the same voltage. 

--- Quote from: Skipperrik on January 11, 2025, 06:33:50 AM ---Low Voltage C4S
IA  193
bRegA  163.7
High Voltage C4S
OA  163.2

--- End quote ---
This is the other side.  You have 163.2V at the output of the high voltage C4S, but somehow 193V at the input of the center C4S board.  This is a bit of a contradiction, as these connections are wired together.  On the A side of the center C4S board (low voltage), +reg, Breg, and IA are all wired together, so they can't have different voltages unless one of those connections is no longer present or you have a very flaky ground connection in the amp disturbing the reliability of your voltage measurements.

My guess is that you have a broken wire that isn't permitting the regulator to work properly.  A buzz in both channels would make me wonder if there's a broken or loose ground wire in the amp, and this is both causing the buzz you're experiencing as well as the regulator malfunction.

Skipperrik:
I believe I found an assembly error that could account for the voltag imbalance. On one of the high current boards I swapped the locations of the 22KΩ 5W and the 47KΩ 2W resistors. I don't think that would cause the buzzing though since it has been dead quiet up until yesterday. I've run a continuity check on every wire in the amp and have found no breaks. I'll reverse the positions of the two resistors and follow up when I get it completed.

Skipperrik:
I found that one leg of a TIP-50 resistor at position B (see photo) has ground continuity. The corresponding leg on the TIP-50 resistor on the opposite board does not. Which is correct?

Skipperrik:
Sorry, the photo didn't get uploaded. Here it is.

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