Noise and wrong voltage readings

[Andrej2]

I built a Crack+Speedball, took the measurements as in the manual which were all as they should, plugged in the headphones, and it played beautifully. Later I plugged in the headphones and static (hiss?) appeared in both chanells, mostly in L, which turns sometimes into a hum.

As the sound got worse and worse I took measurements again:

Joint/measured/required
1   102 75
2   168 170
3   0 0
4   169 170
5   140 75
6   0 0
7   122 100
8 
9   139 100
10  0 0
11 
12  0 0
13  169 170
14  0 0
15  186 185
19  79
20  0
A1  144 75
A2  0 0
A3  1.5 1.56
A4  0 0
A5  0 0
A6  103 75
A7  0 0
A8  1 1.56
A9  0 0
B1  103 75
B2  169 170
B3  122 100
B4  135 75
B5  168 170
B6  140 100
B7  0 0
B8  0 0

Source voltage: 230Vac, heater: 4Vac

What could be wrong?

[davew]

I would first check for shorts around the 9 pin socket pins. Perhaps somehow something was bumped and moved? It is very easy to push 2 adjacent pins together (from personal experience  ). I assume that "Source voltage: 230Vac" is the ac line voltage measurement. I assume you're not in the US and that is the correct transformer primary for your location. The reading of "heater: 4Vac" tells me you likely have something shorting to your heater voltage lines. It should be around 6V. The short is likely around the 9 pin socket as I mentioned first.

Since you say it played fine for awhile then it's also possible your input tube has failed. I assume you're using the same headphones from the first listening session.  Do you have a cheap spare/sub input tube. I would not swap tubes though until after you've confirmed there are no shorts.

I'm sure the more senior forum members will also respond soon with suggestions.

[Paul Birkeland]

The 12AU7 (at minimum) is not heating. 

I agree with Davew, check for shorts around the 9 pin socket, and around the octal socket.

You can also power up the amp without tubes in it (one tube at a time) to see if the heater voltage pops back up to ~6V AC if one of the tubes is removed.

[Andrej2]

First of all, thank you for your help davew and Caucasian Blackplate.

I checked for shorts around pin 9 and couldn't find any. I then removed the 6080 tube and left the 12AU7, and the heater measured 6.08Vac. When I removed the 12AU7 and connected the 6080 tube the heater measured 5.74Vac. Without any of the tubes it's 6.14Vac. What's going on? Unfortunately I don't have any spare tubes.

[Paul Joppa]

5.74 volts is within the +/-10% spec for the tubes, and is just what I would expect with 230v on the 240v primary. It's much better than the 4v reported at first!

It appears from the measurements that the 12AU7 is not emitting enough electrons. When the reported heater voltage was 4v, it seemed the most likely explanation was that the heater was not hot enough. With these new voltages, it may be that the cathode isn't fully formed, in which case I'd let it run for 50-100 hours and see if it picks up - some modern-production new tubes seem to have rushed through this step in manufacturing. If that does not work, then I'd suspect a bad tube - contact Eileen and ask for a replacement.

For what it's worth, I note that the 12AU7 plate at A1 was 144 volts, but fell to 135v at  B4 (they are connected) by the time you checked the 6080. Assuming you tested the voltages in that order, this is a good sign the tube is breaking in (cathode is forming).

[Andrej2]

Hi Paul Joppa, I'll let the tube run for a 2 days as you suggest but what I don't understand is how did it test and play perfectly in the first time when I completed it?..

[Paul Joppa]

Hi Paul Joppa, I'll let the tube run for a 2 days as you suggest but what I don't understand is how did it test and play perfectly in the first time when I completed it?..

I apologize, I had forgotten that aspect. I have no idea how that could happen, nor do I have any idea how you could get 4 volts on the heaters - you would have noticed if the lights were all dim in the house! You're right, there is - or was, at least - something else going on. Keep a watchful eye; you might not want to run it when you are not present until you are sure it's OK.

[Andrej2]

OK, I let it run for two days, now there's hiss in both channels not affected by volume (while the dac is connected) and hum on left channel. I took new measurements and all the red ones are even higher!

1  130 75
5  157 75
7  137 100
9  155 100

heater is 5.63Vac

Do I have to send you the tubes for a check?

[Paul Joppa]

Contact Eileen,tell her PJ said you need a replacement 12AU7. She won't be in the office until Monday, but she's pretty fast when shes there. Let us know how that works out.

[Andrej2]

So far looks like it's working with the new tube and the voltage readings are correct. Thanks. The only issue I now have is imbalance between channels probably due to the potentiometer (I read a few reports of that here).
I'd also like to note what huge difference DACs of different quality in the line makes.