Right channel hum and no signal

[Sp33ls]

So, I've not had the best of luck recently with my kits 

When I went to paint the bell end of my Crackatwoa, I noticed shortly afterwards that a hum in the right channel would appear. After checking ground connections and tightening the nuts, it'd be intermittent.

After removing the paint around the top of the bell end holes, I checked with a DMM on the bottom side of the bell end, and it says I have a solid connection to ground for the bell end.

I passed all of the resistance checks.

But, more recently I noticed that I can reproduce the hum by pushing on the transformer, and now the right channel signal is either nonexistent or when it is present, the low-end seems to be quite muddy.

Also, the type of tube seemed to have an impact. My 6SN7's didn't have a right-channel, but my 12AU7's did, but some were muddier than others.

When doing the voltage check, now one side of the high-current boards doesn't have the LEDs light up, and one half of the low-current boar doesn't either. I also noticed that one of the LED's under the driver also doesn't light up now.

I've checked solder joins and loose connections, and have re-soldered suspicious joints, but nothing.

Any advice? I'm pretty bummed because I just picked up a nice pair of ZMF Atticus, and they sounded phenomenal for a little bit, but then I began to notice these issues.

[Paul Birkeland]

Posting the voltages that don't align with what's in the manual would be really helpful.

An LED on the 9 pin socket that isn't illuminating and seeing something around 150V one one of the "O" terminals on the small PC board are common symptoms of one of a couple of issues.

1.  Do both halves of your 12AU7 light up?  If not, the solder joint between pins 4 and 5 on the 12AU7 socket isn't working.
2.  If both halves of your 12AU7 light up, what is the actual plate voltage on the offending side? (OA on the C4S board above the 9 pin socket)

I would very, very strongly recommend just sticking to 12AU7s until the amp is working properly.  Using an adapter and a bunch of different tubes and attempting to draw any conclusions is just bringing in extra variables that aren't at all helpful.

[Sp33ls]

For sure, the only reason I threw a 6SN7 in there was in order to see if there was something funky with my 12AU7's. I was just surprised to see a difference in behavior.

It appears both side of the 12AU7 light up. One thing I noticed was that actually at first, both of the LEDs over the driver lit up, but then that one in the picture went dark again after a little bit (didn't notice it fade, just looked back after measuring it was dark).

I've swapped the 6005's but that didn't change anything. Unfortunately, I only have this one 6080 power tube on hand at the moment... you don't think it could be the power tube giving out, do you?

Here are the voltages:

Terminal     Voltage (DC)
Low Current C4S
IA        150V   (30)
OA 60-90V   (27)
KregA  3-6V   (.05)
bRegA 150V    (30)
IB        150V   (149.5)
OB   60-90V   (70.4)
KregB   3-6V   (5.8 )
bRegB   150V   (149.5)
High Current C4S (offending channel)
IA         190V   (212)
OA        150V   (31.5)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (26.5)
bB        150V   (31.9)
High Current C4S (working channel)
IA         190V   (202.3)
OA        150V   (149.5)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (93.8 )
bB        150V   (149.5)

Headphone Jack
Tip          0V    (0)
Ring        0V    (0)

Thanks for the quick reply. I'd love to get this back to 100% this weekend if possible   

[Paul Birkeland]

Yeah, so now that I see the voltages, your problem is unlikely to be related to the 12AU7.

Pull the 6AQ5s and let me know how KregA and OA on the high current C4S board change.

[Sp33ls]

Pulled the 6AQ5s & this time both LEDs on the driver tube remained lit. Also, both of the high current boards had two LEDs lit this time (whereas previously one had all 4 and the other had none).
One thing that was interesting was that OA & IA seemed to gradually rise from about 50 to 70VDC in the time period of me measuring the system.

Terminal     Voltage (DC)
Low Current C4S
IA        150V   (66)
OA 60-90V   (57.5)
KregA  3-6V   (0)
bRegA 150V    (66)
IB        150V   (215)
OB   60-90V   (70)
KregB   3-6V   (1.4 )
bRegB   150V   (215)

High Current C4S (offending channel)
IA         190V   (220)
OA        150V   (69)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (60)
bB        150V   (70)

High Current C4S (working channel)
IA         190V   (217)
OA        150V   (215)
bA         0V   (0)
IB          0V   (0)
OB  90-110V   (119 )
bB        150V   (215)

I placed an order for "Code Matched Pair NOS NIB 6005/6AQ5W GE 5 STAR Ruggedized, Precision 640 tested" on eBay just in case one of the tubes is bad. I did notice that there was a blue glow to one of them that I hadn't noticed before. Fingers crossed the NOS pair is decent.

[Paul Birkeland]

It's very, very unlikely that a tube is causing this problem.

Now pull the 6080 and remeasure IA on the low current C4S on the offending board (or OA on the high current C4S).  Also check Kreg again if you can.

-PB

[Sp33ls]

With only 12AU7 plugged in, both of the driver LEDs light up, and all 4 of the low current LEDs have a soft glow. None of the high current LEDs light up.

IA (offending): 226VDC
KregA: 0.004
KregB: 1.45

With 12AU7 and 6005's in, the LEDs are the same as above, but now with two LEDs lighting up on the working channel's high current boards.

IA (offending): 74VDC
KregA: 0.206
KregB: 1.96

[Paul Birkeland]

With only 12AU7 plugged in, both of the driver LEDs light up, and all 4 of the low current LEDs have a soft glow. None of the high current LEDs light up.

IA (offending): 226VDC
KregA: 0.004
KregB: 1.45

This is somewhat informative.  With no 6AQ5s installed, we will have about 220V across the 147K and 2.49K resistors that help the regulator operate.  There is some voltage on the B side Kreg, but nothing on the A side.  This could happen if the TL431 is installed backwards, shorted out, or miswired.  It's also possible that a TL431 an PN2907 got mixed up, but that usually creates a different looking set of problems.


To continue debugging this, we will do some tests on the amp with no 6AQ5s and no 6080. 

What voltages do you get on the "X" side of the 2.2uF capacitor on each side of the center C4S board?

I would also be sure to triple check that the wire which leaves KregA and heads over to the 6AQ5 is properly connected at the 6AQ5. 

[Sp33ls]

Measuring the voltage across the 2.2uf cap, right? So, just placing the leads on both sides of each:

A side: 226VDC
B side: 224VDC

The amp seems to have been working fine for the past year (minus that one cold solder I had on the step attenuator), so wouldn't I have noticed a bad configuration on the TL431 before now? I wouldn't be surprised if I bumped it on accident while handling and caused another issue, though.

[Paul Birkeland]

No, one probe to ground and the other probe on the "X" terminal.

[Sp33ls]

Oops, sorry.

A side: 225VDC (closest to the corner) & 0.008VDC (closest to center)
B side: 225VDC (closest to the corner) & 2.25VDC (closest to center)

So, that's interesting (to me anyways lol).

I tried re-soldering the capacitor just to see if it was a bad connection, but there wasn't any difference.
Any thoughts on where to check given those readings?

[Paul Birkeland]

The 0V on the A side is troubling.  Disconnect the white wire leaving Kreg on that side, then remeasure this voltage.

[Sp33ls]

I disconnected the white wire at KregA, and measured the X closest to the center of the board. No difference in the the voltages of 'X' on the A side.

[Paul Birkeland]

Remove the TL431, then recheck.

[Sp33ls]

Removed the 431 on the A side and rechecked the voltages at X on the A side. This time there's a difference.

225VDC and 3.75VDC.

Coincidentally, when I test the 431 with my Fluke's diode mode, I see 0.008VDC like I was seeing before at X.