Help, my left channel has developed hum [resolved]

[thunderfrenchie]

Hi, total noob here. I assembled my Kaiju with zero problems, every test checked along the way. It worked perfectly at first. I was able to adjust the hum pots and the amp was 'quiet' at idle with no music. I left it on for 72 hours in a row to burn it in. It then developed a low level hum in the left/black channel. The pot is now out of whack. Turning it all the way to the left reduces the noise but meter still reads .123.

Some measurements.
Right (OK side):
53U: 57.6
IA: 504
OA: 297
32U: 174.2

Left:
65U: 23.1 [this seems a problem]
IA: 504
OA: 298
46U 174.0

Thoughts?

Thanks,

John

 

[Paul Birkeland]

Can you post some photos of your build?

Is the 100uF axial capacitor oriented correctly on that side?  Are any of the legs of the hum pot touching the chassis?

 

[Doc B]

Also swap the 300Bs and see if the hum follows the tube and/or if you get different voltage readings

 

[Paul Birkeland]

Your pictures are teeny tiny, I can't see anything in there.  If the forum is bugging you about file size, you can put a couple on a post, then post the rest separately. 

The hum pot is the 10 ohm one with the 22 ohm resistors attached to it. 

-PB

 

[thunderfrenchie]

OK, larger...

 

[Paul Birkeland]

10U doesn't look to be soldered.

What is the AC voltage between A1 and A4 with a 300B in the socket and the amp running?  (A pair of clip leads would be helpful to measure this)

 

[thunderfrenchie]

On the affected side (actually B1 and B4) I'm getting 330v dc

On the unaffected side (A1/A4) I'm getting around 280.

Black lead was on the 1.

 

[thunderfrenchie]

Also desoldered 10U and 14-23U

 

[thunderfrenchie]

That's REsoldered

 

[Paul Birkeland]

Sorry, I meant to ask what the AC voltage is between B1 and B4, not A1 and A4. 

 

[thunderfrenchie]

AC I get 0

 

[Paul Birkeland]

Be sure your meter is set to a very low range.  You can compare your values between B1/B4 and C1/C4.  I suspect you will find a difference between sides. 

 

[thunderfrenchie]

In case it matters also notice I get a little transformer hum from PT9. All measurements with no speakers attached btw

 

[thunderfrenchie]

C1/C4 yields 4.92 V AC

Now getting about .003 v AC on B1/B4

My meter, Sperry DM-6250 is qutorange. Happy to buy whatever meter you recommend.

 

[thunderfrenchie]

B1/B4 now stabilizing around .007 v AC

 

[thunderfrenchie]

Could I have a bad 5670?

 

[thunderfrenchie]

Also in case it matters, when I first discovered the problems, I noticed that the affected side's OT was much hotter than the other.

 

[Paul Birkeland]

B1/B4 now stabilizing around .007 v AC

What AC voltage do you between terminals 5 and 7?

Could I have a bad 5670?

This is incredibly unlikely.  You don't have heater voltage at your 300B.

I noticed that the affected side's OT was much hotter than the other.

The output transformer is on the bottom of the chassis and the plate choke is up top.  The plate chokes will get warm during operation, but the output transformers tend not to. Can you confirm which was presenting a temperature difference?

 

[Paul Birkeland]

42U does not look soldered.

 

[thunderfrenchie]

The OT on the side that was humming. Even after the other side had cooled down the OT was quite warm. At full power it was definitely hotter than the other.