Tracking down a hum that surfaced on my system after speedball install

[12AU7-6SN7]

Hi everyone,

I recently purchased a used Crack amp that was reported by the first time builder to have a hum on the left channel without the speedball boards installed (he wasn't able to get the SB working so reverted to the Stock Circuit)

In my system (Power circuit and source gear) the hum was not detectable at all. Amp sounded great. A bit quieter when cranked up when set up in the living room with a laptop to the DAC vs in the room with our desktop pc and rats nest of power cables and cable modem/router gear near by, but still sounded good at normal volumes. Nice and clean and tubey.

Just got the failed Speedball boards looked over  by my pop who used to be a tech. We fixed a broken LED lead on one of the A / B boards. Then installed the wiring for the C4S and installed the boards. I made a mistake on the left side with the terminal 7 black wire to G instead of O to the big Transistor with heatsink. Weak volume from a 1v input and no sound in right channel. A beer later I dove back in and found my mistake, fixed it and fired it up with a 2v source. Sounds great.



I just started noticing a high pitch hum in the left channel after it had been on for awhile. Doesn't matter if the volume is zero or up high. I guess the hum from the original build that was manifesting for the builder is now apparent in my setup with the speedball boards.

edit: just tapped on the top of the aluminum plate to do a little percussive maintenance, and it worked... the hum is now mostly gone (I think I hear a tiny hint of it though) so, take that fwiw - ah, it came back a little while later. tapping worked again


I have some pics of it before the speedball and will have to get some pics of with the speedball installed later. (https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FHnOZKgi.jpg&hash=4bd7e4e12897e3695fc76f39b917867f2edfaf02)

Any ideas where to start? Reflow everything that we didn't touch today?


 My pop is old, but he is a skilled tech that just can't see as well as he used to, I'm able to see ok and solder well. We didn't go over the thing completely. It was working fine when I got it despite the hum for the original owner. Just want to work this out before installing a new ALPS 100k pot.

Thanks in advance for any insight into this hum

[Doc B.]

The way too long leads on the 22K ohm resistors could be causing oscillation.

[12AU7-6SN7]

we removed those for the install of the Speedball

edit2: as I suspected it might, after it warmed up there is a slight high pitch hum ringing in the left ear even away from the cable modem.

edit: I just tried setting up away from the cable modem and pc power cord jumble at my main listening station and the high frequency noise in the left ear is gone. So, it might just be EMI. Perhaps the configuration is adjustable to mitigate it?

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FtnC85tW.jpg&hash=aceecae3381f6ba92ed255cb05f447ecb828b713)

[12AU7-6SN7]

 I noticed that the hum/whine hasnt presented itself with a different output tube (a cetron 7236)

is it possible I damaged the stock RCA 6080 by accidentally putting the black wire from terminal 7 to g on the left side of the speedball board instead of O? I fixed the misplaced wire and all was well until I noticed the HF noise in the left channel. Or is it just a coincidence that the hum developed after the speedball?

[Paul Birkeland]

is it possible I damaged the stock RCA 6080 by accidentally putting the black wire from terminal 7 to g on the left side of the speedball board instead of O?

Oh yeah!  That will let the 6080 go kind of wild in the circuit, as it will see +75V of bias.  Plate current will be limited by the time it takes the 270 Ohm resistors in the power supply to explode.

Having said that, a 6080 is a tough little sucker that can withstand just about anything besides a concrete floor.

What this sounds like is a flaky solder joint in the amplifier, potentially on the 6-10 5 lug socket or on the octal socket itself.  You can try poking around the circuit with a chopstick and a cheap pair of disposable headphones on your head to see if you can activate the noise by disturbing one specific part of the amplifier.

-PB

[12AU7-6SN7]

Oh yeah!  That will let the 6080 go kind of wild in the circuit, as it will see +75V of bias.  Plate current will be limited by the time it takes the 270 Ohm resistors in the power supply to explode.

Having said that, a 6080 is a tough little sucker that can withstand just about anything besides a concrete floor.

What this sounds like is a flaky solder joint in the amplifier, potentially on the 6-10 5 lug socket or on the octal socket itself.  You can try poking around the circuit with a chopstick and a cheap pair of disposable headphones on your head to see if you can activate the noise by disturbing one specific part of the amplifier.

-PB

I took the speedball out and things seem to be pretty quiet. It's more to my preference that way. Also with the other tube it wasn't making the noise with the SB.  I suppose I should reflow some of those, but it would be hard to tell now without the speedball.