Stereomour II Left 12AT7 issue

[adlehman]

Hey y'all,

After a few months of smooth use, my left channel has developed an issue.

It started as a slight hum that could be diffused by gently nudging/wiggling the left 12AT7 tube, but it has now become a scratching/popping sound in addition to the hum. It comes through sporadically and is loud - when you wiggle the tube now the scratching/popping still persist, sometimes worse, sometimes a bit better.

Tried to look around for anything obvious but couldn't find anything. I also switched the L and R tubes to test if it's the tube itself but it doesn't appear to be that either.

My next step is to really just triple check the solder connections on everything attached to that little terminal for the 12AT7. I gave everything a proper poke/tug and no wires appeared loose.

Attaching some photos but the light was a but low, let me know if a close up of anything is needed and I can provide. If anyone has any tips I am all ears!

 

[adlehman]

Adding more photos here:

 

[Paul Birkeland]

You'll want to get a wooden chopstick and hook the amp up so it's running flipped over with some cheap speakers connected to the outputs. Poke around the circuit with the chopstick and eventually you should find a spot or two where you can poke and create the noise. Do keep in mind that the connections ad the tube socket and the places where those connections land are very much subject to inspection!

 

[adlehman]

You'll want to get a wooden chopstick and hook the amp up so it's running flipped over with some cheap speakers connected to the outputs. Poke around the circuit with the chopstick and eventually you should find a spot or two where you can poke and create the noise. Do keep in mind that the connections ad the tube socket and the places where those connections land are very much subject to inspection!

I'll get to it, thanks Paul!

 

[adlehman]

You'll want to get a wooden chopstick and hook the amp up so it's running flipped over with some cheap speakers connected to the outputs. Poke around the circuit with the chopstick and eventually you should find a spot or two where you can poke and create the noise. Do keep in mind that the connections ad the tube socket and the places where those connections land are very much subject to inspection!

Welp, a lot of things I poke throughout the chain cause a different pop or hum to occur.

I presume I should go back through the channel connections and sure up the solder joints as a next step?

 

[Paul Birkeland]

Anything you poke at that makes a pop needs to have its joints reheated on either end.

 

[adlehman]

Anything you poke at that makes a pop needs to have its joints reheated on either end.

Went back through, reheated everything, and added a little solder to light areas.

The crackling and popping have stopped, but the hum persists.

Nothing changed around the amp recently, no new routers or electronics that could be giving off electrical interference.

 

[Paul Birkeland]

Can you listen to a 60Hz tone and a 120Hz tone and let us know which sounds like what you're hearing? Also, have you adjusted the hum pots on the amp?

 

[adlehman]

Can you listen to a 60Hz tone and a 120Hz tone and let us know which sounds like what you're hearing? Also, have you adjusted the hum pots on the amp?

60hz for sure, and yes to the hum pots! no change in hum when adjusted.

 

[Paul Birkeland]

Is this from both channels or just one channel?

 

[adlehman]

Is this from both channels or just one channel?

Just the left channel.

 

[Paul Birkeland]

I'd poke around the solder joints of the resistors going to pins 1 and 4 on that 2A3 socket, as well as the 22 ohm resistors going from there to the hum pot, as one probably isn't well connected.

I'd also mention that the 100uF/450V power supply cap I see isn't something I'd ever install on my own equipment. If we provided that to you, I'd like to extend an apology and encourage you to contact us regarding obtaining a proper replacement. If you had to source a replacement in a pinch, I'd consider that a part that should be swapped out relatively soon.

 

[adlehman]

I'd poke around the solder joints of the resistors going to pins 1 and 4 on that 2A3 socket, as well as the 22 ohm resistors going from there to the hum pot, as one probably isn't well connected.

I'd also mention that the 100uF/450V power supply cap I see isn't something I'd ever install on my own equipment. If we provided that to you, I'd like to extend an apology and encourage you to contact us regarding obtaining a proper replacement. If you had to source a replacement in a pinch, I'd consider that a part that should be swapped out relatively soon.

Thanks, Paul.

I'll poke around this weekend and see what I can find. Appreciate the insights here!

That part was a replacement in a pinch after one popped. I'll also order a new one that matches the one that came with the kit. unless you have a brand recco. Thanks!

 

[Paul Birkeland]

That part was a replacement in a pinch after one popped.

I'm glad we didn't provide that, I got a little panicked when I saw that! Nichicon, NCC, Panasonic, and Kemet are all trustworthy, provided they come from a reliable supplier.

 

[adlehman]

I'm glad we didn't provide that, I got a little panicked when I saw that! Nichicon, NCC, Panasonic, and Kemet are all trustworthy, provided they come from a reliable supplier.

Thanks Paul, I replaced it with a Kemet.

Double checked and reheated transistors from 1 and 4 and checked the 22 ohm connections as well. No glaring issues.

Still getting a hum in the left channel, same sounding 60hz.

I'll keep poking around in that channel and see if I can't find something with a weird connection.

 

[Paul Birkeland]

If the hum pot has no influence on what you're hearing, then I'd just reheat the joints on both ends of each 22 ohm resistor on that channel. You could have one 22 ohm resistor 100% disconnected and you'd have perfect DC voltages but lots of 60Hz hum that wouldn't change by adjusting the hum pot.

 

[Natural Sound]

If I may add, does the hum go away when you disconnect all your source cables?

 

[adlehman]

If I may add, does the hum go away when you disconnect all your source cables?

Thanks for the idea! Just tried it and the hum still remains.

Gonna poke around more today and reheat some connections.

 

[Paul Birkeland]

You can also split the circuit in half by temporarily soldering in a little jumper wire across the 249K resistor on that side of the amplifier. If the hum goes away, then you have a hum being induced by something going on in the 9 pin driver stage. If the hum remains, then you have hum coming from the 2A3 output stage.