Voltage problem [resolved]

[stochastic]

Hi all - finished building my Crack last night.  It passes all resistance checks, and all of the voltage checks EXCEPT terminals 1 and 7 both read 150 volts, and the LED attached to A8 does not glow (A3 is fine).

This is my first time working with tubes - the 6080 seated very nicely and flush, but I had a hard time getting the 12AU7 in very far - see pics.  I pressed firmly and wiggled, but couldn't get it to go in further, not sure if this is normal.

Thanks for the help!

 

[Paul Birkeland]

It looks like maybe you accidentally cut one of the UF-4007 diodes in the power supply.  Definitely you will want to fix this ASAP.

Can you carefully measure the DC voltage at A8?

 

[stochastic]

Well I wiggled the 12AU7 and everything works now - LED lit, Terminal 1 voltage is 80, Terminal 7 voltage is 105.  Terminal A8 voltage is 1.6, which (as I understand it) is exactly the bias voltage we want the LED to provide...

So that's great!  Thank you kindly for the prompt response.  Any tips for getting the 12AU7 to seat better in the socket?  I don't want to break anything, but I'm also concerned about the finnicky connection...

 

[Paul Birkeland]

If wiggling the 12AU7 brings things into operation, I'd be suspicious of a loose connection in the amp causing these issues rather than the socket itself.

 

[stochastic]

Copy that - thanks!  I just listened to some music - got some hiss out of the right channel, then it cut out completely after about a minute - I'll double check for loose connections - thanks again!

 

[stochastic]

OK - I can get the amp to work perfectly, but just touching the 12AU7 can cause the right channel to pop and cut out completely; wiggling it brings the right channel back.

I just ordered a couple new 12AU7's and a tube saver from tube depot to try to rule out a tube issue (I don't mind having the extras).  Seems like it's definitely a right channel issue, and given HOW sensitive the connection is to tube movement, it's either a tube problem, socket problem, or bad solder joint on the socket - visually everything looks ok to me, and nothing is loose when I wiggle it.

This is the second amp I've built but the first with point-to-point wiring (the other amp was the Akitika GT-108).  Any beginner advice for how to further isolate the issue?

 

[Paul Birkeland]

I just ordered a couple new 12AU7's and a tube saver from tube depot to try to rule out a tube issue (I don't mind having the extras).

This is more likely to complicate the issue than to solve it. 

Any beginner advice for how to further isolate the issue?

Plug in an inexpensive pair of headphones (they can be low impedance) and play music through the amp while it's running upside down.  Poke around the circuit with a wooden chopstick until you find the connection that causes things to drop out when you touch it.  Keep in mind that it could be a wire that connects to the tube socket but the end that doesn't connect to the tube socket may end up being the issue.

 

[stochastic]

Thank you - will do!  Great advice.

 

[stochastic]

LED connection was poor at terminal A8; successfully identified via chopstick poke and fixed.  Wiggling the 12AU7 no longer disrupts the connection - everything works now.

Paul, sincere thanks for your help and patience - this won't be my last Bottlehead project!

 

[Paul Birkeland]

I'm glad you were able to track it down easily, and hopefully the chopstick came with good takeout.