I seem to have a dead tube.

[Tarquinflimbim]

MY 12AU7 doesn't light up.  I checked the wiring, and it looks good.  I swapped it with a 12AU7 from my mainline, and it does light up...

Could you send me a replacement, please?

Thx,

T

 

[Paul Birkeland]

What OA and OB voltages do you get on the C4S board that is mounted above (or below depending on how you look at it) the D socket where the 12AU7 goes?

 

[Tarquinflimbim]

0A - 128v
0B - 155v

 

[Paul Birkeland]

That is unlikely to be the board above the 12AU7 socket.  If it is, then remove the 12AU7 and recheck.

 

[Tarquinflimbim]

Sorry - wrong board as you suspected.  0B = 130V, 0A = No reading (which I guess is an issue)

 

[Paul Birkeland]

Did these checks pass when you build your Eros?  You should have 220V there.  You should also get a voltage reading on OA.

 

[Tarquinflimbim]

All of the resistance checks were spot on.  I'll get the magnifier out and see what I can find.

 

[Paul Birkeland]

No, this was a voltage check that is done part way through the manual to check that the board above the 12AU7 is operational before proceeding with the build.  It's a lot easier to debug this board before continuing to build out more of the amp.  This check is at the bottom of page 55. 

As far as the 12AU7 goes, sometimes it's not all that easy to see whether the tube is glowing or not, so we would rely on electrical checks to ensure that it's conducting properly before calling it dead. 

 

[Tarquinflimbim]

OK - found a wire that was "attached" but not soldered.  I now have 218V on OA and unfortunately still 130V on OB.  I'm tired and will check it out in daylight tomorrow.  I remember both of the metal tabs reading correctly when I was on page 55, but now OB is Down 90V.

 

[Paul Birkeland]

OK, you don't need a 12AU7 then.  Is this an Eros 1 or an Eros 2 kit?  We will need to undo some of the build to see where the problem is, and the instructions to do so will depend a bit on which kit you have.

 

[Tarquinflimbim]

Eros 2.  I disconnected the PSU board from the rest of the circuit, and all of the numbers measure well.  Next step?

 

[Paul Birkeland]

Can you post the IA/IB, OA/OB, and KregA/B voltages on the board above the 12AU7 that you have? 

With good voltages, you would reconnect the forward C4S boards and run it without tubes to see if it's still pulling down the regulated HV rail. 

 

[Tarquinflimbim]

Which connections to I remove to disconnect the C4S board?  I'm getting 319VDC on IA/IB, OA/OB, and KregA/B.  Oddities noticed: 6.3V DC is now floating +47V - i.e. there is a 6.3V difference between terminals, but they read 53.3 and 59.6V.

All tests with no valves.

 

[Paul Birkeland]

To disconnect the forward C4S board completely, you just pop the wires out of IA and IB.

The 6.3V DC rail should be about 51V above ground.

 

[Tarquinflimbim]

Thx - will take a look tomorrow.  I disconnected the IA and IB and a quick probe around and got 300V more often than not.  I will play with those voltages in the morning when the light is better!

 

[Tarquinflimbim]

Ended up watching a lot of football.