Shorted During Voltage Checks [Solved]

[ssenesy]

I've assembled my Crack (without Speedball) and the initial check of my resistances were all good.  When doing voltage checks on the input tube (around A2 or A3), I heard a snap, after which I immediately shut down the amp.  I did a visual inspection and all looked ok, so I restarted it and got normal voltages except for the following:

1 - 117vdc
7 - 126vdc

A6 - 118vdc
B1 - 148vdc
B3 - 147vdc

the LED from A8 center pin did not light (I don't know if it ever lit), but the one from A3 to center pin did.  I confirmed orientation on both LEDs.  The 3K resistors got really hot while doing the checks.  After I powered off and pulled the tubes, I re-ran the resistance checks and now got the following abnormal readings:

1 - open
2 - open
4 - open
5 - open
13 - open

everything else was normal.  I was thinking that the LED from center to A8 was bad, but with all the opens now on resistance checks, I'm not sure.  Any help would be appreciated.

TIA

Stan

[Paul Birkeland]

Run a wire from A3 to A8 and retest the amp.

The 3k resistors are supposed to get hot.

-PB

[ssenesy]

I apologize for basically taking forever to get back to this, but life intruded and the amp got forgotten on the shelf during the holidays.

I soldered a jumper between A3 and A8 as you requested.  I'm attaching a PDF of my voltage and resistance checks.  The one big difference I did note was that both LED's lit after the jumper was soldered.

Stan

[ssenesy]

FWIW, here's a few pictures of the wiring.  I've checked everything more than once and also tried re-flowing my solder joints (along with a mechanical connection check).

[Paul Birkeland]

Are both halves of the 12AU7 glowing?

[ssenesy]

Paul;

The glow is very faint (I have to cup my hands around the tube to block out light) but I think I can see an orange glow at the base for both heaters.  Nowhere near the brightness of the 6080.

Stan

[Rocketman248]

We're you able to get any further on this?

[ssenesy]

Not so far.  I'm totally at a loss as to what to check next.  I backtracked over my wiring several times now, so if it's a mistake, its really subtle (or blatantly obvious so I'm not seeing it).

[Paul Birkeland]

If both halves of the 12AU7 are glowing and you are able to get both LED's to light with the jumper, the next step is to measure the voltage (carefully) at A7 and A2.  It's possible that maybe the ground jumper on the volume pot isn't correctly installed, which would allow the grid to develop a voltage potential, and the tube to not conduct properly.

-PB

[ssenesy]

Paul;

The voltage reading on both A2 and A7 is 0 VDC.  I checked the resistance on the black wire running from the input ground, through the volume pot, to A9, and over to the output jack and there is continuity from one end to the other and all points in-between.

FWIW, I did Granger's ground path continuity check (from the stickies in this topic) when I built the amp and everything was fine.

I really appreciate all your help so far.  It's been a long time since I put a VM to use

Stan

[i luvmusic 2]

A7 and A8 are they shorted?

[ssenesy]

I luvmusic 2;

A7 and A8 aren't shorted.  I checked A2 and A3 and they aren't shorted either.  During voltage checks on the amp, I've got:

87 VDC on A1
146 VDC on A6

which, as I understand are the anode voltages.  Interestingly, the voltage on A6 was 118 without the A3-A8 jumper but jumped up to 146 after.  I'm not sure what that means.

Appreciate the help.

Stan

[Paul Birkeland]

See if you can borrow a different 12AU7, just be sure it isn't the problem.  Since the amp was working pre-Speedball, this seems unlikely, but it may be possible.

[i luvmusic 2]

By looking at the picture i thought it is shorted.Sorry!

[Rocketman248]

See if you can borrow a different 12AU7, just be sure it isn't the problem.  Since the amp was working pre-Speedball, this seems unlikely, but it may be possible.

This is all pre-Speedball.  He hasn't installed that yet.