Shorted During Voltage Checks [Solved]

ssenesy · 4166

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Offline ssenesy

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on: November 21, 2014, 07:19:02 AM
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
« Last Edit: March 04, 2015, 03:17:06 PM by ssenesy »



Offline Paul Birkeland

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Reply #1 on: November 21, 2014, 09:58:11 AM
Run a wire from A3 to A8 and retest the amp.

The 3k resistors are supposed to get hot.

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline ssenesy

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Reply #2 on: February 19, 2015, 02:33:07 PM
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



Offline ssenesy

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Reply #3 on: February 19, 2015, 04:44:34 PM
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).



Offline Paul Birkeland

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Reply #4 on: February 20, 2015, 07:02:00 AM
Are both halves of the 12AU7 glowing?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline ssenesy

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Reply #5 on: February 20, 2015, 07:13:17 AM
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



Offline Rocketman248

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Reply #6 on: February 23, 2015, 03:39:21 PM
We're you able to get any further on this?

Nick DeBrita
Yokosuka, Japan


Offline ssenesy

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Reply #7 on: February 23, 2015, 04:41:50 PM
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).



Offline Paul Birkeland

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Reply #8 on: February 24, 2015, 07:55:31 AM
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

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline ssenesy

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Reply #9 on: February 24, 2015, 03:28:57 PM
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
« Last Edit: February 25, 2015, 01:34:17 PM by ssenesy »



Offline i luvmusic 2

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Reply #10 on: February 25, 2015, 02:31:45 PM
A7 and A8 are they shorted?



Offline ssenesy

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Reply #11 on: February 25, 2015, 02:54:52 PM
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



Offline Paul Birkeland

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Reply #12 on: February 26, 2015, 10:59:37 AM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline i luvmusic 2

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Reply #13 on: February 26, 2015, 11:11:14 AM
By looking at the picture i thought it is shorted.Sorry!



Offline Rocketman248

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Reply #14 on: February 26, 2015, 01:33:54 PM
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.

Nick DeBrita
Yokosuka, Japan