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Tell me what these readings mean...

1/22/2004 9:55 PM
Julien Tell me what these readings mean...
I am trying to fix my Cust. Vibrolux Reverb which took a lightning hit. These are some readings I just took, power on, reverb and tremelo off, everything else at 50%.  
It has no voltage at TP1 which is at R1 and R2, the two 68k resistors at the inputs so there seems to be no grid voltage ...but here is what I am measuring...  
V1: 12ax7.. pin1 190v; pin2 1.59v; pin3 1.59v; pin(4 and 5) 0v; pin6 149v; pin7 0v (with a hum); pin 8 1.41v; pin 9 0v.  
V2: 12ax7.. same except for pin 6 which is 185v.  
V3: 12ax7.. pins 1 and 6 bridged 430v; pins 2 and 7 bridged 7.2mv; pins 4 and 5 bridged 0v; pin 9 0v.  
V4: 12ax7.. same as 1 except for pin 6 which is 190v.  
V5: tremelo tube is all over the board...  
The power tubes are 6L6 and they test like this..  
pin 1and 8 0v; pin2 0v; pin3 426v; pin4 426v; pin5 40.5v; pin 7 drops to 0 when I test it.  
It look's like I have no heater voltage but the tubes light up when I turn it on stand-by.  
The schematic is here if you have time to help me...  
Thanks, i hope that this isn't too jumbled up.
1/23/2004 1:30 AM
Jack Collins

It would help if you told us what is wrong with it.  
The heaters voltages are ac, not dc.
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1/23/2004 2:17 AM

Is there any sound at all? hiss, hum?  
Are your speakers OK?  
your voltages don't sound so bad w/o checking the schemo...  
1/23/2004 2:49 AM
Yes the speakers are ok. pin 9 to pins 3 and 4 is 6.2v a/c  
pin 9 to ground is 3.07v a/c.  
The problem is that I had a lightning hit and since then the amp has just sounded thin and really kinda weak.  
I can't seem to find the problem. I have swapped out all the tubes one at a time. I think i'll try swapping them all out at once. I don't know what else to do.  
thanks, J
1/23/2004 7:49 PM
Rob Mercure

Your heater voltages sound OK - generally the tubes are the last things that would be damaged from a lightening strike and semiconductors (diodes) the first thing. Does this amp have any sort of varistor on the PT primary input?  
1/23/2004 8:34 PM
I don't know what that is. Tell me and i'll check it.  
Thanks, J
1/23/2004 8:34 PM

I recommend resoldering the ground joints, especially the ones near the input jacks (for sensitivity) and the filter caps (for current handling).  
This is especially considerable when lightning strikes very near to your building, resulting in a brief spike where the green earth wire gets a large jolt of something other than the stable earth ground it is normally attached to.  
Without series resistance to slow current inrush, the big filter caps will briefly draw or discharge as much as they can from the ground wire during the extreme pulse. Seems to me there must be some micromelting, recrystallization, and perhaps oxidation in the heart of the solder joints, since they don't look any different. Remelting just might do the trick.  
I have had input jack grounds go bad from sparks unrelated to lightning, so they might not be subject to the current induced solder damage unless you have something plugged in that can sink some energy, but lightning is not fully understood and there is an outside chance that a guitar pickup could produce and inductive kickback under extreme conditions.  
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