weird voltages

coors · 9302

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Offline coors

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on: March 21, 2010, 08:22:17 AM
Hello, I need some help with troubleshooting.
Everything seems to be wired right
When I measure the voltage at lead B5 I get 7 volts, should be 72.  The leads going into and out of it are correct, lead 36=359v and lead 34 is -16 volts.
On the corresponding leads on the opposite side i get all the appropriate voltages
A5=71.2 v
14=-15v
16=353 v
Everything lights up and plays.  I hear more of an image in the right channel of the headphone.  The headphone is a grado SR225.
Prior to installing all of the resistors I did check them on the multimeter and they were within tolerances. 
I have played the amp a few hours per session without problem. 
also the bass is not extremely taught, however I am not sure if this is a problem with the headphones or just the nature of the amp.  The low level retrieval is excellent. 
Any thoughts?  I am not sure what to do.  the connections are all soldered well and the mechanical connections were good prior to soldering. 
Thanks
Sam



Offline Paul Joppa

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Reply #1 on: March 21, 2010, 11:17:18 AM
First test is to swap the tubes and see if the problem follows the tube. A bad tube is not especially common, but it's so easy to check that this test is very cost-effective.

If it's not a bad tube, then let's look at the voltage on the other pins of the driver triode - B4 (grid) and B6 (cathode). That will help figure out what is going on. While you are poking around there, inspect the tube socket and nearby wiring carefully in case there are any wire whiskers shorting to places they shouldn't or other unexpected problems. You'd be surprised how many times I've traced a problem in my own builds to a terminal I forgot to solder, for instance!

Paul Joppa


Offline coors

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Reply #2 on: March 21, 2010, 11:38:30 AM
well it must be a bad tube. 
When I swapped the tubes I know get
B5= 73.3 v
A5=6.1 v
Do I just call for a new tube?
Thanks for your quick response. 
The tube lights up fine and looks exactly like the other tube, there does not seem to be any broken connections
I am completely new to tubes so I am not sure if visual inspection would be helpful
So in tubes you could have one lead that is not working well yet the tube works to some degree?
thanks again
Sam



Offline coors

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Reply #3 on: March 21, 2010, 11:53:48 AM
Oops
I looked at the tube again that is bad and there is a difference.  There is a horizontal plate that the leads go into that separates the leads from the anode and cathode of the tube.  It looks like a small piece of laminate has peeled away from this metal horizontal plate.  I am not sure if this makes any difference but that is the only thing visually I see different. 
S



Offline Paul Joppa

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Reply #4 on: March 21, 2010, 03:40:11 PM
Yes, that's probably the problem. Call or email Eileen and she'll send out a replacement tube, no problem.

Paul Joppa