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Capacitor question

Kcz5o

Member
Messages
21
I recently started workingon a 71 plush super 450 and am not sure about one of the capacitors. I am assuming it is a 20uf 250vdc cap but have not run across them being labeled like it is. Also it appears to be hooked up backwards. Can anyone verify what it is and if in fact it is hooked up backwards.

Thank you for any help.

View media item 2128View media item 2127
 

RiftAmps

Member
Messages
207
Yup, looks like a 20uf 250v - can you measure it with a capacitance meter to confirm?

What part of the circuit does the -ve end connect to?
 

Kcz5o

Member
Messages
21
Thanks for the reply. I have a meter I can measure it. I ordered a 22/500 cap becuase I couldn't find the 20/250. The negative end goes to a blue wire on the power transformer. I found a plush schematic for a bass amp and it shows a 50/100 cap with a positive to ground. I am kinda new to this. I thought that electrolyic cpacitors always had psotive to ground?
 

Kyle B

Silver Supporting Member
Messages
5,096
You don't need a meter to check that cap. It's about 40 years past it's functional life....it needs replaced as does EVERY 'vintage' electolytic cap in there. Probably between 5 and 10 caps overall.

can you post or link the schematic? A bias cap would correctly be connected + to gnd. Schematics can be wrong. So could the label on that cap....
 

Kcz5o

Member
Messages
21
Yes I planned on replacing it I just wanted to make sure I replaced it with the correct value and was confused about the postive side going to ground. I think it is part of a bias circuit.
 

Kyle B

Silver Supporting Member
Messages
5,096
if the amp is currently functional, you can determine the polarity with your voltmeter ;)
 

Kyle B

Silver Supporting Member
Messages
5,096
in honesty my friend, if you have to ask that question, you probably shouldn't be poking around inside a tube amp. Those warnings about tube amps having the power to electrocute you are not urban legend...
But... you'd do it with the meter set for DC volts, and have you red & black leads correctly plugged into the meter. With the amp running, probe across the cap. Then note whether the meter reports positive or negative voltage.
 

Kcz5o

Member
Messages
21
I am new to this and have been reading a lot and am extra careful. I've been very careful to drain the caps and then check with a voltmeter to make sure they are drained. I aslo make sure if I have to work on a live amp I keep one hand in pocket but prefer not to work on it live. Still makes me nervous but have been checking and double checking to be as safe as possible.

So with the positive end of the probe on the postive side of the cap and the negective probe on the negative end of the cap I should be reading a negative voltage if it is postitive to ground?
 

Kyle B

Silver Supporting Member
Messages
5,096
I am new to this and have been reading a lot and am extra careful. I've been very careful to drain the caps and then check with a voltmeter to make sure they are drained. I aslo make sure if I have to work on a live amp I keep one hand in pocket but prefer not to work on it live. Still makes me nervous but have been checking and double checking to be as safe as possible.

So with the positive end of the probe on the postive side of the cap and the negective probe on the negative end of the cap I should be reading a negative voltage if it is postitive to ground?
With the black lead on the cap (-), and the red lead on cap (+) (i.e. chassis GND), you would read a POSITIVE voltage if the cap is marked correctly, the amp working "properly", and that cap is indeed the bias-supply cap

I understand the concept of (-) not being 'gnd' on one of these caps seems counter intuitive. But it's exactly the way it has to be. A tube requires a voltage on it's control grid which is more NEGATIVE that the voltage on it's cathode. If it's not, the tube would pass unlimited current until something melts from the heat. (There's lots of no-math explanations on the web of how a tube works, if you didn't already know that little tidbit :) ... It's pretty critical to understand how tubes work, at least generally, if you want to be fixing amps)
I don't have a schematic, but presumably the cathodes of the power tubes in this amp are connected to chassis gnd. That being the case, you need some voltage that is more negative than gnd to be made available for the control grids. That negative voltage would be present on the (-) of the capacitor. (Again presuming it is indeed the bias cap!) So the cap (+) connected to gnd is "right" as we know GND is a more positive voltage than the bias (grid) voltage.

Does that help???? :)
 
Last edited:

Kcz5o

Member
Messages
21
I thought I had replied to the last post but it looks like I had not. Yes that helps. Thank you.
 




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