blown resistor

[mozarti]

Dear Bottleheads,

The resistor connecting C7 and C9 (this is for one of the power supply tubes) blew up.  I replaced it with 220ohm carbon resistor (they did not have any metal film resistors) from Radio Shack and as soon as I turned the preamp on the resistor caught on fire.  The only measurement that is different is that point 6 measures 188 instead of 147 volts.  Should I just buy a 215 ohm metal film resistor and try again? Or is there anything else that I should check to figure out why the original resistor blew up?  Many thanks in advance.

 

[Paul Birkeland]

How were your resistance checks?  Try powering it up without that particular EL84 in the socket.  If the resistor holds without that tube in the socket, try the other EL84 from the functioning channel on that side.  (Note - the BeePre can be run without the EL84's installed, but should have 300B's installed when powered up)  If you accidentally swapped the wiring between pins 2 and 3, I could see that particular resistor exploding.

-PB

 

[mozarti]

Thank you, BP.  I did as you suggested.  The resistor is fine when I put in the other tube, but the A side of my board does not light up. As to the resistance measures, everything is fine, except 22, 25, c7, and c9 measure at around 18.5kohm instead of over 100kohm. As to voltage, 6 now measures about 157volts, which is better than 186 but still not around 147.

 

[Paul Birkeland]

I'd recommend posting some detailed photos of your build.  There's certainly a miswire somewhere.

-PB