What could possibly be damaged by powering an amp with a bias cap lifted?

Status
Not open for further replies.
Jump to Latest
#3
Good question. I'm a newbie, and am gonna follow this thread, but from the layout pic, I see it as the bias for the P-P power stage, from rectified (by diode) AC from the PT (but you knew that). Hmm...wish I could see the schematic. What kind of PI is it? Hard for me to visualize it from the layout. I would think just lifting the cap wouldn't change DC voltages, but it is in parallel with a resistor, so has some filtering effect (which would be lost without the cap). Any help? Schematic?
 
#4
I'm trying to back track a problem that's now blowing the mains fuse. It doesn't blow immediately, it only blows once the amp has warmed up about a minute past the point of passing guitar sound.


I've been through all the voltages with a light bulb limiter attached and everything looks normal. I've been scratching my head on this one off and on for a few weeks. I'm to the point of just replacing the whole bias section and see if it fixes the issue. This is a fairly fresh build but it did work fine.
 
#5
LATUBEGEEK said:
Good question. I'm a newbie, and am gonna follow this thread, but from the layout pic, I see it as the bias for the P-P power stage, from rectified (by diode) AC from the PT (but you knew that). Hmm...wish I could see the schematic. What kind of PI is it? Hard for me to visualize it from the layout. I would think just lifting the cap wouldn't change DC voltages, but it is in parallel with a resistor, so has some filtering effect (which would be lost without the cap). Any help? Schematic?
Click to expand...

Just to clarify, my build is using a 5AR4 tube as the rectifier.

I used this schematic but used a PT with a bias voltage tap and incorporated the adjustable bias circuit found at the bottom of this schematic


Perhaps I'll work on drawing up the exact schematic as implemented, seeing as I've recently learned how to use some schematic software....
 
#6
I found the schematic...that 100uF is just a filter cap....Kevinkr is right - you would get ripple hum... You should use a DMM and look at voltages. My experience (little that it is) shows voltages to be HIGH as the when power is first applied, then lower as tube start conducting/amplifying. Look at B+ points A thru D for changes before the fuse blows. Will help identifying the stage where the problem is. Then use a different tube, check for hot resistors (very carefully - VOLTAGES - keep yourself ungrounded!).
 
Status
Not open for further replies.
Top Bottom