Tweet
http://img104.imageshack.us/my.php?image=biasvo1.jpg
I recently went to fixed bias and it sounds great. But i have issue with it and i can't see where i went wrong. Sorry about the sloppiness, but the schematic above shows how i wired my bias circuit which is copied from a marshall. I put 1 ohm R's from each EL34's cathode and pin 1 to ground. I measured them and they read .9 ohm according to a fluke meter, so i think thats close enough. When i check pin 5 things are fine as it reads 44vdc with the pot fully one way. But when i go to measure current across the cathode R's i get about .005 mv. It's supposed to be around 35-40, but at the highest setting i can get it with the pot, it's only about .005. (autoranging fluke, and even manual at 600mv reads the same) So i changed the R that is noted on the schematic down from 56k to 33k and finally to 10k. At 10k it read 24mv and the fuse blew ! So i can't even get it up to where it is supposed to be, yet even at 10 or more mv LESS that where i should be able to get it to read it blows a fuse ! In fact, i think that killed a tube because it's blown a couple more since then even after i put that R back to 56k, and i had played it for a good while with no blown fuses before i started trying different R values. Any ideas as to what's wrong with using the bias circuit from a JCM 800? And yes, the diode and caps are oriented right.
I recently went to fixed bias and it sounds great. But i have issue with it and i can't see where i went wrong. Sorry about the sloppiness, but the schematic above shows how i wired my bias circuit which is copied from a marshall. I put 1 ohm R's from each EL34's cathode and pin 1 to ground. I measured them and they read .9 ohm according to a fluke meter, so i think thats close enough. When i check pin 5 things are fine as it reads 44vdc with the pot fully one way. But when i go to measure current across the cathode R's i get about .005 mv. It's supposed to be around 35-40, but at the highest setting i can get it with the pot, it's only about .005. (autoranging fluke, and even manual at 600mv reads the same) So i changed the R that is noted on the schematic down from 56k to 33k and finally to 10k. At 10k it read 24mv and the fuse blew ! So i can't even get it up to where it is supposed to be, yet even at 10 or more mv LESS that where i should be able to get it to read it blows a fuse ! In fact, i think that killed a tube because it's blown a couple more since then even after i put that R back to 56k, and i had played it for a good while with no blown fuses before i started trying different R values. Any ideas as to what's wrong with using the bias circuit from a JCM 800? And yes, the diode and caps are oriented right.
Comment