So, I rewired the Earth Revival to the original schematic. When I powered it up, everything seemed OK. I did not check the B+ voltage, which in hindsight I realize I probably should have done first, but I did check the voltage at the grid and it was around -50V or so when the bias pot was adjusted to set it to a minimum, which seemed like a reasonable number to my inexperienced head.
I then put in the power tubes to complete the bias. I initially used the primary shunt method, with a lead clipped to the center tap and the other to one side of the OT primary, and when I turned the amp on, the current started out reading around 80mA and slowly drifted down around 40mA as the amp warmed up. I'm not sure if the drifting is normal, but it seemed reasonable that it would take a minute to "settle in" as all of the capacitors in the amp were new. The current seemed reasonable, as it's a 4 x 6L6 design, so it'd be about 20mA/tube with the bias pot at min.
During this time, I decided to instead measure the voltage at each end of the primary and also at the center tap and do a little math to set the bias, as I already knew the impedance of each side of the primary and the drifting current made me second-guess my measurement. The shunt method also makes me a little nervous, even though I clip the meter leads to the points. I turned off the amp, and moved both of my meter test points to ground + primary, changed the meter to read DC Volts, and turned the amp back on. Shortly after this, the fuse blew. I removed the meter, put a new fuse in, and it immediately blew as soon as I turned the power on. Tried fidgeting with a few other things, replaced the bias pot, since it was about the only thing besides solid-state components I hadn't replaced (all new caps, not just the electrolytes, all new wire as the old was a rat's nest of cheap wire and the filter circuit needed complete rebuilt...).
I've tried all I can think of, but the fuse still blows immediately when I turn on the power. I've triple-checked the grid & screen resistors, the choke resistors, & all of the resistors in the filter circuit, which were all new, and they are all still correct value. I saw no smoke from anywhere other than the fuse when it initially blew.
Any advice/suggestion as to where to begin looking for the problem would be greatly appreciated. The only thing I haven't checked and/or replaced since I started working on the amp are the contact relays & diodes (I think, solid state component w/ 3 legs, labeled "Q" in schematic), mainly because I didn't know how to check them and there were so many other problems I didn't think they were likely to be the source of trouble.
Aaron
I then put in the power tubes to complete the bias. I initially used the primary shunt method, with a lead clipped to the center tap and the other to one side of the OT primary, and when I turned the amp on, the current started out reading around 80mA and slowly drifted down around 40mA as the amp warmed up. I'm not sure if the drifting is normal, but it seemed reasonable that it would take a minute to "settle in" as all of the capacitors in the amp were new. The current seemed reasonable, as it's a 4 x 6L6 design, so it'd be about 20mA/tube with the bias pot at min.
During this time, I decided to instead measure the voltage at each end of the primary and also at the center tap and do a little math to set the bias, as I already knew the impedance of each side of the primary and the drifting current made me second-guess my measurement. The shunt method also makes me a little nervous, even though I clip the meter leads to the points. I turned off the amp, and moved both of my meter test points to ground + primary, changed the meter to read DC Volts, and turned the amp back on. Shortly after this, the fuse blew. I removed the meter, put a new fuse in, and it immediately blew as soon as I turned the power on. Tried fidgeting with a few other things, replaced the bias pot, since it was about the only thing besides solid-state components I hadn't replaced (all new caps, not just the electrolytes, all new wire as the old was a rat's nest of cheap wire and the filter circuit needed complete rebuilt...).
I've tried all I can think of, but the fuse still blows immediately when I turn on the power. I've triple-checked the grid & screen resistors, the choke resistors, & all of the resistors in the filter circuit, which were all new, and they are all still correct value. I saw no smoke from anywhere other than the fuse when it initially blew.
Any advice/suggestion as to where to begin looking for the problem would be greatly appreciated. The only thing I haven't checked and/or replaced since I started working on the amp are the contact relays & diodes (I think, solid state component w/ 3 legs, labeled "Q" in schematic), mainly because I didn't know how to check them and there were so many other problems I didn't think they were likely to be the source of trouble.
Aaron
