Tweet
Pickup up an "As-Is" Rebel 20 (ver. 1) - No output
Initial problem - bad main fuse (owner couldn't find the "real" fuse in the IEC socket, which led to the blown C127 (100uf/450v) 1st stage cap and suspect diodes. (When I say blown - I mean the ground plane was blown off the board around the V+ solder pad which was blown off as well.
Good voltages now, works, but very odd bias problem
New tubes - good sound, but 6V6 alone about 2/3rds EL84 volume.
Step 2 - Check bias
Good voltage to sockets (Pin 5 6V6 / Pin 2 EL84) can't recall exact reading but it was solid negative and different
Meter in EL84 reads right at 40 mvdc
Meter reads 10 vdc (not milivolts) at 6V6 test point. Tubes play, not red, just low output.
Looking at the schematic, cathode "sits" on a 1 ohm resistor to "System" Ground to read bias. But, if the schematic is correct, there is a parallel resistance of 10K to the Bias Ground. Reading the EL84 TP to ground gives me 1 ohm, which would be fine. Reading the 6V6 TP to ground gives me about 10 ohms. I haven't gone back in and removed the Output jack board to see the full Power Tube board but I'm wondering if either the 1 ohm cathode resistor has gone bad, or did it get replaced with the wrong one.
Since the Bias circuit for the EL84's works, something in the 6V6 side isn't right. I've got 400 V on the Plate and Screen connection with the tubes out and, there is good sound out of the 6V6's, just low. If the reading on the TP was correct, the tubes show glow and then burn out.
Wanted to get some idea as to why it is reading so weird before I pulled things apart. When switched to all EL84 it sounds great.
Thanks
Initial problem - bad main fuse (owner couldn't find the "real" fuse in the IEC socket, which led to the blown C127 (100uf/450v) 1st stage cap and suspect diodes. (When I say blown - I mean the ground plane was blown off the board around the V+ solder pad which was blown off as well.
Good voltages now, works, but very odd bias problem
New tubes - good sound, but 6V6 alone about 2/3rds EL84 volume.
Step 2 - Check bias
Good voltage to sockets (Pin 5 6V6 / Pin 2 EL84) can't recall exact reading but it was solid negative and different
Meter in EL84 reads right at 40 mvdc
Meter reads 10 vdc (not milivolts) at 6V6 test point. Tubes play, not red, just low output.
Looking at the schematic, cathode "sits" on a 1 ohm resistor to "System" Ground to read bias. But, if the schematic is correct, there is a parallel resistance of 10K to the Bias Ground. Reading the EL84 TP to ground gives me 1 ohm, which would be fine. Reading the 6V6 TP to ground gives me about 10 ohms. I haven't gone back in and removed the Output jack board to see the full Power Tube board but I'm wondering if either the 1 ohm cathode resistor has gone bad, or did it get replaced with the wrong one.
Since the Bias circuit for the EL84's works, something in the 6V6 side isn't right. I've got 400 V on the Plate and Screen connection with the tubes out and, there is good sound out of the 6V6's, just low. If the reading on the TP was correct, the tubes show glow and then burn out.
Wanted to get some idea as to why it is reading so weird before I pulled things apart. When switched to all EL84 it sounds great.
Thanks
Comment